As the load goes up for snow in northern climates or for heavy roofing material like tile the span length decreases.
Roof rafter snow.
Slipperiness of the roof.
The truss could be designed based on the assumption that the 25 psf roof snow load is a factored ground snow load in which case a ground snow load is back calculated using asce 7 based on the specified roof snow load pg 25 psf therein lies the problem with specifying only a roof snow load.
If your roof is located in windswept areas or even north of the treeline the wind will cause snow drifting making less of it accumulate on your roof.
For roofs that will experience heavy snow or wind and will span the same distance use 2 6 rafters.
Consult local building codes for snow load capacity and reference the span tables for joists and rafters published by the american wood council awc as a general rule roofs that have a high human occupancy rate should be framed with rafters every 16 structures such as sheds or barns can be frames with rafters every 24.
If your roof is 1 000 square feet the total snow load is 15 000 pounds of snow.
Snow increases the overall weight of your roof adding to the forces acting on your rafters.
You can use either 2 6 or 2 4 rafters for roofs with a 3 12 pitch or greater but it depends on the overall size of your shed.
Let s look at the sample table below which shows maximum rafter spans for a snow load of 31 3 pounds per square foot psf.
The roof deck collects the snow load and transfers the weight to the rafters.
Depending on your region you may need to design for a maximum snow load of anything between zero to 65 pounds per square foot or roughly 0 3 kpa.
According to the american wood council the maximum span for rafters is about 30 feet and that s with a relatively low load of 20lbs per square foot.
Less snow will accumulate on a slippery roof it will slide off instead.
For any rafter the portion of the roof deck that transfers this load is the tributary area.
Visit the roof framing page for more information on cutting roof rafters and visit the roof pitch calculator for determining rafter lengths based on rise and run.
Strengthening existing roof rafters is a difficult process that often requires professional help.
If the snow weighs 10 pounds per cubic foot and there are 1 5 feet on the roof each square foot of the roof is getting 15 pounds of pressure.
It extends outward in both.
Rafter span tables use these tables to determine lengths sizes and spacing of rafters based on a variety of factors such as species load grade spacing and pitch.
