Making a perfect square is HARD!

Any tips ya'll? I'm trying to get the perfect placement for the posts of my greenhouse. It's going to be 24 feet by 8 feet. So a nice long rectangle. The problem is it HAS to be square because my plan for the build is to buy standard 2x4x8 boards, make the wall frames and then screw them into the posts. I did that with the kids clubhouse but it is not on a permanent foundation so it was easy to get square.

I'm going to post some clubhouse pics so you can see what I'm talking about. This was my test last year to see how I would do with framing something and adding windows. My test greenhouse you could say. Except we COULD NOT dig holes in the area this is located as water and electric lines run there so I did a cement pad to help prevent it from blowing away.

I purposefully faced the windows on the clubhouse away from the sun so it wouldn't get too hot in there for the kids. The greenhouse is going to have one wall of windows and the rest of it will be metal siding similarish to the clubhouse.

Well , standard practice is to measure diagonal and adjust your posts to be perfectly matched, as well as plumb in both directions. Brace them solid and finish set them.
Another method (not carpenter recommended) would be get the posts  close but don't pack them, just brace.  Then start attaching your dimension lumber. Soon it will be square enough to pack your posts in.

I'm not a carpenter so close, is good enough for me...  after all, when I build its certainly not going to be a piano:)
You however may want to use the first method.

thomas rubino wrote:Well , standard practice is to measure diagonal and adjust your posts to be perfectly matched, as well as plumb in both directions. Brace them solid and finish set them.
Another method (not carpenter recommended) would be get the posts  close but don't pack them, just brace.  Then start attaching your dimension lumber. Soon it will be square enough to pack your posts in.

I'm not a carpenter so close, is good enough for me...  after all, when I build its certainly not going to be a piano:)
You however may want to use the first method.

We have excessively high winds so I intended to cement the posts in. We did measure on the diagonal and all but the very last one was perfect. For whatever reason the last hole was 8'6" apart and we could NOT figure out why as the diagonal measurements were correct. I am not a very good carpenter. lol

A couple of tips:

1. Always measure to the same point - internal measurement or external measurement. That is, not forgetting the thickness of the timber.
2. Think in triangles - a bit of Pythagoras makes it perfect.
3. Measure twice, cut once. (If you're really bad, measure thrice, write it down, then get an assistant to remeasure it!)

Did you use cross braces? If not, then I suggest you put some in - metal strip bracing would be okay.

F Agricola wrote:A couple of tips:

1. Always measure to the same point - internal measurement or external measurement. That is, not forgetting the thickness of the timber.
2. Think in triangles - a bit of Pythagoras makes it perfect.
3. Measure twice, cut once. (If you're really bad, measure thrice, write it down, then get an assistant to remeasure it!)

Did you use cross braces? If not, then I suggest you put some in - metal strip bracing would be okay.

sooooo What do you mean cross braces?

Not only will diagonal measuring help, but also keep in mind the 3-4-5 rule ... another way to think in triangles.

Measure down one leg of the triangle three units (such as 3 feet), then measure down the other leg of the triangle four units. Then, with your mark at 3 and your mark at 4, measure diagonally from these two marks and that should equal exactly 5 units. If not, push out or pull in to adjust. This is the technique I use to start off a fence corner to ensure I have a right angle (square) corner.

I remember when I was a kid, I helped my father build a pole constructed a 12×24 run in shed.  To make sure it started off square, we made battre boards, then measured corner to corner of the string lines to make square.  At age 12, I thought it amazing.

elle sagenev wrote:soooo What do you mean cross braces?

In the wall frames, to ensure they stay upright over time and don't lean, bracing is used to keep it upright. For example, you've probably seen an old farm building with a severe lean? It's likely they didn't have bracing.

In construction they either use timber or metal strapping. In your building metal strapping is fine to use.

It's simply nailed at, say, the top corner of the frame and run out diagonally to the opposite bottom corner and nailed/screwed into place.

It keeps everything square so doors and windows won't jamb.

This is one commercial brand, most hardware stores should carry similar products:

http://www.mitek.com.au/Products/Building-Products/Building-Product-Range/Bracing/Structural-BracingStrap/Structural-BracingStrap/

Hope that assists.

I have heard this referred too as hurricane strapping.  Keeps roofs on in high winds (try's too anyway).

Looks like everyone's got a good advice on making a square. Just don't remember that dimensional lumber varies in dimension! 8ft boards are almost never 8ft, 2x8s can be 1.5" or 1.75" depending on the miller, etc. Making stuff square can feel tedious, especially if you're in it yourself. It usually means walking back and forth a dozen times inching things into place. For posts, definitely use temporary braces to help figure it out until you concrete them in.

