If your Seattle house was built before about 1950, there is a meaningful chance the roof sits on spaced boards with gaps between them rather than a continuous surface. It is called skip sheathing, and it is the single most common reason re-roof budgets move after work begins.
It was not a shortcut. It was the correct assembly for the roof it was built to carry.
Those homes originally wore cedar shake, and cedar must dry from both faces or it rots from within. Spaced boards provide the airflow across the underside that cedar needs. Builders in the 1910s, 20s, and 30s were building the right roof for the material available. The problem arrived decades later, when those roofs were converted to asphalt composition, which has entirely different requirements.
Two reasons, and the second is the important one.
First, support. Asphalt shingles are flexible and need continuous backing. Spanning a gap, they flex under foot traffic and load, and over time that flexing cracks the mat.
Second, and decisively, nail holding. Roofing fasteners must penetrate at least three quarters of an inch into the deck or fully through its thickness. On spaced boards, a meaningful proportion of nails land in the gaps between boards, where they hold nothing whatsoever. The shingle is attached at fewer points than the manufacturer's specification assumes, its wind rating no longer applies, and in a sustained Puget Sound windstorm that is where tabs lift.
This is why a roof can look immaculate from the street and still lose shingles in a November blow.
The correct remedy is normally not to strip the original boards. They are structurally useful and removing them is expensive and disruptive. Instead, new sheathing panels are installed over the existing boards, creating the continuous nailable surface asphalt requires.
On a typical 20 square Seattle roof, that means roughly 60 to 70 sheets of material plus the labour to cut, fit, and fasten them, and it commonly adds several thousand dollars to a project. That is a large enough number that discovering it on tear-off morning is a genuinely bad experience for everyone involved.
This is the practical point of the whole article, and it takes about ten minutes.
Go into the attic on a bright day with a flashlight and the lights off. Look up at the underside of the roof. If you see continuous panels, you have plywood or OSB and this issue does not apply. If you see individual boards running across the rafters with visible gaps between them — and often thin lines of daylight at those gaps — that is skip sheathing.
You now know the most expensive variable in your project before you have paid anyone anything, which puts you in a completely different position when the quotes arrive.
Tell every bidder what you found and ask each one to price the overlay explicitly as a line item rather than as a contingency. That converts the largest unknown into a comparable number across bids.
Ask specifically about panel thickness and how it relates to your rafter spacing, since thicker panels give better fastener holding and a stiffer deck for a small incremental cost when someone is already installing them. Ask whether panel edge clips are included where panel edges fall between rafters.
And ask what happens to ventilation. On many of these homes the original assembly relied on the spaced boards themselves for a degree of airflow. Sheathing over them changes the assembly, and it is the right moment to confirm that soffit intake works and that baffles keep insulation from blocking the eave path.
It is worth saying that this is not a disaster, it is an expense. A roof overlaid with proper sheathing, correctly fastened, with functional ventilation, is a better roof than it has ever been — genuinely improved rather than merely resurfaced. Homes that have had this done properly do not have to have it done again.
The bad outcome is not paying for the overlay. The bad outcome is a contractor who quotes a roof-over on a house with skip sheathing, nails a new layer into a substrate that cannot hold it, and leaves you with a roof that looks new and performs like neither.
This post is part of our in-depth coverage of roof replacement in Seattle.
Go into the attic on a bright day with a flashlight and the lights off, and look at the underside of the roof. Continuous panels mean plywood or OSB. Individual boards running across the rafters with visible gaps between them, often with thin lines of daylight, is skip sheathing.
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