Membrane roofing is not a typical DIY item (in my opinion). Simple to accomplish with a truss roof, or a properly stick framed assembly. Why must you have a low-slope roof? I think you'd be better served with a higher pitched roof that would be able to shed snow. Should you have snow in your area I would recommend consulting a local professional engineer to evaluate your roof to properly size the framing members. With all that said would I frame my low-slope roof (your example in my area) with 2x12 #2 SPF at 16" o.c. A couple of winters back we had numerous flat roofs collapse from unusual snow loading. It takes experience and knowledge to evaluate anticipated loading properly. Of course should this roof be adjacent to the house, with a wall that extends above the garage roof then you would have to take into account snow drift (where snow piles up against the wall increasing load).Īlso with a low-slope roof you can have a build-up of snow which can melt and turn into ice, which weighs more than snow. for a rafter without an attached ceiling (deflection L/180) to span 20' using #2 grade spruce-pine-fir (SPF) lumber (typical for my area) I would be allowed to span 21'-5" at 16" o.c. See Table R301.7 for Deflection Limits.įor example in my area our ground snow load is 30 psf. Also the type of ceiling to be attached, if any. You will need to know what species and grade of lumber you will use for the rafters, and again your ground snow load. This link will provide you with the Prescriptive Rafter Spans from the 2009 International Residential Code. Typically at a minimum live load is 20 pounds per square foot, unless your ground snow load exceeds it.Īlso, should you plan on using the "flat roof" as an elevated deck you would need to account for the live loads associated with this use. Live loads are based upon loads that may change, such as snow and people. This would require a call to your local building department to determine what your ground snow load requirements are, whether you need a building permit, and what drawings you would need to submit.ĭepending on your roofing materials a typical low-slope membrane and framing will be around 10 pounds per square foot for dead load. Penetrations can be troublesome.Īs tony.g said you have to know what your loads are. Joe is correct, if it is a surface that people normally and routinely walk on it will require guard rails for fall prevention. Just let the truss manufacturer know you are using it as a deck. Your deck would be stronger in my opinion going this route. have to account for the deflection between the trusses and one layer of subflooring typically won't get it done. maximum, though most truss companies prefer 24" o.c. your walls would be built level all around with the trusses on top. the top chord could be sloped to provide your 1/4" per foot slope for drainage. You may want to contact a local truss manufacturer and discuss with them using floor trusses for your project. I did miss the part on your OP about the BBQ and people, even though I did mention about the possible use of the roof as a deck. I would not do anything that is less than code (remember that MINIMUM part) just because it may be allowed. I just want something that will last and won't collapse if we get a foot of snow (which happens every couple of years).Ĭlick to expand.doesn't matter what he let's go through, it's your and your family's safety and investment that count. The roof will be treated like a deck - I plan on fiberglassing it and maybe have the odd BBQ on it with 5 people.ĭoes this sound ok? The inspectors in our city seem to let anything go through in respect to sheds and garages. I'll run two rows of blocking to keep twisting down and glue and screw t&g 3/4 plywood on the roof. There will be a slight pitch to the roof as I plan on having an extra 2 or 3 top plates on one supporting wall, to make it around 5" taller than the other. I've been reading (googling) about spans and from what I can tell, I should be able to run No 2 Douglas Fir 2x12 "joists" the full 20 feet span of the ceiling without any problems, preferrably spaced at 12" oc. I want to build a relatively flat roof for simplicity's sake and to keep the overall building height low. I have the walls up - 2圆 8' studs spaced 16" oc. I'm a long time reader, first time poster.
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