Assuming your doing outside measurements on your 24 x 8.  Applying pythagorus gives you 24^2+8^2 = c^2  Solving for c^2= 640  c= 25.298.   So you know you have 25 ft and some odd inches.  So subtract the 25 ft off so you can just deal with the remainder.  So how do you solve for inches?  That is a simple ratio problem  .298 ft/1 ft = z/12 in.  cross multiply and divide and you now have units of inches.  So you get 3.576 inches.  Now you could use ratio and proportion to solve for the fractions but it is easier to simply look that much up in a fraction to decimal table.  So you end up with a diagonal of 25 ft 3 9/16 inches.  A good double check here is are both diagonals equal.

Or you can short cut it and simply google a calculator that coverts decimal feet to feet and inches.  Here is one such.

decimal feet to feet and inches calculator.

But even this is a lot of work to go to.  As long as both sides and both ends are exactly the same length the building is square when the diagonals are equal.  Usually in framing construction if you can get the diagonals accurate to withing 1/8" it is good enough.  But be sure that your pairs are exaclty the same length or you can have problems.


There are dozens of other ways to reach the same results.  You can use various trig short cuts.  For example using 45 degree diagonals.  So for this you would go out 8 feet on each long side and then measure the diagonal back.  8 ft by 8 ft is a 45 degree angle.  Using sine  or cosine on this you find that c = 1.414 a = 1.414 b = 11 ft 3 3/4 inches.

Now if you are setting posts stringing it first and then setting posts to the strings is the most accurate method I have found.

PS on 3 4 5 triangles remember you can multiple all 3 by any number and maintain the ratio.  so a 6 8 10 or a 9 12 15 are also perfectly valid .

Ed Belote wrote:I remember when I was a kid, I helped my father build a pole constructed a 12×24 run in shed.  To make sure it started off square, we made battre boards, then measured corner to corner of the string lines to make square.  At age 12, I thought it amazing.

Very interesting and I'll have to try it. We did have boards we were using in an attempt to help but corners would help a lot more!

F Agricola wrote:

elle sagenev wrote:soooo What do you mean cross braces?

In the wall frames, to ensure they stay upright over time and don't lean, bracing is used to keep it upright. For example, you've probably seen an old farm building with a severe lean? It's likely they didn't have bracing.

In construction they either use timber or metal strapping. In your building metal strapping is fine to use.

It's simply nailed at, say, the top corner of the frame and run out diagonally to the opposite bottom corner and nailed/screwed into place.

It keeps everything square so doors and windows won't jamb.

This is one commercial brand, most hardware stores should carry similar products:

http://www.mitek.com.au/Products/Building-Products/Building-Product-Range/Bracing/Structural-BracingStrap/Structural-BracingStrap/

Hope that assists.

So our pole barn and garage do have cross braces. I don't know that I will do that on this. It's going to be pretty small. But maybe. We'll see how sturdy it is when I get it up.

@ elle - Let me tell you a story: When I got married, he came with a house and shed built by some previous occupant of the land. One day I asked my husband when he was going to fix the shed, because I thought sooner would be better than later. He looked at me totally puzzled, "there's nothing wrong with the shed". Then I pointed out that it was leaning a good 20 degrees off vertical. The builders had used standard framing methods and clothed the shed in wide clapboards, but strength in buildings comes from using materials that create a diagonal. Over time, snow load, wind load etc, it started to lean. It doesn't have to be specific "braces" - after using ratchet straps to straighten the building, my husband covered some of the inside walls with 4 x 8 ft sheets of plywood which if fastened at each stud creates diagonal support.

In other words - I'm backing F Agricola. If you want this building to last, I would plan on covering sections with something like plywood, buying specific metal bracing, or in some locations you can use a pair of wire rope with turnbuckles in a cross pattern (I've done that to shore up a gate support and to square up a door that had sagged - amazing how many things sag with age...) Bracing key areas will do the job - it doesn't have to be top right corner to bottom left and vice versa - just large enough areas that you get a solid, triangular effect. Do an experiment with your kids and small scraps of lumber, pushing and pulling on the frame so that you will have a feel for the difference it makes and so your kids will learn a valuable life lesson that they won't likely forget!

thomas rubino wrote:I have heard this referred too as hurricane strapping.  Keeps roofs on in high winds (try's too anyway).

