Cedar Planks for Lean-To
potton
| Posted in General Questions on
Need help to confirm if a 2:12 lean-to roof (10’x30′ long) in zone 6 is strong enough if constructed with 1” thick (actual 1”) tongue-and-groove red cedar 6” planks screwed to nominal 3”x6” BC Fir (Douglas Fir) open rafters spaced 24”.
[with weather membrane and steel roofing]
Thanks !!
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Probably (maybe) strong enough, depending on snow and wind loading in your area, type of building, and other systems.. The T&G would be acting as the structural roof sheathing, which is typically done with structural panels (plywood, osb) to create a diaphragm. The T&G would not be as strong in this application as structural panels, and you may need to beef up other structural aspects of the building to compensate. Western red cedar is very soft and not your best wood for structural applications.
It's also a question of whether it's graded for structural applications. Some cedar is and some is not.
I'm not an engineer, but I worry that with a 2-12 pitch, you'd have problems with snow load. Why not OSB or plywood, and then clad the T&G on the interior for looks? You might be able to use an exterior sheathing+insulation product too, which will add performance.
There's also something to consider about the labour required. The time needed to nail in all those thin boards starts to stack up against the 8x4 sheets which go down fast. Imagine it being a 2-day job that turns into a 10-day job because of the attention to detail — you've lost whatever savings you made on material.
Thanks for the reply. This is roof above a patio and a three-season screened veranda, so no need for insulation.
Since we want exposed rafters, cutting and fitting T&G between each rafter would be quite time consuming, that is why I wish to aim for the planks to do both jobs: roof structure and underside look...
(I'm building myself)
potton,
The determining factor in sizing rafters is almost always deflection - that is the amount it will bend under load, not break. So practically I think you are fine with the 3"x6", but what your code wants may be a different question.
Your big problem is that metal roof screws are typically 1" or longer, which will show on the ceiling. I would consider adding 1"x3" @ 16"oc strapping over the t&g for that.
I would check your local code. For 24" OC, ours simply requires lumber to be 3/4" and no more than 11 1/4 wide, no other rating is required.
It's very common in this application to use 2x tongue and groove. The fasteners for your roofing need to be at least 1-1/4" and they will poke through anything thinner. You can also use the 1x T&G and put a layer of plywood or OSB over it to give thickness.
There's two parts to your question about structural strength, the sheathing and the rafters. The sheathing has two requirements: it has to be thick enough not to sag between rafters, and it has to provide shear strength to keep the rafters from racking. Since 3/8" plywood is adequate to span a 24" space I have no doubt the 1x has adequate sag resistance. It may not provide adequate shear strength. However, if you layer it with plywood or OSB that will solve the problem.
Whether the rafters are adequate depends entirely on your snow load. That is local. You need to ask your local building department what the local snow load is. Then you need to find a rafter span table that includes 3x6 Douglas fir and see if it can support a 12' span at 24" on center at your snow load.
Thanks
That is my issue, I just can't find Douglas fir joist span tables for other than 38 mm (1.5'') width..
The snow load in my area in Quebec is 2.5 kPa (52psf)
That's probably because those tables all assume graded lumber, usually #2 or better, and that tends to mean standard dimensional lumber. If you have an "honest" 3x6, that's going to be pretty close to a sistered dimensional 2x6, which would be about 3x5.5", so I'd design from tables and pretend to be working with sistered 2x6s as rafters.
Bill
The strength of wood goes up linearly as its width increases. Same with reducing the span. So a 3x6 at 24" spacing is basically the same as a 2x6 at 16" spacing (actually a little better). A quick google tells me that a Douglas fir 2x6 16" OC can span 9'6" at 50 PSF snow load. Even at 12" spacing it only spans 10'7".
So it doesn't seem up to the task.
DC,
It's a bit unclear, but I think the addition is 10 ft from the existing house, so the span is probably close to 9'-6" Maybe potton can confirm.
For some reason I was thinking it was 12'. Yeah, I think you could make 10' work.
"The strength of wood goes up linearly as its width increases. Same with reducing the span."
True for load bearing capacity, but not true for deflection, which varies by the cube of the span (doubling the span increases deflection eight-fold).
I misspoke, I meant spacing instead of span.
The actual 'width' of the roof (in the 2:12 plane) would be 9'7'' including an overhang past the 6x8 BC Fir beam supporting the rafters. The horizontal distance between the ledger and the center of that beam is near 8'4''.
