Junkwood Timberframing?

I'm looking for the nearest on-ramp to the timber framing superhighway. I've done some traditional timber framing through sessions at the Low Tech Institute. Very cool and very helpful but very labor intensive and I could care less if my buildings have tidy, square-looking frames. Having read a bit here about roundwood, I asked Scott the director what the benefits of squared timers for his project were.  He didn't have an answer and the way his face made it look like wheels were in motions I'm wondering if the next construction there might have a more raw, rounded look to it.

I recently borrowed Ben Law's book and it is highly impressive. After looking through it, I feel like I viewed the Mona Lisa and am standing with a paintbrush in my hand and nothing but an overwhelming sense of inferiority in my mind. I'm looking for something that will take me from a property of raw timber to one with some cool buildings on it, with as few shortcut 'cheats' along the way as possible.

My concessions to convenience and promptness are the use of free pallets from a local business (in ample supply and destined for a disposal burn presumably), and the use of cordless tools and reusable construction screws. Also, metal roofing as something reusable in the face of extremely skill and time intensive alternatives.

I have 4 walls up with a salvaged door to complete the lower structure. It is a 16'x16' foundation of pallets with an 'H'-ish structure of walls extending 16' on the long sides and two 10' cross walls defining a 10' square interior space. The walls are roughly 8' tall. The final step is to take the 5 3x16' sections of standard metal roofing I've purchased and making a roof out of it. It would be relatively easy to buy some 16' 2x4s and throw up some rafters and purlins for mounting, but I have a large supply of timber on the property of roughly the correct dimensions.

The surplus materials are mostly from red pines. The primary stand has 60-70' trees that are about 12" at the base. There is another stand of younger trees maybe 20' tall max that have never been thinned. I am clearing the larger trees in an area and have the tops left that are approximately the correct size. The smaller stand has infinite material through thinning. Both tops and saplings seem to have a bit of a taper, though I haven't inventoried through the stand that carefully. There might be some undergrowth saplings spread throughout that would be around 15' of consistent diameter, but I'm not sure if I could find a half dozen that are the same.

Looking at examples around Wheaton Labs, The Willow Bank looks pretty close to what I'm looking to construct in layout, although a little smaller. No need to get fancy around the sides, just a moisture-proof sheet floating a bit over the square layout with plenty of overhang all around. I've been told a simple sheet of the metal would work, precipitation wouldn't draw under the top edge and leak down. The original roof framing was sparse and seemed to be stressed out. I don't see updated pictures of how it was rebuilt, just a diagram of Mr. Haasl's suggested improvements.

So point of discussion is basically, is it a good idea to try and make this roof out of these thin red pine pieces? What needs to be done to make it reasonably solid? I'm pretty sure the overall effort will be less economical than simply purchasing dimensional lumber, but my priority is to have a product with more personal inputs, as well as to learn a technique that would be useful for future projects and perhaps hint at more robust timber framing tasks.

Not sure if this needs to be mentioned, but I will be limbing and debarking all of the sections. One consideration would be if hewing 2 sides would be practical and helpful for even dimensions. My thought is it would be difficult to do consistently, that the taper could be built into the design. Minor dimensional variations seem inevitable either way, the metal roof should flex enough to accommodate them. Perhaps just spot hewing if some pieces are obviously out of spec with the rest?

Another idea I thought was to assemble two pieces screwed together, base to tip, to counter the taper and make something that would be more like a 2x4. My main concerns would be that, used the flat way, it would be easy to miss the meat of the 'board' with a screw; used on end, any screw would need to be long enough to include both pieces. Any thoughts on this working as a rafter. My understanding is that purlins just need to have some substance for a screw, structural strength isn't a big consideration?

The other main consideration is slope. I built a similar shed last fall (using dimensional lumber) and used what I read was the minimal slope for a "shallow" sloped roof. I think it was 15*, works out to be 12:1? I put simple vertical 2x4s on the high side screwed to 2x4 'rafters' about 24" on center as the roof supports (no purlins). The slope was not enough to shed any significant snow, but the supports held for the whole season. The riser 2x4s show a slight racking, but I think a few diagonal supports would have taken care of that. Bottom line, I think 15* was more than I needed since it didn't shed snow, any slight slope that will shed fluid would work the same while drawing less stress on the risers.