Hurricane straps, from what I've used, are embedded into the concrete foundation and wrap over the sill plates to keep the sills attached to the foundation.  That's in addition to the threaded rod embedded that the sill plates are bolted on with.

Diagonal, or cross, bracing is used to keep the wall square and plumb.  If you screw plywood or OSB to the wood frame, it does the same thing and gives you an exterior.  When construction around here switched from frame and OSB to frame and insulation, the strapping had to be added to keep everything square.  I've done 1x3s on a 45* angle, every 2' and also used a T-shaped strapping that required you to run your skilsaw down the line to cut out a notch and bang the strapping in, nailing at every stud.  I don't like the ones linked above as they can buckle.

When sinking posts, I mark out the holes, squaring them with a tape, bore them, drive a stake a few feet out towards each of the next posts, place and plumb the first and screw boards to it and the stakes.  Do the same with the other corners until they're all square and plumb.  Sometimes you have to shave out a hole to move the last post in or out.  Once they're all good, then cement them in place.  Any interior posts can be set to a line on the outside posts, at the right spacing.

As others have said, bracing (either with wood, metal straps, or plywood/OSB to make shear panels) is important. I wouldn't count on the corrugated metal siding to do this job by itself. The diagonal bracing works best in tension, so having at least one diagonal in each direction on a wall is good practice. Plywood/OSB should be well nailed (and construction adhesive) to all the studs it covers to get the most strength as a shear panel.
Since the South (sun-facing) wall will be all glazing, you'll maybe want to make up for the lack of structure there by making the other three walls a bit stronger (and it's still a good idea for some wind braces on the South wall anyways...)

Kenneth Elwell wrote: The diagonal bracing works best in tension, so having at least one diagonal in each direction on a wall is good practice.

If you're using T-strapping, code here is one diagonal, but I'd use two if it's flat strapping.

Timothy Markus wrote:

If you're using T-strapping, code here is one diagonal, but I'd use two if it's flat strapping.

This, of course is the elephant in the room... Is this building subject to building codes/inspection? If it is, then you'll need to follow the code, but make sure they know what you are building, and how & where you are building it...

This was a recent email on a local farm group:

"Hi all,

Short story: the Building Department rescinded the need for a $1650 building permit for my hoophouse and I only needed to pay for an electrical permit. Thanks for all the help, CRAFT!

Long story: Two useful pieces of information in getting that building permit requirement rescinded were:

1. the fact that other MA towns don't require one. With (name removed)'s help I was able to cite that farmers in Dracut, Lincoln, Concord, Andover, Beverly, and Westford aren't required to get a building permit for a hoophouse (as long as there isn't a concrete foundation).

2. the fact that the MA and IRS tax code classifies greenhouses and hoophouses as equipment, not real estate. That was info I got from the Farm Bureau's legal advisor.

My electrician put in a panel and now I'm just waiting for meter installation from National Grid and I'll be powered up!"

I'd be sure that you are in the clear to build what you are planning to, and that you won't run into trouble/hassle/aggravation after you have begun. From the sound of your plan, it's almost a pole barn... only difference being the plastic/glass to the South and your "intent to use as a greenhouse", which might not change a Building Department's seeing a pole barn.

Kenneth Elwell wrote:

Timothy Markus wrote:

If you're using T-strapping, code here is one diagonal, but I'd use two if it's flat strapping.

This, of course is the elephant in the room... Is this building subject to building codes/inspection? If it is, then you'll need to follow the code, but make sure they know what you are building, and how & where you are building it...

Yeah, I wasn't clear.  It's code to use 1 diagonal of the T-strapping; you couldn't get away with just one diagonal of the flat strapping, as it doesn't resist buckling.  You could likely, however, use two diagonals of the flat strapping and have it pass code.  

The building code has evolved into being for good reasons, though I'd agree that it can lead to inflexibility.  I think it's best to build to code or, if you don't, know why you don't have to and be able to convince an inspector that your method is OK, even if you don't get it inspected.  I've seen inspectors pass things not to code when an explanation was supplied as to why it was done that way.  

If you know what you're doing and why, and can explain it to the inspector, you can often not meet the 'letter' of the building code.  If your knowledge isn't sufficient to explain why you've deviated from code, but why it still meets the intent, I think you're likely best off sticking to code.  

Kenneth Elwell wrote:

Timothy Markus wrote:

If you're using T-strapping, code here is one diagonal, but I'd use two if it's flat strapping.

This, of course is the elephant in the room... Is this building subject to building codes/inspection? If it is, then you'll need to follow the code, but make sure they know what you are building, and how & where you are building it...

