I've been an occasional lurker at Permies for quite a while, going back uncounted pages of threads to read good information. I finally registered, looking for some guidance on a couple of issues we are currently having. While I have more than a little experience in conventional stick framing as well as post frame building.
We were working towards building a small timberframe cabin. Having hewn about 1/3 of the needed timbers, from white oak, harvested onsite (about half of that with a chainsaw mill and half of it with a broadaxe...the broadaxe actually ends up being considerably faster for me than the chainsaw mill, as well as producing much straighter dimensions), I'm actually legitimately running out of time, because of time constraints.
While having the timbers milled elsewhere and/or purchasing already milled timbers has proved to be prohibitively outside of my budgetary limits, but having found a local mill that can provide me locally-sourced dimensional lumber, the decision was made to alter our plans, and stickframe the cottage/cabin, using those beams we already have as "functional highlights," if you will. This is where my issues arises.....
1) The original timber frame plan was to use 6x8 rafters, 10' o.c. with purlins in between, and metal roofing over that, for rainwater catchment considerations. I was discussing my altered plans today, with two friends who have spent their entire working career in the construction industries (we're all in our early 40s. One of those is the VP of a post frame building company. The other is a senior field supervisor for a multi-state construction company....in other words, at least within conventional construction concerns, I certainly value their expertise and concerns). My altered plan involved simply doing a conventional post-frame-type roof structure: dimensional rafters at 10' o.c., with purlins in between those. While I recognize that up to 24" o.c. is normal, I've specifically looked at over 20 post frame barns in the last month, in the local area, and in regions with significantly higher snowloads (Idaho, Oregon, and northern Nevada, to be specific), that were built with the 10' o.c. and purlins method. And, of course, larger o.c. spans are typical in traditional timberframing.
One of the local friends pointed out that this worked in timberframing, because of the greater load-bearing strength of the timbers, versus dimensional lumber...i.e. a 4x8 or 6x8 has significantly greater load-bearing capacity than a 2x8. So, part one of the first question is, should the planned design carry roof loads in a place with no significant snow load? Part two of the first question is, if not, would it be possible to glue and screw multiple 2x dimensional pieces together to, in effect, create a 4x or 6x beam, to replicate the load-bearing (or close to it), of a timberframe beam to carry the load, and do this this way?
While trusses would be an option, because of roof design considerations (we need one portion of the under roof to provide loft space, and the rest will be a vaulted/cathedral type roof area, for aesthetic and climate management reasons), they really won't work (and are prohibitively expensive at this point).
2) Because of the desire for the vaulted/cathedral ceiling, and the 10/12 pitch of the roof design, we can't do normal stick frame rafter ties, or we lose a significant portion of the height of the vaulted ceiling area. My idea was to use one of the hewn timbers as a single "collar tie," at the top of the walls (adequately supported, of course). Would this be adequate to prevent rafter spread, even if I had to do normal rafter spacing at 16" o.c. or 24" o.c., or do I HAVE to have a ceiling joist/collar tie at the same intervals?
3) Third question involves a completely different area.....
The plan with the timberframe was to use clay slip straw for infill, with earth and lime plasters. I would still like to do this with the stickframed structure. Unlike the timber frame however, with its normal knee-braces, etc, for lateral stability and windbracing, typically in the stick framing, that is coming from the sheathing and/or siding. Can I "frame" in knee braces of some sort, or does the clay slip straw infill somehow offer lateral bracing, in a way I cannot even begin to imagine?
Overall design goals for the structural aspects of the house include:
four person family, with one or two more children in the not-too-distant future, and one large (Gladiator Mastiff) dog. Hot climate area (western edge of the Ozarks, near Stillwell, OK area), so white, limewash walls will reflect some heat, while adequate ventilation to allow good breeze access to the house, and heat escape (thus the vaulted ceilings), should keep the temperatures inside bearable.
Beyond that, between Gaelic heritage on both sides of the family, and the strong Gaelic cultural impact on the region, we would like to keep the design aesthetics in line with traditional vernacular architecture of small farm cottages of the British Isles. Eventually, the exterior walls will also be covered with limewashed stone, in pursuit of this as well, further improving thermal properties of the structure. While the Gaelic vernacular style is not completely appropriate for the area, I'm trying to blend the local environmental requirements with the vernacular of that region, tastefully.
