Snow Load Flat Roof Design

After you obtain the roof slope factor from those sections in asce 7 10 the balanced design snow load for the sloped roof can easily be calculated using equation 7 4 1.

Snow load flat roof design.

The balanced snow load is applied everywhere where the roof structure is located. Step 4 determine the flat roof snow load. This rain on snow augmented design load applies only to the balanced load case and need not be used in combination with drift sliding unbalanced or partial loads. For locations where 0 pg 20 psf all roofs with a slope w 50 with w in ft shall have a 5 psf rain on snow surcharge.

Determine the transient load of the roof. This may also be true in some more moderate climates as well. In between snow events roofs may lose snow mass due to either wind scour or by melting from building heat loss. The sloped roof snow load p s shall be obtained by multiplying the flat roof snow load p f by the roof slope factor c s.

Load types loads used in design load equations are given letters by type. D dead load l live load l r live roof load w wind load s snow load e earthquake load r rainwater load or ice water load t effect of material temperature h hydraulic loads from soil f hydraulic loads from fluids. This is the snow load calculated in the previous section. The thermal factor c t from table e 3 determines if a roof is cold or warm slippery surface values shall be used only where the roof s surface is unobstructed and sufficient space is available below.

Flat roof snow loads of 30 psf 1 44 kn m 2 or less and roof live loads of 30 psf 1 44 kn m 2 or less need not be combined with seismic loads. Snow loads acting on a sloping surface shall be assumed to act on the horizontal projection of that surface. This is all of the natural stresses imposed upon a roof such as snow loads rain loads and wind loads. Flat roofs must be installed on a very specific mild slope so that the water can drain off properly and if the drains and scuppers aren t cleared regularly this increases the chance of a leak.

Regardless of the recommendations of this snow load calculator there are some cases in which you should always remove the snow from your roof. This is highly regional as weights of snow for example range from 10 to 300 pounds per square foot based on moisture and volume of the snow. Where flat roof snow loads exceed 30 psf 1 44 kn m 2 20 percent shall be combined with seismic loads. For ground snow loads greater than 20 pounds per square foot the roof load is equal to 70 of the product of the exposure thermal and importance factors the ground snow load and 20 pounds.

A flat roof is no more or less likely to leak than a pitched roof it all comes down to the design and the competence with which it is installed. In colder regions the design snow load represents snow deposited by multiple snow events. Used but they typically defined the load cases or combination stress or strength limits and deflection limits. Per asce 7 05 section 7 10.

If your structure has a sloped or flat roof you still need to calculate the flat roof snow load. For roofs that are flat and with ground snow loads lower than 20 pounds per square foot the roof snow load is equal to 70 of the product of the exposure thermal and importance factors and the ground snow load.