The store I bought the roof from said standard purlin spacing is 20" on center when I asked about number of screws that would be needed. I think at one point Mike advised 24" for rafters. What is the general consensus on guidelines like this? I know I saw a table of equivalents for roundwood to square timber, was that in Ben Law's book? How about standard guidelines for roof framing like this, any 'authoritative' references?

I have to admit I read your whole presentation and I am confused.

I'm looking for the nearest on-ramp to the timber framing superhighway.

I dont think there is a shortcut.
I have a few questions though;
-  why are these tools important, " use of cordless tools",
- purlins and RAfters descriptions seem to be mixed up
-

My understanding is that purlins just need to have some substance for a screw, structural strength isn't a big consideration?

Each part has to have the strength to carry out its function, what did you mean with this comment?
I would put it to you the advantage of squared timber are as follows;
- consistent sizing standardise's dimensions, fasteners and uses the timber in a more efficient manner. But if you are using trees off your property it may not be an issue.
- fastening of metal panels to square timber is a lot easier.
- assembly is easier
- lining to exclude wind and water is easier.
can you post some images?

John C Daley wrote:I have to admit I read your whole presentation and I am confused.
]
I dont think there is a shortcut.
I have a few questions though;
-  why are these tools important, " use of cordless tools",
- purlins and RAfters descriptions seem to be mixed up
]
Each part has to have the strength to carry out its function, what did you mean with this comment?
I would put it to you the advantage of squared timber are as follows;
- consistent sizing standardise's dimensions, fasteners and uses the timber in a more efficient manner. But if you are using trees off your property it may not be an issue.
- fastening of metal panels to square timber is a lot easier.
- assembly is easier
- lining to exclude wind and water is easier.
can you post some images?

Cordless tools are important because of a need to get this shed up to move items to from other leaky and for-pay storage space. I have drills and screws on hand and don't want to take time to do M&T or twine joinery at this point.

From what i gather, the rafters will run the full 10' span of the walls as well as the overhangs outside that. The purlins will only need to span 16", 24", or whatever i space ther rafters at. I don't see the small pieces I would use for purlins as being any sort of support problem, but having to go blind through the roof and find them with a screw might be.

I don't think any lining to exclude moisture will be needed. The screws include some sort of conforming washer under the head that seals against the metal. The edges would be the same as the willow bank from the looks of it, which I've never heard of leakage problems with. The only difference besides the attempt to use roundwood would be to space it 3-6" or so in from the edge to avoid getting that wood wet.

I have to switch phones to post pictures from construction, will take some new ones also...

Here's the few pics from this phone. The first two are from the current build, just the foundation of pallets, carefully chosen and placed on leveled ground.

The last two are from the first iteration, thrown up on top of snow with whatever pallets I could find of roughly the right size. The roof was some old salvaged metal that was all wrinkled and had holes here and there. The simple 2x4 risers and rafters took all the snow load for the winter without any help. The whole structure started to keel to the side after the snow melted, so I'll need to redo everything once I can move stuff to the new shed.

I just picked up a load of 50+ pallets.  I find them handy to have on a homestead.  If it were my build, I would be concerned about those pallets in contact with the ground rotting out in a few years.

John F Dean wrote:I just picked up a load of 50+ pallets.  I find them handy to have on a homestead.  If it were my build, I would be concerned about those pallets in contact with the ground rotting out in a few years.

The main town I drive to regularly for all my non-self-sufficiency needs has a place that has been giving massive numbers of them away free. I've heard any unclaimed ones will just be burned to get rid of them, so I guess repurposing them offsets the carbon of transporting them at least somewhat. Definitely with the price of lumber lately, this is the best option money-wise at the moment.

