Roof Underlayment Explained

What Is Roof Underlayment?

Roof underlayment is the waterproof layer fitted over the roof deck and under the shingles, metal panels, or other roofing material. You cannot see it once the roof is finished, which is why most homeowners never think about it — but it is an important part of the roof system. It is the second line of defence: the roofing material on top keeps most water out, and the underlayment protects the deck if water gets past that first layer.

Think of a roof as a system, not just a surface. The shingles or panels, the underlayment, the flashing, and the ventilation all work together. If any part of that system is missing or damaged, the roof is more vulnerable. This guide explains what underlayment does, the main types, and when it matters most.

What Underlayment Does

Underlayment has several jobs, and they all relate to protecting the roof deck and the home underneath.

It sheds water that gets past the roofing material. Shingles overlap to direct water down the roof, but wind-driven rain can get underneath them, and a damaged or missing shingle can let water through. Underlayment provides a waterproof barrier over the deck, so water that gets past the shingles runs down the underlayment and off the roof, rather than soaking into the wood.

It protects the deck during installation. When a new roof is being installed, the underlayment is fitted first, and it protects the deck from rain while the shingles or panels are being laid. That matters in Phenix City, where a shower can arrive with little warning.

It adds a layer of protection against wind-driven rain. In heavy rain and strong wind — both common in Alabama — water can be driven up under shingles. Underlayment helps the roof resist that.

It can help with ice and water in vulnerable areas. In valleys, around chimneys and skylights, and along eaves, a self-adhering waterproof membrane (often called ice and water shield) can be used as an extra layer of protection. Even in a mild climate like Phenix City's, valleys and penetrations are vulnerable points where extra protection is worthwhile.

It can provide a temporary weather barrier. If a roof is damaged in a storm and the shingles are missing in an area, the underlayment may still be in place and may limit water entry until a repair can be done. It is not a substitute for the roofing material, and it should not be relied on for long, but it is better than exposed decking.

When Underlayment Matters Most

Underlayment matters on every roof, but it matters most in certain situations:

• After storm damage: If shingles are missing or lifted, the underlayment is what stands between the rain and the deck. See our storm damage roof checklist.
• On low-slope roofs: Water drains more slowly from a low-slope roof, so it has more time to find its way under the roofing material. Underlayment is particularly important here.
• In valleys and around penetrations: Valleys, chimneys, skylights, and vents are where water concentrates, and where leaks most often start. Extra protection in these areas is worthwhile.
• During a roof replacement: A replacement is the time to fit new underlayment. Old underlayment is generally removed with the old roofing, and new underlayment is fitted before the new roofing goes on. See our guide to roof replacement cost factors.
• In heavy rain and wind: Phenix City's climate — heavy rain and strong wind — makes the second line of defence more important, because wind-driven rain is more likely to get under the roofing material.

Underlayment and a Roof Repair or Replacement

In a repair: If a repair involves removing shingles in an area, the underlayment underneath may also need to be replaced in that area, especially if it is torn, worn, or water-damaged. Ask whether the estimate includes underlayment for the repaired area, or whether it only covers the shingles. See our guide to roof repair cost factors.

In a replacement: New underlayment should be part of the replacement. Ask the roofing professional what type of underlayment will be used, whether a self-adhering membrane will be used in valleys and around penetrations, and whether the cost is included in the estimate. Our guide to questions to ask before accepting an estimate has a fuller list of questions to take into the conversation.

Frequently Asked Questions