Welcome, so to answer your questions basically the internet is probably not the correct place to get the right approved or safe answer. Your rafters should be designed to deflection allowables found in code or by the inspection office BSO, usually not to exceed 1" lower chord based on clear span....same for any building method or DIY. Find a local truss designer that uses MiTeck or some CAD software that has loading software as in dead and live loads that include snow. A PE is usually behind the software to approve and stamp your drawing's. For Timbers I use CAD too, hybrids the same. Timbers are far too expensive here so I offer non-structural interior beams.
If the lower chord pitch ends up too low, raise the wall heights or decrease the spans by adding load bearing interior walls, or raise the upper pitch or a piggy back may help....10/12 is high if you have no snow loads......If it is attic head height reconsider an A-Frame.
I don't think you'll find a hybrid heavy type 4 timber/light frame code path that drives you into hiring a PE. All the PE or any software use these days needs is an Architectural fully dimensioned floor plan. The software will id the issues, then redesign.
Earth or any "structural" infill will take down braced shear wall requirements IF you can provide data to a PE since there is no code path other than the exterior stucco code path method in the IRC wall chapter but, getting a PE or AHJ to agree may be like pulling teeth. Ask a PE if he can take the cost of construction or lumber down by using it to satisfy the safety factor (~ 2) to 1 knock-ups. He'll want to see approved mechanical properties, or I would before stamping.
John, not sure where you are, but snow load is something to consider. And if you don't have enough snow to worry about, if a tree falls on your house (you'd be surprised how often it happens in storms) the ability of that peak to hold up lots of extra weight is crucial.
I agree that you need to research the specifics of what you want to do and the type of lumber you are using. It wouldn't hurt to run your plans past an engineer.
One basic rule of thumb I've learned in building from scratch for years is Overbuild your Foundation and your Frame. If you try to get away with something, or you find yourself asking that question, "Do I HAVE to _________________ (fill in the blank)" You do. You will never regret an overengineered building, especially if you plan on a heavy cast iron stove for heating, and another one for cooking. You want it as solid as possible during unusually heavy storms, high winds, snow load, saturated soil, earthquakes, etc.
One of the most depressing things to watch is to 3 or 4 years down the road see the building starting to sink in one section, or torque, windows and doors not shutting properly. Make sure you leave plenty of crawl space underneath the building to get around and make any adjustments that might be necessary.
Don't fall for the My-Place-Is-Special, It-Won't-Happen-Here Syndrome.
Just how big is the span you're dealing with here?
If I was set on a peaked roof, and wanted to avoid putting rafter ties near the base of the rafters to preserve headroom in the loft and an open feel in the full height area, I'd be thinking about a structural ridge beam. In a relatively small building with a loft, a central support post could fit right in a the boundary between loft and open area. Depending on size of structure this centre post might be all the extra support the ridge beam needs... (plus either posts or reinforced stickframe at each end, and the usual bracing)
As Terry suggests, someone with the appropriate background/software/alphabet soup, located in local meatspace, is likely the 'right approved or safe' way to get your design finalized.
In order to have a design that hopefully doesn't need much help once you find that person, I'd be looking around the web for span calculators, and at state/regional standards for live/dead loads. Also would seek published strength info for your specific wood options. The strength difference between your oak and whatever the local place is making dimensional lumber from may be pretty substantial.
I completely agree with Cristo's remarks on overbuilding, too.
Some general thoughts you might keep in mind, forgive me if it's obvious stuff:
The strength of a beam or rafter, all else being equal, increases in a linear fashion with additional width. However, strength is proportion to the square of the beam height.
If you look at wooden I-beams and box beams... the above is why they work.
So, in theory a 6x8 is is exactly the same strength as a beam built up from multiple full dimension 2x8s of the same material, adequately affixed together. In practice I would guess the latter would come out a bit stronger as flaws in one piece won't align across the whole beam.
However, a 4x10 beam would be able to support a heavier load, despite being lighter and of smaller volume. A 3x12, even more so... (At some point the beam is too narrow and needs bracing to avoid failure by buckling sideways, I have no idea where this point is.)
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I'll give a classic example of politics and under designing to code minimums. Moore, OK has been hit for years with EF 4-5 tornado's. Outside of tornado and hurricane alleys wind loads are 90 MPH, Moore raised it to 150 after being hit and destroyed consecutively back in 11-13. I called them and tried to make them aware of earth mass and concrete but they refused to listen since the only walls standing are made of OPC concrete that can withstand 150+ MPH from flying light studs, metal, etc ballistics. If designed to 200-300 mph they would wipe out the fast known light wood industry, perhaps heavy depending, and they refuse at the expense of lives. Go figure!