This was a recent email on a local farm group:

"Hi all,

Short story: the Building Department rescinded the need for a $1650 building permit for my hoophouse and I only needed to pay for an electrical permit. Thanks for all the help, CRAFT!

Long story: Two useful pieces of information in getting that building permit requirement rescinded were:

1. the fact that other MA towns don't require one. With (name removed)'s help I was able to cite that farmers in Dracut, Lincoln, Concord, Andover, Beverly, and Westford aren't required to get a building permit for a hoophouse (as long as there isn't a concrete foundation).

2. the fact that the MA and IRS tax code classifies greenhouses and hoophouses as equipment, not real estate. That was info I got from the Farm Bureau's legal advisor.

My electrician put in a panel and now I'm just waiting for meter installation from National Grid and I'll be powered up!"

I'd be sure that you are in the clear to build what you are planning to, and that you won't run into trouble/hassle/aggravation after you have begun. From the sound of your plan, it's almost a pole barn... only difference being the plastic/glass to the South and your "intent to use as a greenhouse", which might not change a Building Department's seeing a pole barn.

I don't require a permit as it's less than 200 sq ft. :)

@Elle - it's great that you don't need a permit. Where I live, we only get ~100 sq ft and it's *really* hard to make useful things with that limitation. That said, I think the points being made are that the "permit rules" can be useful teaching tools as to what makes a building safe. They aren't always right. One year our region got a statistically deviant 3 ft of snow in 3 days. Glass greenhouses built to snow standards still collapsed (our snow tends to be *very* wet - so heavier than average/inch) creating a dangerous clean-up job. We try to keep that in mind on our farm and often build "above code" or "stronger than code" to keep ourselves safe and to feel confident that when we get unusual weather, that we won't have to cope with collapsed buildings. (Despite our efforts, my wood shed did *not* survive a tree landing on it - so sad!)

elle sagenev wrote:
I don't require a permit as it's less than 200 sq ft. :)

I'll echo what Jay said about being stuck with about 100 sq ft.  It does suck.  Your building looks fine, but you should also be aware that not needing a permit doesn't mean you don't have to build to code.

North of here, we have unincorporated townships where you don't need a permit to build anything except a septic bed.  You are still required to build to code, even though there's no permit or inspection.  However, if any thing ever happened that caused the officials to show up, anything not built to code could be condemned.  Add to that, if you build something that fails and someone gets hurt, you're in a world of trouble if it wasn't built to code.  Just something to be aware of.

Timothy Markus wrote:

elle sagenev wrote:
I don't require a permit as it's less than 200 sq ft. :)

I'll echo what Jay said about being stuck with about 100 sq ft.  It does suck.  Your building looks fine, but you should also be aware that not needing a permit doesn't mean you don't have to build to code.

North of here, we have unincorporated townships where you don't need a permit to build anything except a septic bed.  You are still required to build to code, even though there's no permit or inspection.  However, if any thing ever happened that caused the officials to show up, anything not built to code could be condemned.  Add to that, if you build something that fails and someone gets hurt, you're in a world of trouble if it wasn't built to code.  Just something to be aware of.

I hear you guys. I really do. The assessors will certainly take pictures of and tax us for the thing. I built a tiny little duck hut and we get charged shed taxes for that thing. So I'm sure the government will be documenting my greenhouse. I'm feeling confident about the structural soundness of my design though. I did test build, albeit smaller, with the kids clubhouse. We have had some of the worst winds ever and I really was concerned it would blow over (it's not really on a foundation just held down by the weight of cement). It had 0 issues. I'll be using the same design principals with the greenhouse it'll just be 2x longer than the clubhouse.


We have some really long scrap 2x4's so I think it would be possible to do cross bracing without any extra cost and barely any extra effort. I will do that where needed.


My husband dug the holes. We'll put the posts in and nail some 2x4's on to see how square the whole thing is before cementing anything. Let ya'll know how that goes.

Make sure you cement in the posts correctly

Most likely your wood may not be for ground contact
Not all pressure treated is.  Even if it is cement or not. A poorly installed past will only last 5. To 7 years.  It usually breaks off at ground level
Cementing in a post does not mean it will resist more wind
Cross brace first

A mini update for ya'll, we finally have all the posts cemented in. We had to redig all but 1 hole. HA! I ended up digging one hole at a time, screwing the framed wall to it and then starting on the next hole. It's up, it's a big ol' rectangle and that was the easy part. (I find sheet metal to be hard to work with because it's so heavy and it's so windy here.)