I'm going with the metal roof and screws so that in a few years when the foundation rots, the roof section could just be transferred over to something the same dimensions built out of roundwood timbers and/or lumber milled from my trees.

My thought is that the addition of maybe 18 concrete blocks to lift the deck off the ground might substantially add to the lifespan of the deck.  Although not as easy to find in my area as pallets, scrounging 18 concrete blocks is certainly doable.  

Some current pics: the first 3 are of the front of the new shed with a salvaged screen door. The last is the current old shed with an idea of how the walls will end up- drying space for cordwood under the overhang, providing a barrier to most wind and anything else...

Shout out to Arthur "Two sheds" Jackson for the homesteading inspiration.

There is a world wide shortage of wooden pallets at the moment, its amazing those pallets are not put back into the system that moves them around.
Anyhow, you are using them in a clever manner.
Purlins are run across the roof rafters, but you dont seem to have any on that shed, is that correct?

John C Daley wrote:There is a world wide shortage of wooden pallets at the moment, its amazing those pallets are not put back into the system that moves them around.
Anyhow, you are using them in a clever manner.
Purlins are run across the roof rafters, but you dont seem to have any on that shed, is that correct?

Most encounters with economic reality I've had in the last 2 years, intensifying this last year, seem to fly in the face of what is being portrayed by the general 'global media', these pallets being at the minor end of the list.

The old shed has rafters only. I shingled the sheets of metal horizontally as they had no structure built into them. The new stuff has ridges pre-formed and is designed to fit together along the slope and shed fluid that way, so purlins will be needed. Nothing is done for the new shed roof yet, I also need to design something for the high side of the shed to mount the rafters on. The high side will be to the left in the pictures I posted, to the SE. The prevailing wind is from the NW so the downslope of the roof is aimed that way (to the right in the pics) to take on blizzardy weather.

John F Dean wrote:My thought is that the addition of maybe 18 concrete blocks to lift the deck off the ground might substantially add to the lifespan of the deck.  Although not as easy to find in my area as pallets, scrounging 18 concrete blocks is certainly doable.  

Too late for this iteration, but what are you thinking with the number 18?

The 4x4 pallet pattern has a 5x5 grid of corners. To support just the 4- way intersections would be the interior 3x3, leaving the outer edges cantilevered with 9 blocks in the middle. Doing all intersections except the outer 4 corners would need 21 blocks.

It seems to me that adding metal strapping with a decent number of screws holding it on the diagonal of each of your walls, would decrease the risk of the structure slumping. The place you get the pallets from, might even have some of the strapping that's used to hold heavy things to the pallets and it would possibly do the job.

Everything I've read about roofing is that if you go lower than 2 to 12, it's considered "low slope". Even a metal roof is more likely to leak under those conditions because it's the "speed" of drainage, not just that it will drain. If I was going that flat, I'd be seriously considering beefing up the walls, putting EDPM on the roof and making it a "green roof" with pumice and plants.

Jay Angler wrote:It seems to me that adding metal strapping with a decent number of screws holding it on the diagonal of each of your walls, would decrease the risk of the structure slumping. The place you get the pallets from, might even have some of the strapping that's used to hold heavy things to the pallets and it would possibly do the job.

Everything I've read about roofing is that if you go lower than 2 to 12, it's considered "low slope". Even a metal roof is more likely to leak under those conditions because it's the "speed" of drainage, not just that it will drain. If I was going that flat, I'd be seriously considering beefing up the walls, putting EDPM on the roof and making it a "green roof" with pumice and plants.

I built a very low slope roof over a walk-in cooler. 1/2 to 12 pitch (6":12') and I had leaks. I did not caulk the corrugated roof panels at the laps, and that was the main problem. Another is that flat of a roof collects leaves (ours has an overhanging tree) and doesn't get flushed clean even with a heavy rain. Although now five years on, it doesn't seem to have leaked beyond a few events at the outset? Maybe that seam got "caulked" with detritus?

