The 1 4 in 12 design slope is reduced more from live load and long term dead load creep to exacerbate water retention.
Low slope roof live load.
Therefore a roof designed and installed to a 1 4 in 12 slope deflects downward and needs to be analyzed as a susceptible bay according to asce 7 10.
The membrane will outlast the shingles and seal around all nail penetrations.
Other geometric discontinuities may have increased pressures as well especially those at greater height on the structure.
For low slope roofs wind pressures typically have the greatest magnitude at the roof perimeter and corners.
Where deflection from snow or other live loads is a concern a greater slope will be needed to prevent any ponding.
And ice reduces a roof s snow load and dries more quickly than a light colored roof.
Larger flat roofed applications low slope such as residential condominium complexes row homes and town houses and smaller applications such as residential porch and garage transitions from the main home where the slope of the roof changes dramatically.
On a roof with a slope greater than 4 to 12 the live load limit is typically adjusted downward from 20 psf to 15 psf to allow for the relatively greater dead load on the steeper roof.
Resilient synthetic rubber membrane widely used on low slope roofing projects for over 50 years in the united states and worldwide.
Technically you should use the actual rafter length when adding up the weight of roofing materials.
Boca also allows you to reduce the snow load for roofs with slopes greater than 30 degrees presumably because snow will slide or blow off steeply pitched roofs.
Self adhesive flashing membrane under the shingles prevents leakage on low slope roofs.
Steep slope roofs have the greatest pressures in the perimeter and corners as well as the roof peak and eaves.
Then apply standard asphalt shingles over the membrane.
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.
For dead loads you are correct.
Finding the right products for your low slope roof.
However in my practice i typically use the horizontal run of the roof for both types of load.
To do this i use conservative too heavy dead loads and full snow loads regardless of pitch.
Minimum slope for low slope roof systems with any roofing material a slope of at least 1 4 inch per foot is recommended to promote drainage and minimize ponding.
For a low slope residential roof there are two main options.
For example in the rafter span table below the highlighted cell in the 40 live load table indicates that 2 x 8 southern yellow pine joists that have a grade of 2 that are.