However, too much dead weight and over designing for freak mother nature events drives up the cost of construction, the foundation, soils, and all structure has to resist.
Strength-to-weight ratios, light is better on foundations and weight distributions, or r-value and more plays a big part, unless you live in a high humid environment I'm not sure the dead weight works out comparatively. That is where design pros shine, that have experience with designs. not usually builders. I'd take a uniform single or double stud wall design loads to a spread footings any day of the week before heavy timber point loads. The 10% of compression deflection should be better. We don't design foundations to compression like most think, its also is designed to deflection or a flexure modulus, that is why 4000 psi is preferred and far exceeds compression loads needed for walls and roofs over 2500 code min but, there is limit or the cost sky rockets, especially today with all the foam insulation's that can't handle 10% deflection or dead weight.
Code, has a safety factor of 2 embedded in the span tables that considers errors and omission and that is all many PE's need to cover their asses, right, wrong, or indifferent, one would probably never touch them in a court of law push comes to shove. Find a good conservative PE that understands strength, dead weights, hygrothermal mass. It IS pay that good pe now or pay failures later. Pick your poison.
I forgot here is a heavy timber/stick hybrid we sub-contracted GC to do the stick framing connection to an existing building, roof, etc..... The Timbers were designed with Auto Cad and sent to an Finite Element Model (FEM) to do the stress analysis that included the foundation. The timbers are wrapped in a 6.5" thick non-structural SIPs that has decent strength to weight ratio per insulation values of 2/inch or ~ R-30 the mfg claims although never tested in the installation. Stick built to code.
This is smack dab in the middle of tornado alley Enid, OK. I got no idea how it would do in a EF5, the weight of the bents in theory should resist up lifting forces and wind and at the same time are producing higher concentrated loads than a stick frame would have. You can see the foundation attach in my blog.
A monolithic clay slip wrap in framework would have performed better, but not on the roof too much weight, in this hot humid climate but the foundation would need to grow and cost. At best an r-value of 1/inch BUT plenty of hygrothermal mass if done right that if a dynamic mass benefit is added equals or exceeds SIPs walls without seam issues.
A light insulation like mineral wool weighs far less, add a plaster, more, but puts less load on the foundation than clay-slips weight.
We pulled out didn't get final pics, beautiful building, I'm guessing he has 10 million in this one and the SIPs is toxic junk! I tired to tell and show the owner the documented issues, did not want to listen, too late. Local Architect firm had no idea what they were doing. When the building fails down the road and it will in the seams there is nothing they can do.
BTW: I beams are the strongest cross sectional area due to the centroid being closest to the neutral axis. An extruded monolithic beam would be strongest. Fastened or glue lams are only as strong as the fasteners or glue. Wood has it's strongest mechanical properties along the grain direction, that is how it should be loaded.
They are designed to a 2x4 web/top chord live load (TCLL of 25 PSF), Top dead(TCDL 10 PSF), 2x6 BCLL 0, BCDL 10 PSf you find in code or at the local BSO. Attic storage 10 PSF is above the ceiling in the garage too lazy to model
Trusses cost around $10,000, all framing around $40,000 which is about 15% of total cost or $18 SF. Timbers over double that.
Drop TCLL to 10 PSF for walking loads I don't think saves much money.
Some don't model trusses I do to get an idea.......the 50' span piggy back scissor looks like this.
Obviously if you substitute dimensional lumber for 6x8 timbers directly in a design, you need to sister enough 2x rafters to equal the strength of the 6x8. This includes factoring in the strength difference between oak and softwood. As others have mentioned, the spans involved will determine how practical different arrangements are. If you can support a ridge beam, there is no need for trusses, and collar ties become irrelevant. If you can position full-width tie beams at the base of the roof eaves holding the wall tops from spreading, again collar ties are irrelevant except if the rafters are too small to span from eave to peak... doesn't seem likely in anything I would describe as a "small cottage". What overall dimensions are you thinking of, anyway? This information would help focus the advice on solutions relevant to your particular situation instead of "all conditions". Also including info on the sizes and lengths of timbers you already have would be helpful in giving useful advice.
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