Jay Angler wrote:Everything I've read about roofing is that if you go lower than 2 to 12, it's considered "low slope". Even a metal roof is more likely to leak under those conditions because it's the "speed" of drainage, not just that it will drain. If I was going that flat, I'd be seriously considering beefing up the walls, putting EDPM on the roof and making it a "green roof" with pumice and plants.

The Willow Bank doesn't look like it has that steep of a slope to the roof. Do we know what it is?

Coydon Wallham wrote: The Willow Bank doesn't look like it has that steep of a slope to the roof. Do we know what it is?

I certainly don't. However, I'm prepared to take an educated guess and then we'll see how close I get!
1. The building appears from the accoutrements to be approximately 6 feet wide or 72".
2. The planks on the side view, appear to be standard 1x4's which are really 3 1/2 inches wide.
3. That means that the roof rises ~11" over 72" which would be maybe 1 3/4 " over 12.

Personally, considering the winter picture shows the snow sliding off, I'm guessing that my "guestimate" is on the low side. The building could be narrower, or the planks could be wider. There just isn't enough scale.

What I do know is that the chicken coop that the previous owners built here doesn't look like its roof has that much slope, but when I tried to measure it, I got about 3" over 12". Good enough for regular shingles, yet feels fine to walk on it when I need to clear the cedar debris off it. Our house roof is much steeper, and even 10 years ago, it made me feel uncomfortable. Now I refuse without a seriously expensive climbing/fall arrest kit on. I told Hubby if he wanted my help, he had to buy me something that fit my small size and I was worth the expense. The lady who fitted me at Hazmasters, insisted Hubby needed the same size and style because the cheap one he had bought was totally too large for him also.

If you want to go "flat" roof, I'd reinforce the roof supports with corner "timberframe" posts, decide where you want the water to run off, ( ie you still want some slope if you want to keep the ground under your pallets dry ) and design it to be flat. Multiple layers of tarp with water-proofing such as that Latex/concrete mix, or splurge on EDPM, then go for dirt and sedums. A roof like that can easily last 75 years or longer. Former owners of my Mother's house had made a giant flat-topped dormer on the north side of her roof. She had leakage problem repeatedly until she found a roofer who said what I've said - Not enough slope, need to treat it as flat, shingles won't work, needs a sealed membrane. Now my sister owns the house and there hasn't been a leak since.

WR to fastening the roof rafters to the high side, why not run a top plate across the studs you have protruding out from the pallets?
As Jay suggested some bracing may help, even as two diagonals across the roof structure, above the purlins but below the iron roofing.
I see some rolled roofing in the corner, do you realise that is wall sheeting, its ribs are lower than roofing iron.

John C Daley wrote:WR to fastening the roof rafters to the high side, why not run a top plate across the studs you have protruding out from the pallets?
As Jay suggested some bracing may help, even as two diagonals across the roof structure, above the purlins but below the iron roofing.
I see some rolled roofing in the corner, do you realise that is wall sheeting, its ribs are lower than roofing iron.

Don't know what studs you are referring to, do you mean in the older shed?

Seems to me putting something between the purlins and the roofing would mess with the basic function of the purlins...

What impact on the structure would the height of the ribs have?

You're right, diagonal bracing in the roof plane would need to be below the purlins, or at least not above them.

The lower the ribs on metal roofing (at fastener or lap joint locations), the more likely you will get leaks. Also, low ribs will require more frequent purlins to avoid sagging.

Glenn Herbert wrote:The lower the ribs on metal roofing (at fastener or lap joint locations), the more likely you will get leaks. Also, low ribs will require more frequent purlins to avoid sagging.

Is there some sort of guideline on what sizes these ribs should be for various applications?

Come to think of it, I could really use to know what size 'timbers' it would take to 'officially' support the roof. I remember seeing a table of roundwood equivalents recently, would it have been in the Ben Law book I borrowed? Would such a table show small things like 2x4 and 2x2 equivalents, is that a thing?

Coydon Wallham wrote:

Glenn Herbert wrote:The lower the ribs on metal roofing (at fastener or lap joint locations), the more likely you will get leaks. Also, low ribs will require more frequent purlins to avoid sagging.

