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Stephen Tashiro
07-22-2009, 11:32 PM
What is the basic reason that certain roofing materials like metal roofing panels and shingles require more slope than the materials used on "flat" roofs? I notice shingles and metal roofing panels will might a seam that runs down the length of the slope. Is it the puddling of water that would cause a leak if it rose high enough to force water into those seams?

Frank Hagan
07-23-2009, 2:01 AM
Yeah, that's my understanding. Shingles rely on gravity to pull the water off of them. They do have different types of asphalt shingles for lower sloped roofs, but I don't know what the minimum slope is. 2:12 I think, but it could be 4:12.

Ben Franz
07-23-2009, 12:00 PM
All (most?) roofing materials come with manufacturer's instructions re minimum slope, required underlayment, etc. A good material supplier can help with selection of an appropriate material. Your local building department is often a good resource as well.

Stephen Tashiro
07-23-2009, 1:53 PM
Ben,

I'm familiar with the literature on minimum slope for the metal roofing panels at the local hardware store. But I want to know why there is a minimum slope. What causes leaks on roofs less than the minimum slope.

Cliff Rohrabacher
07-23-2009, 1:56 PM
Flat roofs are usually built up of several layers of molten asphalt and fiber cloth with a top coat of light colored river rocks to reflect the heat of the sun.

Sloped roofs have a run of sloop per foot. I forget what my local code is but there is a code specification. These can be tar paper and mineral felt or torch down or rubberized material with some adhesive.

Asphalt shingles are a terrible roofing material when compared to slate but they last a few years if ya treat 'em nicely. There's new materials like Propylene roofing components that looks exactly like slate. I've seen one roof done in it it looks fabulous but, how long it'll last being colored black is anybody's guess. Most polymers unzip and get white and dusty when left out in the sun.

Rick Moyer
07-23-2009, 9:44 PM
Ben,

I'm familiar with the literature on minimum slope for the metal roofing panels at the local hardware store. But I want to know why there is a minimum slope. What causes leaks on roofs less than the minimum slope.

Well, for shingles, wind could be an issue on a flatter roof; as well as if snow (not where you are;)) were to acumulate and then freeze/thaw causing an ice dam... water could easily back up under the shingles and get into the roof. This is less likely on a more steeply pitched roof. I can't answer about the metal panels.

Brian Effinger
07-23-2009, 10:10 PM
Well, for shingles, wind could be an issue on a flatter roof; as well as if snow (not where you are;)) were to acumulate and then freeze/thaw causing an ice dam... water could easily back up under the shingles and get into the roof. This is less likely on a more steeply pitched roof. I can't answer about the metal panels.
Ice is exactly the reason that shingles really shouldn't be used on a slope less than 3-1/2 : 12, unless there is ice and watershield under them (in which case you can go down to a 2 : 12). As snow melts off of a roof it runs down towards the eaves, and when it gets there it re-freezes and will get under the shingles.