In parallel, I have difficulty finding decorative rafter hangers that can accomodate 2-1/2''... So I might have to limit rafter's width to 2'' (actual)...
potton,
Nothing from Simpson? https://www.strongtie.com/decorativehardware_woodconnectors/category
You can also just cut down the seat of the rafter where it sits in the hanger and leave the rest full dimension.
Malcolm,
Exactly, the maximum width for their light hanger is 2-1/16'' for their APLH26R and the minimum with the heavy is 3-9/16'' for the APHH46. (And it's not clear if those dimensions are interior or exterior...)
Cut down the seat... good idea. How would you do it?
I've use standard hidden flange hangers and spray them black. After install touch up the nails/screws with a black paint and looks almost as good as the decorative stuff for much less cost.
Probably an HUC46 would work for you if you can get the rafters as actual 3" wide.
HUC46 has a width of 3-9/16''. An actual 3'' wide would be loose, wouldn't?
potton,
I'd probably make a jig and use a router.
thanks!
An additional detail: my rafters and beams will be sawn from repurposed utility poles, so I don't know which grade of Douglas fir they would have been if they were 'regular' lumber.
(Same thing for my T&G cedar planks)
Treated utility poles?
yes; they remove the treated outer portion.
http://www.tredsi.com/EN/index.html
I'm in the midst of building a fence around the park at our community hall using utility poles salvaged from nearby road construction. When you cut them you can see the treatment is heaviest on the outside, but penetrates to some extent right to the middle. I'd find out what treatment was used before committing.
Typical treatment goes all the way through, or at least it's supposed to. A lot of old utility poles used creosote too, which isn't the nicest stuff. There have been various compounds used over the decades though, some of which were pretty similar to regular treated deck lumber. Just removing the "outside" is NOT going to completely remove whatever was used to treat the wood.
You may want to reconsider the use of this material in your living space, or possibly make sure it's sealed up well. I would suggest some 1/2" (or thicker) foil faced polyiso under the rafters, with seams taped with foil tape. The nice thing about the foil is that metal foil is pretty much an "everything barrier", so anything remaining in your timbers won't be able to diffuse through it over time. With a vent channel in the assembly too, you probably wouldn't have anything to worry about.
Just keep that in mind as you proceed with your project since you're not using normal materials here.
Bill
Appreciate your reply. My use is outside however, simple roof over a patio and a screened area, with the goal of exposed rafters...
potton,
It's probably fine. If you ever convert the porch into living space you can always encapsulate the beams with some sort of finish.
Considering how often questions come up as to how you can convert three season porches into the conditioned area of houses, it might be a useful lens with which to look a the building assemblies you are proposing, and see if minor changes might not make that easier down the road.
Malcolm
Indeed.
Thanks
Without having grading info for the Doug fir it requires guesswork to determine whether the 3x6 24" O/C are acceptable. Perhaps you can get some engineering design values from the company you're working with.
Just looking at the rafters only and making some assumptions for the Doug Fir (#2, Fb = 900 psi "base value" and Mod E = 1400000), using a 9ft span, 60psf, 2.5"x 5.5" dimensional: I believe the rafters are acceptable in terms of strength. Deflection would be ~3/8" or ~L/300.
Qualifier: I have a little background, but I'm not a structural engineer. These are just "back of the envelope" numbers and do not factor in any other roof assembly elements. The Doug fir design values vary significantly depending on grading.
Steve,
Would it still be acceptable if rafters were 2.0 x5.5 (so they fit APLH26R hangers), but spaced 20’’ ...?
For rafters, the critical measure is whether they sag unacceptably, the standard is 1/180 of the span, which is about 5/8" for a 10' span. There is a website called Sagulator that will calculate the sag of a piece of wood given the dimensions, the loading and the species. It's at: https://woodbin.com/calcs/sagulator/
I would use a loading of 60 pounds per square foot -- 50 for the snow and 10 for the roof itself. With 24" spacing that's 120 pounds per linear foot.
Potton,
I think you're good either way, but again without grading info for the Doug fir there is no absolute certainty. Using the same assumptions as previously made but changing to 2x5.5 dimensional, 20" OC, deflection would be similar at ~3/8" over the span.
There are other Simpson hanger options for 2.5" wide members, both concealed and non. Simpson does a lot of custom stuff, maybe they can just coat the size you need, assuming you're looking for the black finish.
(corrected deflection value)
The rafters being sawn from 15 year old utility poles there is no grading info…
Thanks a lot!
potton,
Are you sure Tred'si doesn't grade them?
In Canada you can't use ungraded lumber as part of a house.