Is there some sort of guideline on what sizes these ribs should be for various applications?

The ribs on the ones they sold me as 'metal roofing' are 1" tall.

Sifting through web articles, I've found that:
A) In Australia and the UK, they seem to call over-purlins "battens", and "purlins" would refer to under-purlins (not that I knew what any of those terms meant a week ago)...
and
B) Around here at least, .75 to 2.5 inch ribs are all used for standing seam roofing. Nothing I found has a guideline on how to determine various strengths, but the guy at the store that was demonstrating where to place the screws said that purlins were 20" standard. Their roofing and some other ribbed metal I found at the dump from someone who dismantled their roof are all 1" ribs and look identical to anything I can recall seeing before, so I'm guessing that is the most common standard in the US at the DIY level?

Common metal roofing does have ribs about 1" tall. It is intended that a soft sticky rubber gasket goes between layers at the joints for waterproofing. This would be more important the lower the slope.

Standing seam roofing may or may not be designed to require a gasket; it was developed before there was reliable/durable gasketing material available.

Battens are spacers to stand off from a solid surface, while purlins span between rafters or other structural members and require some strength of their own.

To add to my growing glossary list of clarifications/corrections, standing seam is not synonymous with metal ribbed roofing, it is a sub type that only has ribs at the seams and is secured without visible fasteners...

Talking to an experienced installer at the store about the "classic" ribbed metal roofing they sold me (looking at my stack of panels again, I'd say they have .75 inch ribs actually), the standard figures recommended at 2:12 minimum slope are 2x4 purlins at 20" (might have been 24" with him) on center. I asked about using 1x material for purlins on a shed like this, he thought that would work as long as the rafters weren't more than 24" on center. [Edit: according to the official literature I picked up this time, it is 3/4" ribs and meant to have a 3:12 minimal slope.]

Talking to a local permie expert about hacking something together with round wood, his suggestion was roundwood for the rafters and dimensional lumber for the purlins to have a more reliable mounting surface. I asked if hewing the roundwood high spots under the purlins made sense, he said better to shim the low spots. He advised 2x4s for the purlins.

I'm thinking that with (free) roundwood rafters at 24", 1x3 purlins would provide enough strength. So i think my decision is between making 8 roundwood rafters to support 1x3 purlins, or 6 to support 2x4 purlins. Do they both sound like they will support a heavy snow load well enough? I'm not sure what the price difference in the lumber would be, but just having more timber off the property in the structure seems like a win...

Btw, I'm thinking 3" for the roundwood rafters, spanning the 10' between the pallet walls. Also 2x4 corner bracing between pallets at the ceiling joist level.

I haven't seen you mention what the snow loading actually is in your area. It's hard to determine what's going to be strong enough without knowing that. Local building inspectors would be able to tell you.

A distant neighbor has offered 60lb/ft as a snow load estimate.

I was starting to think about how I could use some stout timbers to make risers for the high end of the rafters and it occurred to me I could have just built the one wall (all the way left from the door in the pics above) that much higher with the prolific supply of various pallets I've picked up. At this point removing a couple dozen screws to take down the old one and replace it with pallets that stand 20+ inches taller seems like less work resulting in a more solid structure than anything else I've been considering. I think it will end up rising either 24 or 26" inches higher this way, over the 10' span

I have never seen so much discussion about a roof!
The diagonal bracing I spoke of is flat steel straps that come in a roll, with holes every 2 inches.
It takes up no room and stops a twist and collapse movement.

John C Daley wrote:

I have never seen so much discussion about a roof!

Yes - I think it's awesome. I expect Coydon's learning a lot. It always amazes me what ideas people can come up with. Some day, I would sooo... love to have a shed with a barrel roof.

@ Coydon: Is there a reason you're set on a "shed" roof?  Have you considered making it a gable roof? I've read somewhere that mechanically speaking, the gable roof is stronger than a shed roof. The fellow that built the log cabin up north years ago, used thin round wood rafter tightly set.  

