The short simple answer is 2 12 has traditionally been considered the absolute minimum acceptable roof slope suitable for asphalt shingles.
Bituminous roof minimum slope.
This number corresponds to the number of inches of rise in a 12 foot run.
The minimum requirement is 2 12.
The idea is to provide positive drainage toward roof drains so there is no ponding water 48 hours after a rain storm.
That works out to about nine degrees if that works better for you.
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Building elements a minimum design slope of 0 25 inches per foot is recommended for all membrane roof systems.
Some people recommend not installing asphalt shingles on slopes below 5 12 but a skilled roofer may well be able to make this work.
The minimum roof slope allowed by code for any type of roof is in 12 applicable only to coal tar pitch roof systems.
The following table provides the minimum required allowable roof slope for each type of roofing material mentioned in the code.
But shingle roofing is not watertight in the sense that standing water or wind driven water on a low slope roof can work its way up between the laps which is the reason for the minimum 2 12 slope.
Shingles of all types need to have a slope to function properly.
All asphalt shingle roofs with a pitch of 4 12 or more have an underlayment of 15 lb.
There are differing opinions as to the minimum slope for a roof but the generally accepted minimum is 3 12.
For example the international building code 2018 edition ibc 2018 prescribes a 1 4 12 minimum roof slope for asphalt built up polymer modified bitumen thermoset and thermoplastic single ply and liquid applied membranes in new construction.
Modified bitumen roofing minimum roof slope roof pitch r905 11 1.
What is the minimum pitch slope of an asphalt shingle roof.
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The 2 12 number is rise over run or two units of vertical for every 12 units horizontal.
This 1 4 12 minimum roof slope requirement also applies to spray polyurethane foam spf roof.
The minimum slope for a flat roof is 1 4 per foot or 1 4 12 but some owners or insurance companies require a minimum slope of 1 2 per foot.
Asphalt felt paper that is tacked to the roof sheathing.
Let owens corning help you determine when it is time for a new roof and then let us help you choose the shingles for your roof that are right for you.
On occasion codes will permit aggregate surfaced coal tar roofs down to 0 125 inches.
This means for every 12 horizontal units the roof must rise a minimum of one fourth vertical unit.
