Calculating mono-pitch roof addition loads
slugboy6000
| Posted in General Questions on
Hello,
I was hoping that someone could point me towards a resource that would help me calculate the ledger board and beam loads on a small mono-pitch (or lean-to) roof addition.
I can make some general assumptions, but I’d prefer to do the math or use a calculator (A.I. can’t seem to do it yet).
I understand how to calculate loads on a flat deck, but the pitched surface is trickier.
Thank you
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Replies
slugboy,
Pitched roofs that are supported at the top and bottom have the same live loads as a flat roof spanning the same horizontal distance.
Thanks Malcolm.
You're answer defies my intuition.
Surely the bottom end of a pitched roof exerts more downward force as the tilt approaches vertical?
I'm baffled, please expound if you can.
Slugboy,
It may feel counter-intuitive - and you aren't alone. My experience has been that these seemingly simple pitched roof structures - whether supported at both ends, or just a the eaves - are probably the least likely to be understood, even among designers and builders.
Leaving aside lateral and dynamic loads (things like winds and snow) which can vary by pitch, as long as they are supported at both ends, the bearing points carry loads equally. All the loads are exerted vertically and uniformly over the length of the rafter, so they are shared equally. There is no mechanism to move them down towards the eaves as the pitch increases.
The way this was demonstrated to us in our first year structures class was with a small teeter-totter supported at mid-span. If the load became greater at the bottom as the pitch was increased, then the teeter-totter would always stay with the lower portion on the ground. But it can be left balanced at any pitch.
The dead load of the structure itself does increase as the length of the members and the amount of sheathing, etc. increases with pitch, but that usually has no affect on the sizing of the structure.
The other characteristic of these roofs is that unlike those only supported at the eaves (with no beam at the ridge), they exert only horizontal loading on the exterior walls and don't need rafter ties to counteract spreading.
Very interesting Malcolm, thanks for the explanation.
I should take a basic engineering class before I build anything else.
Slugboy,
In practice not much of this matters. Most structural sizing of things like rafters and headers in houses comes from building code tables that are very general and don't require calculations.
What often determines (or should determine) the size of roof structures is the need for adequate depth to accomodate insulation.