The correct term is 'skillion' and then gable.
Both have their advantage, the gable has two lots of guttering c.w. one for the skillion,
but the gable will span a greater distance with reasonable sized materials c.w. a skillion.

Yeah, I saw that skillion is the down under name for what we tend to call shed or lean-to roofs in the US. That seems to be a big chunk of the discussion so far: navigating through this tower of Babel that is the internet. The English diaspora is one big group of people divided by a common language.

I also had my dad tell me, when he was having a contractor do a bunch of stuff in his attic, something like 'if you think it is expensive to build a house from the ground up, try building one from the roof down.' For me that goes along with an Abraham Lincoln quote i just read: 'If you give me 8 hours to chop down a tree, I'll spend the first 6 sharpening my axe.' Only with the benefit of hindsight I'd get the axe sharpened in an hour, then research the tree to make sure no one will think I'm a tyrant for chopping it down.

Trust me, it all makes sense if you have enough tick and mosquito venom following through your veins...

Jay Angler wrote:
@ Coydon: Is there a reason you're set on a "shed" roof?  Have you considered making it a gable roof? I've read somewhere that mechanically speaking, the gable roof is stronger than a shed roof. The fellow that built the log cabin up north years ago, used thin round wood rafter tightly set.  

The gabled roof seems like triple the work at this point. It would require preparing twice as many rafters, plus the ridge beam and joinery there. The added strength and efficiencies of the second slope aren't much benefit with my needs for this building. I'm also sticking with the basic design from the first shed and making improvements only where I've witnessed a need for them, don't want to introduce new, superfluous opportunities to screw up.

Plus, having visited Wheaton labs and read many threads here, i think Paul has had great success with these roofs. I like the look of those buildings.
It even incorporates a bit of the FLW prairie architecture local to me with the big overhangs. I think i just need to anticipate more snow load than Montana here...

A shed roof is definitely simpler and has fewer joints and possible leak points. Depending on situation and climate (and intended use), either can have advantages. A shed roof can have the high wall to the south for maximum solar gain, and only has runoff on one side if that is a benefit, but in high wind exposures it can catch more wind and be at greater risk. 10' is not a huge span, but long enough that it needs substantial rafters. (I don't think 3" at the small end is safe unless the wood is oak.) Splitting that with a substantial ridge beam would allow small rafters to be perfectly safe. The ridge ends would need concentrated support, more than pallet framing.

I tried out some different search terms in a general web search and found a similar existing discussion- lo and behold, it is from some place called permies.com.

The info there on roundwood equivalents only starts at 6".

I was also looking at a Wikipedia page on traditional timber framing and they mentioned that at one point the milled lumber was tapered toward the high end without loss of strength to the structure. It seems any weight or material savings wasn't worth the effort to mill specialty lumber so it didn't become widespread, but helps inform what the strength of tapered roundwood will be in a final structure.

This is a great thread, building with pallets is very economical.
I totally get spending time on the technique and design of a building in order build well without spending money.

Since you have "free" long timbers  and pallet wood you could build trusses from pallet wood and timber,  to vastly increase the strength of your roof.
A simpler solution is to use a massive number of timber rafters, like one at every foot.
Same goes for the purlins, with a supply of pallets you can afford as many you want, at whatever spacing.
It would of course add weight along with strength,but your load bearing walls are arguable the cheapest and easiest part of your build.

Speaking of walls, for sheathing I encourage you to staple  pallet deck boards to your framing, diagonally.
Put over your vapor barrier and the it could be bracing and furring for the rain screen.
Finish up with board and batten siding made from the same pallet deck board.

For my rafters, I have a stand of 20' trees I can thin as much as needed, a half dozen I've cut down this spring, the tops of ~60' trees I'm in the process of cutting down, and old tops spread throughout the property from past timber harvests over many years. Will a moisture meter tell me all I need to know about their usefulness? Would the old tops be useable as long as they weren't noticeably punky? Will punky wood start to read more moist than when it was solid? Does the 'drop' test work better (drop the limb on the ground hard, if it doesn't snap it is usable)?

