Synthetics: Do you recommend synthetic underlayments?

Synthetic underlayments are relatively new in the roofing industry and have been used in the Winnipeg market since approximately 2010. These synthetics are being sold as a substitute for #15 felt tar paper as a shingle underlayment. The benefits to synthetics are increased tear strength, quicker installation, easier handling and the ability to withstand being exposed to the weather. In the United States, where roofs are left exposed for days, weeks or even months on end, the ability to be to left exposed is a benefit. In Winnipeg, roofs are not left exposed to the weather so most of the synthetic benefits are not realized in our market.

There are many brands of synthetics such as Owens Corning’s Deck Defense, IKO’s Cool Grey, BP’s Deck Guard, CertainTeed’s Diamond Deck and many other generic brands. What all of these products have in common is they are all non breathable and are therefore considered to be vapour barriers.

There is concern and discussion in the roofing and construction industry that putting a non breathable synthetic underlayment on the complete roof may cause moisture to be trapped in the attic or in between the synthetic and the roof decking. Nobody is completely sure as these products do not have a long enough track record in the Winnipeg market. What we do know is that all manufactures state that it is critical that synthetics only be installed on properly ventilated roofs. Many homes in Winnipeg, due to their construction, are not and cannot be properly ventilated so synthetics, based on the manufacturers’ recommendation, should not be used in these cases.

Dr Roof believes that any benefit a synthetic underlayment may provide should be weighed against the potential problems a non breathable underlayment may cause. Until these products have a proven track record and there is a better understanding of their performance on roofs in our climate Dr Roof will continue to promote #15 felt as the best option as a roof underlayment. Felt is breathable, has been used for decades, has a proven track record and properly installed, #15 felt will perform as well as a synthetic without the risk of putting a vapour barrier on your roof.

One synthetic product that is breathable is GAF’s Deck-Armor. If you insist on using a synthetic this is the product you should consider as it is the only breathable synthetic underlayment in our market as of right now. As you might expect Deck-Armor is more expensive than the other non breathable synthetics but we believe it is worth it.

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