![]() ![]() Strength and stiffness are equally important. Beams, studs, joists and rafters act as a structural skeleton and must be strong enough and stiff enough to resist these loads. The house acts as a structural system resisting dead loads (weight of materials), live loads (weights imposed by use and occupancy), like snow loads and wind loads. This article will focus on how simple beams like joists and rafters react to loading. If, when the loads of the house are combined, the house weighs more than the soil can support – the house will sink until it reaches a point at which the soil can support the load. Remember when your science teacher said: every action has an opposite and equal reaction? Well every building load has an equal “reaction load”. The structural goal of a house is to safely transfer building loads (weights) through the foundation to the supporting soil. A complete analysis of wood’s mechanical properties is complex, but understanding a few basics of lumber strength will allow you to size joists and rafters with the use of span tables. Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base (foundation), supports the weight of its branches (column) and bends as it is loaded by the wind (cantilever beam). First world problems.Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. There's a 6 foot easement on that side to the fence, so the building wall will be reasonably close to the fence. OTOH, the north side where it will actually go did have the advantage that it could be closer to the "effective" line because the actual property line is about 7 feet into the neighboring property because of what had to be done when subdividing it years ago. It would be "logistically" and visually a lot better if it was on the south side rather than the north side but because of the required 15' side setback, a very large and mature willow tree would have to be destroyed. SWMBO is to not have the structure require removing any live trees. Positioning my own building has been an exercise because a "prime directive" for both myself and Professor Dr. Every property has its upsides and downsides. I'm sure that what you put up will meet the immediate need and you've mentioned that you do have a plan to move on in a few years to larger quarters. No way that would pass code now, but it's been there for 100 years and is still pretty straight. The funny thing i that the house is also 16 feet wide and has rough cut 2x4 rafters on 16" centers and a 12/12 pitch. If anyone knows of a rafter span table that goes up as high as 90# I would be very grateful to hear of it. It seems to me on a back of the envelope basis that full size 2x10 should be about right, but I am not an engineer. I think there is some extra strength in the full size rafters, too. It looks on the 70# chart like that would be okay, but I would need to go bigger for 90# presumably. They will be at 6/12 pitch, 24" centers and the span will be 8'9". I had specified full size rough cut 2x8 rafters on 24" centers. I would be happy to use whatever size rafters she told me to, but since neither of us knows what is sufficient I seem to be stuck unless I hire a structural engineer, which I would rather avoid if possible. The inspector also doesn't seem to know what size rafters are wanted. I can find rafter span charts for up to 70 psf, but nothing higher. The building inspector says that the roof must be designed for a 90 pound snow load. I have run into an obstacle in getting my workshop design approved. ![]()
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