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What is Synthetic Underlayment?

Different types of synthetic underlayment have been around since the early 2000’s. Most synthetics are woven or spun from either polyethylene or polypropylene. While the ingredients are similar, the manufacturing process and thickness of each type of underlayment can be fine-tuned to produce different properties, such as exposure time and walkability. One way of comparing synthetic underlayment is by weight, similar to the way that felt is compared. Instead of pounds-per-100-square-foot, synthetics are rated by grams-per-square-meter (GSM).

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Price: Synthetics vs. Felt

The price of synthetics is based on weight and performance. Heavier products typically have more high-performance qualities built in, like improved walkability, durability, and longer exposure times. The cost of the thinner synthetics right now is comparable to a high quality 15 pound felt, but still more expensive than the cheaper felt. Thicker synthetics cost about 30%-40% more. Some thinner products are actually cheaper than felt, but Dale Walton, a Product Manager at CertainTeed, warns against going too thin.  CertainTeed sells two synthetic products, RoofRunner which has a GSM of 110 and DiamondDeck which weighs 185 GSM. Dale said that their testing has shown that products with a GSM less than 110 are prone to tear outs and puncture holes from imperfections in the decking surface.

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Synthetic Comes in Larger Rolls 

Roofing felt rolls are 3-feet long and cover about 400 square feet. Synthetic underlayment rolls are 4-feet long and cover closer to 1000 square feet. It does not take a roofing scientist to figure out that a roof can be covered faster with larger rolls, and more coverage per roll translates to fewer trips fetching additional product.

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