Tar Roof Slope

Originally Posted By: Kevin Blackwell
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This is Kevin from Houston,TX


What is the required slope for proper drainage on a tar roof only?


Originally Posted By: kmcmahon
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Assuming you mean a built-up roof, then 1/4"vertical in 12’ of run



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Originally Posted By: jpeck
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A built-up roof can be dead level, as long as there is no ponding water.


It 'should' have a slope to promote better drainage, but a built-up roof is approved for dead level flat roofs. Depends on the asphalt used, and on a flat roof, you would use 'dead level' asphalt for hot mopping.


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South Florida

Originally Posted By: roconnor
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The IRC requires the slope on built up roofs to be al least 1/4" per foot (2% slope) per R905.9



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Originally Posted By: jpeck
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From the 2003 IRC.


R905.9.1 Slope.Built-up roofs shall have a design slope of a minimum of one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage, except for coal-tar built-up roofs, which shall have a design slope of a minimum one-eighth unit vertical in 12 units horizontal (1-percent slope).


Okay, the IRC does not have dead level slope, but it gives the option down to 1/8" per foot slope.

I am a firm believer that dead level flat roofs are how architects try to get even with everyone else. They KNOW it is virtually impossible to make a dead level roof not pond water, but, hey, that is not their problem. Their line on their drawing is straight and flat, why can't you make the roof that way?


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Jerry Peck
South Florida