Do any research into roofing materials, and you'll quickly come across the fact that the slope of your roof determines what types of roofing materials you can use. You'll find lists of materials and the range of slopes they need along with explanations of how to calculate slope. One thing that's harder to find is why different materials are appropriate or inappropriate for different roof slopes. It helps to understand the logic behind advice that says you can't use such-and-such material if the roof slope is over or under certain values.
Rain Is a Huge Factor
Roofs that use shingles or tiles have to have a slope of at least 4:12 or more, depending on the precipitation in the region. This ratio is formed using the rise, or how much the roof slope rises vertically, over the run, or how long it is horizontally (if you remember your geometry classes, the rise and run are basically sides A and B of a right-angle triangle). A 4:12 slope means the roof rises 4 inches for every 12 inches of vertical length.
A roof slope that is shallower than that has to use materials that can be rain-proofed and can't use shingles or tiles. First, for the shingles and tiles, if you place those on a low-slope roof, strong winds could push rain under the shingles or tiles. That rain can then damage your home. For that reason, shingles, tiles, and anything else where there might be an opening for rain needs to go on roofs that are considered steep-slope, which is generally 4:12 or steeper.
Another issue is waterproofing. On steeper roofs, more rain simply rolls off instead of pooling, like it might on shallower roofs. On roofs with shallower slopes, you need to use materials that resist letting that pooled water soak through. Unfortunately, if you try to combine materials, like a base layer of a material meant for a shallow roof and then ornamental tiles on top, water can become trapped under the tiles. Even if it doesn't soak through the roof, it can become stagnant and allow mold to form.
Moisture Removal Is Another Factor
Materials like tiles and shingle roofing use gravity to get rid of moisture. In other words, they need rain to run off the material. Low-slope roofs may have drainage channels instead or may allow some moisture to simply evaporate. For an almost-flat expanse of roll roofing that has a waterproofed layer underneath, evaporation can work well. It's not so helpful for shingles that might trap some of that water.
Your house has already been built to have a certain slope. While this can be modified, it's easier and less expensive to look at materials that your current roof slope can take. A residential roofing contractor can help you find the material that will work best for your house.
Share1 February 2021
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