Synthetic vs Felt Underlayment: Which Is Better for Treasure Valley Roofs?

For most homes in the Treasure Valley, synthetic underlayment is the better choice. It holds up to Boise summer heat and UV exposure far better than asphalt felt, resists tearing in wind, and stays flat under the shingles for the life of the roof. Dodd Roofing and Exteriors installs synthetic underlayment as standard on roof replacements in Boise, Meridian, Nampa, Caldwell, Eagle, Star, Kuna, Garden City, and Middleton, with ice and water shield added at eaves, valleys, and penetrations.

What does roof underlayment actually do?

Underlayment is the water resistant layer installed directly on the roof deck before the shingles go on. Shingles shed most of the water, but wind driven rain and any shingle that lifts or cracks can let moisture through. The underlayment catches it and carries it to the eaves. Idaho code requires underlayment on every asphalt shingle roof, so the question is not whether to use it but which kind.

How does felt underlayment perform in Idaho?

Asphalt saturated felt, sold as 15 pound or 30 pound, has been used for a century and still works. Its weaknesses show up in our climate. Felt dries out and gets brittle under high desert sun, and it wrinkles when it absorbs moisture, which can show through thinner shingles. It tears easily in wind if a crew cannot cover it the same day. Thirty pound felt is the minimum we would consider, and only on a roof that will be shingled right away.

Why is synthetic underlayment the better fit for the Treasure Valley?

Synthetic underlayment is a woven polypropylene or polyethylene sheet. It weighs a fraction of felt per square, does not absorb water, and will not wrinkle or rot. Most brands are rated for weeks or months of UV exposure, which matters when a roof is opened ahead of a weather delay. It is also safer for the crew because the surface has traction on steep pitches. The material costs more than felt per roll, but on a full roof replacement the difference is a small percentage of the total, usually less than a few extra sheets of decking.

Where should ice and water shield go instead?

Neither felt nor synthetic replaces self adhering ice and water shield. In the Treasure Valley we install it in valleys, around chimneys, skylights, and vents, and along the eaves where winter snow can melt and refreeze. Homes in the Boise foothills and north facing eaves in Eagle and Star benefit from a wider band. The rest of the deck gets synthetic underlayment.

What should you look for on your roofing estimate?

A good estimate names the underlayment product and says where ice and water shield is going. If it just says felt or leaves the line blank, ask. Ventilation matters too, since a hot attic ages every layer of the roof. See our guide to attic ventilation requirements in Idaho and our overview of roof replacement cost in Boise for what else drives the number. Dodd Roofing and Exteriors includes synthetic underlayment on every shingle roofing estimate. Call (208) 295-9421 for a free inspection anywhere in the Treasure Valley.

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