Synthetic Underlayment, Perm Ratings, and… | Shepley Wood Products
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Synthetic Underlayment, Perm Ratings, and Wood Roofing

Over the past several years synthetic underlayments have become a popular substitute for traditional felts (#15 and #30) in common roofing practices for both asphalt and wood roofing. The use of asphalt felts (semi-permeable products) has long been the underlayment of choice. Typically #15 felt for asphalt shingles, and #30 for wood roofing were the common practice. Todays homes are larger and more detailed than in the past, making it unlikely the job will be started and completed the same day. Asphalt felt is paper based, and has no memory, meaning once it attracts moisture (by being left on the roof overnight, or if it gets wet) it will ripple and curl and will not lay flat once it dries. When then used with asphalt shingles, especially traditional 3-tab shingles, it sometimes telegraphs the ripples through the finished product, giving the roof an uneven appearance. This is less likely with wood roofing due to the weight and strength of the shingles and fasteners. To compensate for this liability, synthetic underlayments have become a popular alternative, as they can be left exposed on the roof for 30 days to 6 months (when fastened correctly), depending on the product, some boasting even longer exposure limits, and will not ripple or curl. Builders of large homes have gravitated to these underlayments, as they can now cover the entire roof of a new home without worry that the felt will ripple and blow off with weather. This allows the builder to continue working inside the house without fear of everything getting wet (as long as the underlayment is applied correctly, usually with cap nails). For that reason alone, the synthetic underlayment is a huge advantage.

Now for the disadvantage. Most synthetic underlayments have a perm rating less than 1, meaning that they are impermeable. Whereas asphalt felts have a perm of 5–7, making them semi-permiable, meaning they allow something (such as water or light) to enter or pass through. It has been documented that the use of synthetic underlayment, combined with insufficient ventilation of the roofing material, may cause problems with wood roofing. External heat is not able to pass through the underlayment, causing the shingles to “cook” from the underside up, causing premature aging and shortened lifespan of the wood shingles. The Cedar Shake & Shingle Bureau (CSSB) has long warned of this in their Roof Manual installation instructions. is a great source for everything relative to cedar roofing. Some contractors also use a self-adhered ice damn protection in place of a nailable underlayment, but this is also an impervious product that will not allow your cedar roofing shingles to “breathe” nor acclimate (see warning on page 3 of the CSSB Roof Manual). If you like these products you can use them providing you ventilate the deck adequately. The use of a continuous vent product (like Cedar Breather) is not sufficient when used with either synthetic underlayments, or self-adhered ice dam protection , as they are deemed inadequate by the CSSB for not providing enough ventilation for the wood shingles/shakes to “breathe”. Installation options are clearly listed in the CSSB Roof Manual, starting on page 3. Cedar Breather is acceptable, however, when used with asphalt felt underlayments or other semi-permeable or permeable underlayments, such as “GAF Deck Armor”, which has a perm rating of 16. I`ve heard many times, “I`ve been covering my roofs with self-adhered ice damn protection and Cedar Breather for years and have never had an issue.” This may be true, but doesn’t mean it is the best way to maximize the lifespan of your roof. How many contractors go back to a cedar roof 10-15 years after installation to check?

On October 24th, Shepley hosted a great seminar featuring Tony Bonura of the Cedar Shake & Shingle Bureau, who was here to educate over 80 contractors on proper installation of cedar shingles and shakes. Tony is an expert in his field, with experience and qualifications too vast to list, and one of the most interesting speakers you will find on this topic. We plan to have him back in the future to speak to the contractors that missed out this last time. If that`s you, I strongly suggest you try to make the next seminar, it is well worth the 2 hours you will spend learning.


Asphalt felt, continuous vent, cedar shingles = acceptable

Synthetic Underlayment (or self-adhered ice damn protection), continuous vent cedar shingles=not acceptable (without proper ventilation of roofing material)