William Bronson wrote:This is a great thread, building with pallets is very economical.
I totally get spending time on the technique and design of a building in order build well without spending money.

Since you have "free" long timbers  and pallet wood you could build trusses from pallet wood and timber,  to vastly increase the strength of your roof.
A simpler solution is to use a massive number of timber rafters, like one at every foot.
Same goes for the purlins, with a supply of pallets you can afford as many you want, at whatever spacing.
It would of course add weight along with strength,but your load bearing walls are arguable the cheapest and easiest part of your build.

Speaking of walls, for sheathing I encourage you to staple  pallet deck boards to your framing, diagonally.
Put over your vapor barrier and the it could be bracing and furring for the rain screen.
Finish up with board and batten siding made from the same pallet deck board.

I like the truss idea to an extent, but this is a quick practical shed. I'm planning along the lines of what you say about lots of these timbers for rafters, just need a better idea of how many would give it a fair chance to survive a few winters before it is replaced. I'd like to get used to using some roundwood by hacking this together while moving more toward regular timber framing in further structures, IE traditional joinery. Are trusses practical for that?

It is also going to be an outdoor storage shed, no fancy finish to the walls. Stacking cordwood around the perimeter as I show in the picture of my older shed above, along with 2.5' roof overhangs (3' on the high side, 2' on the low side) should make it water proof to anything but a hurricane, while providing drying space for the infill for a cordwood structure I intend to build a few years down the line.

For structural reinforcement of the pallet frame, I am thinking of 4 corner braces, parallel to the ground at the top of the short walls, about 1/3 of the way along each wall. Quick and simple with the materials on hand, plus will offer supports to hang storage items from. Does that sound like a useful addition?

When I'm building with dimensional lumber of questionable strength I test it with my body weight, either hanging from it or standing on it, and sometimes for good measure, I bounce up and down.
This tends to lead to overbuilding, since most structural members don't need to support 280 plus pounds of weight.
For your purposes, two 5 gallon buckets of  water will weigh more than 80 lbs., so maybe hang them from your timber and bounce them up and down?

What species are the older tops, and how far off the ground have they been sitting? Some will become unsafe after just a couple years on the ground. Testing anything that is not new would be wise, as William describes. Moisture doesn't necessarily tell you anything about soundness.

If you just want an outbuilding to use for a few years, 3"+ poles spaced at a foot should do fine. That would be close enough for pallet wood to serve as purlins. The thin wood would be quite bouncy at wider rafter spacing, and I wouldn't necessarily trust it with serious snow loads.

Diagonal bracing on the horizontal plane as you describe would not do much; the metal roofing screwed to the framing will keep the deck square. The issue is the walls leaning, and at least one diagonal brace on each wall is what will help there.

Glenn Herbert wrote:What species are the older tops, and how far off the ground have they been sitting? Some will become unsafe after just a couple years on the ground. Testing anything that is not new would be wise, as William describes. Moisture doesn't necessarily tell you anything about soundness.

If you just want an outbuilding to use for a few years, 3"+ poles spaced at a foot should do fine. That would be close enough for pallet wood to serve as purlins. The thin wood would be quite bouncy at wider rafter spacing, and I wouldn't necessarily trust it with serious snow loads.

Diagonal bracing on the horizontal plane as you describe would not do much; the metal roofing screwed to the framing will keep the deck square. The issue is the walls leaning, and at least one diagonal brace on each wall is what will help there.

The tops are all red pine. There is a sprinkling of hardwood saplings that they take down as they are thinning the pines. I've identified birch, poplar, cherry, and maple so far. Not sure if I could tell the downed saplings apart or know which ones would be usable (cherry and maple?), but easy enough to sort them from the tops...

I'm thinking of just doing new 1x3s for the purlins. Lumber prices are coming down around here and I gather 1xs would be fine at 12" spacing on the rafters. I need to get the shed done and move on to my yurts soon, more messing with pallet wood at this point I think will put me too far off schedule.