Roofers often wonder what the differences are between granular and smooth roof underlayments. There are several differences between the two, and a few considerations that you’ll need to account for before choosing smooth vs. granular underlayment. Let’s start with a definition of each of these types of underlayments, and then we’ll get into what you’ll need to know to make the right choice for your next roofing project!
What is Granular Underlayment?
Granular underlayments are peel-and-stick underlayments with a granulated surface. This generally means that the underlayment is comprised of two layers: rubberized asphalt topped by a layer of fiberglass, on top of which a layer of granules, like sand or minerals, has been added. To look at it, granular underlayment often has a similar design and texture to asphalt shingles.
What is Smooth Underlayment?
Smooth underlayment, whether it’s a peel-and-stick variety or one that requires conventional fasteners, doesn’t have that granular surface. It can either be asphalt saturated underlayment, or it can be a synthetic product with no granules. Either way, a smooth underlayment will have no sand or minerals coating that upper layer. Epilay’s underlayments would all be considered smooth underlayments.
How to Choose between Smooth and Granular Underlayments
There are lots of considerations to make when choosing between a smooth and granular underlayment. Walkability is a big factor. Roofers often choose granular underlayments because that granulated surface provides high traction when walking on the underlayment. However, there is a downside to this, which is that the granules are easily shaken loose. They’re prone to collecting on the roof’s deck or other uncovered surfaces which means that while the underlayment has high traction, uncovered surfaces coated with loose granules are more slippery. Meanwhile, many smooth synthetic underlayments have anti-skid surfaces without the granules—like Epilay’s GripWalk surface. 
Thickness is another reason some roofers choose granular underlayment. These underlayments are typically 40 to 60 mils, which gives them extra durability when compared to thinner conventional underlayment. But, here again, synthetics, while being lighter weight, are often even tougher than granular underlayment because they feature a multi-layer design that drastically increases tear resistance while keeping the underlayment’s carry weight very low.
There are some other disadvantages to granular underlayment, too. The granulated surface can make it difficult to seal seams between each piece of underlayment, and because of their design, these underlayments become soft on hot days, which means oil can soak through to the roof deck or even the roofing material installed over top of the underlayment. In addition, when installing a new roof over existing underlayment, it isn’t advisable to reuse existing granular underlayment, which means removing it to install new before the roof itself goes on. However, with smooth underlayments, provided the underlayment is intact, roofers can generally reinstall over top of existing underlayment without worrying about a time-consuming removal job.
Granular underlayment can offer a cost savings compared to smooth underlayments, but it also comes with its own set of considerations that may not make it the best fit for your next roofing project. Safety, durability and ease of installation are big concerns that offset cost, so make sure to keep these factors in mind as you choose between underlayments!
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