Restoration Guide: Roof Underlayments

Editor's Note: This is article 5 of 13 in Chapter 3: The Roofing Guide of Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original materials in the U.S. Housing and Urban Development (HUD) Rehab guide.

Underlayment: the Basics

Underlayment is the layer of protection under the finish roofing material. In the past, builders usually used #15 roofing felt, an asphalt-impregnated felt. For heavy roofing, such as slate and concrete or clay tiles, builders usuallay used #30 felt.

Lay your underlayment from the eaves to the peak, overlapping in the same method as the shakes or shingles. Staple it to the sheathing to hold it in place until you install the finish roofing.

Many roofers feel roofs need a stronger underlayment than roofing felt provides, especially in areas of extreme weather. Many builders in hurricane areas assume a hurricane will breach the top layer of roofing. So, the underlayment assumes the final integrity of the roof.

As your old house nears its time for a roof replacement, you may want to consider the newer products for your home renovation.

Techniques and New Materials

1. Replacing old roofing felt with new

   This is a major home renovation and usually is done in conjunction with replacing the roofing on an old house. This is good because you must remove the old roofing to get to the underlayment. If extreme weather is an issue--that is, you don't live in Palm Springs or such--you should double-up the underlayment over the eaves and 36" up from the exterior wall to protect against ice damming. Then, you seal the edges with mastic.

   The weight of the felt (#15 or #30) depends on your climate and roofing material. You must take care during installation so weather doesn't damage the felt. Because the felt is only stapled to the roof deck (sheathing) before the roofing is installed, wind and rain can damage it. Careless installers can also gouge and rip the underlayment, requiring you to replace it before installing the finish roofing.

2. Using reinforced underlayment to replace old felt

   Reinforced underlayment is relatively new and is much less likely to tear than roofing felt.

   One product, Typar 30:

   • is made of spun-bonded polypropylene
   • has been around since the late 1990s
   • is used in place of #30 felt with concrete and clay tiles
   • is stronger than roofing felt
   • resists tearing

   Unlike ice and water barriers, reinforced underlayment is not self-sealing.

3. Ice and water barrier to replace old felt

   Like reinforced underlayment, ice and water barrier is a fairly recent development.

   The material:

   • is usually made with fiberglass and rubberized asphalt
   • resists tears and cracking
   • is slightly elastic
   • holds up well over time
   • does not dry out
   • will not rot
   • is usually self-adhering
   • seals around nails
   • is coated to make it skid resistant

   Apply an ice and water barrier to the roof deck at the eaves, extending at least 36" inside the exterior wall to protect against water encroaching the roof at the eaves. The barrier also provides an extra level of protection when used under flashing at skylights, vents, chimneys and valleys. But, if you use an ice and water barrier over the entire roof, take extra care with attic and roof ventilation, as the barrier may trap moisture in the attic.

   Though more expensive than roofing felt, the material is a cost-effective guard against leaks and protects against ice-damming and wind-driven rain.

   Not all ice and water barriers are temperature-safe. So if you live in a hot, desert climate, make sure your barrier is designed for the temperature.