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How a Roof Works

Lesson 4. What Is Under Your Shingles

The layers under the shingles are what stop an Iowa ice dam, and they are also the layers a cheap bid quietly leaves out.

At a Glance

  • Decking is the wood. Soft decking must be replaced
  • Underlayment covers the whole deck as a second line
  • Ice and water shield is the sticky one, and it matters most here
  • Two feet inside the warm wall is the rule to ask about

The Deck

Under everything is the wood. On houses built since roughly the 1970s it is plywood or oriented strand board. On older Davenport houses it is often individual plank boards with gaps between them.

Plank decking is not a fault. It has held roofs up for a century. It does mean the gaps have to be handled, because a shingle nail landing in a gap holds nothing, and it means the roof may need a thin layer of sheathing over the planks.

Soft or rotten decking has to come out. You cannot nail a roof to wet wood and expect the nails to hold.

Underlayment

A sheet covering the whole deck under the shingles. Traditionally felt paper, now usually a synthetic sheet, which is stronger, lighter and does not tear in wind during the install.

It is a backup. If water ever gets past the shingles, the underlayment is what stands between that water and your decking.

Ice and Water Shield, the One That Matters Here

A self adhering rubberized membrane that sticks directly to the deck and seals around every nail driven through it. It is the only layer that stops water moving uphill, which is exactly what an ice dam makes water do.

It belongs at the eaves, in every valley, around every chimney and skylight, and up any wall the roof runs into.

Worth knowing. The rule worth knowing. At the eaves it should reach at least two feet inside the warm wall line of the house, measured horizontally. On a house with a deep overhang, two feet from the edge is not enough, and that is a common shortcut.

How to Spot the Shortcut

Read the estimate. It should say what underlayment is going down, what ice and water shield is going down, and where. If it says shingles and nothing else, the difference between that bid and a higher one is probably buried right here.

This is the cheapest question you can ask and it is worth more than any other question on this list.

Next. Lesson 5. Flashing, the Part That Fails First
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