Deck Panel Edge Gaps and H-Clips Belong in Your Roof Spec

A roof deck is made of panels, and panels move. Plywood and OSB take on moisture from the attic, from rain that gets under the underlayment during a tear-off, and from humid St. Louis summers, then dry out and shrink through the winter. Each change in size pushes on the panel next to it. If the panels are laid tight against each other, that push has nowhere to go, and the panel edges can lift or buckle. The damage often shows up later as a ripple running across the shingle courses, usually along a seam line, and it can look like a shingle installation problem when the cause sits one layer lower.

A shingle roof is only as flat as the deck under it. Shingles are thin and bend to whatever is beneath them. When a deck edge rises even a fraction of an inch, the shingle courses above it rise too, and the wave follows the seam across the roof. This is one reason the deck is part of the spec, not just a surface the crew covers up.

Material and spec

The common fix is a small gap at each panel edge and at each panel end, usually about 1/8 inch, set during the install. The sheathing maker's instructions and the APA guidance that many builders follow both give this figure, though the exact spacing depends on the panel type and on the conditions the day the deck is laid. A panel laid in cold, dry weather will grow when humid weather arrives, so the gap is there to absorb that growth. Ask the roofer whether the gap is written into the scope or left to the crew's habit.

Where two panels meet between rafters, the edge of each panel has nothing solid under it. That edge can flex under foot traffic during the install and under the weight of a stack of shingle bundles. H-clips are small metal pieces that sit between two panels at an unsupported edge and hold them level with each other. Not every roof needs them. Where the panel edge sits on a framing member, the edge is supported, and a clip adds little. Where the edge falls between rafters, the clip keeps one panel from lipping above the other, and a lip under the shingle shows up as a dip in the rows above it.

The gap and the clips work together with the nailing. A panel that lifts at its seam can pull a nail sideways, and a nail that does not bite into solid wood holds less than the shingle maker's nail schedule assumes. St. Louis winters add another load. Moisture from ice damming at the eave can soak the lowest panel edges, and that edge may swell and push against the next panel in spring. A roof with the gap and clips specified in writing has a better chance of staying flat through that cycle than one where the crew assumed the panels would behave.

What it costs

H-clips are inexpensive per piece. Most of the cost is the time a crew takes to set them, and that time depends on how many unsupported panel edges the roof has. The roofer can count those edges from the framing layout or from the rafter spacing, and the count should appear in the scope. A bid that says deck work or sheathing without a clip count leaves the cost open, and the number can change once the tear-off exposes the framing.

If the tear-off finds a soft, swollen, or cracked panel, the replacement is usually priced per sheet. That rate should be in writing before the tear-off begins. A bid that prices the gap and the clips inside general labor makes them hard to check against the finished job, and a homeowner cannot tell later whether the clips went in. Ask for the clips and the gap as separate lines, with the count and the unit rate shown.

What to ask

Before you choose a roofer, put these questions to each bid and compare the answers in writing:

• Does the spec call for a gap at each panel edge and each panel end, and what is the stated width?
• Where a panel edge falls between rafters, are H-clips in the scope, and how many are counted?
• Were the unsupported edges counted from the framing layout, or estimated from the total roof area?
• If a panel is soft or swollen at the edge during the tear-off, is the replacement a unit price per sheet written before work starts?
• Does the crew check the deck for ripples and lifted edges before the underlayment goes down, and will you receive photos of the seams?
• Does the sheathing thickness match the rafter spacing on this roof, according to the sheathing maker's span table?

A roofer who answers these questions in writing is giving you a deck spec along with the shingle spec. The shingle name, the nail schedule, and the underlayment all depend on a deck that lies flat. If you want to talk through the deck layout on your house before you decide, Keys Roofing can be reached at the office at (314) 220-2333.