Ask most people what a roof is and they’ll describe shingles. Fair enough, that’s the part you can see from the street, the part that comes in colors, the part on the estimate. But shingles are only ever as good as what’s under them. That layer is the decking, plywood or OSB panels nailed across the rafters, and it’s the piece that fails quietly while everyone’s looking at the surface.
How Roof Decking Fails
How does wood up there go bad? Moisture, mostly, and it doesn’t take a dramatic leak. A nail that backed out. A pipe boot with a hairline crack. Water finds the wood, the wood swells, and in OSB the glue holding the strands together starts letting go. Attic humidity does the same job in slow motion. An attic that can’t breathe keeps the underside of the deck damp for months on end, and mold and soft spots follow. Sometimes it’s just age. Decking that was borderline thin when it went on in the eighties may simply be sagging between rafters now.
Signs You Can Spot Without Climbing Up
You won’t spot any of this from the driveway. The clues are indirect. A roofline with waves or dips in it. A ceiling stain that keeps coming back after the shingles were already patched, which tells you the water’s getting into the wood, not just past the surface. The clearest sign is a spongy spot underfoot, but please, leave that test to somebody with proper footing and a harness.
When Decking Problems Usually Surface
Now the part that surprises people. Decking problems almost always get discovered mid-job, after the old shingles come off. The crew tears down to the wood, finds a few bad sheets, and calls you over. This is normal. It is not a shakedown. A trustworthy contractor will have warned you up front that some replacement might be needed and will show you the actual damaged panels before swapping them. Companies like STX Roofing lay out on their own site what this part of the job looks like, and reading something like that before you sign a roofing contract is time well spent.
What Good Decking Installation Looks Like
If new panels do go in, it helps to know what a proper install looks like so you can ask the right questions. Panels get laid with a small gap between them, about an eighth of an inch, so humidity swings don’t buckle them. Nails go on a tight schedule at the edges and through the field, and into the rafters, not the empty space between. The APA, the outfit that writes the standards for plywood and OSB, has a short guide to proper roof sheathing installation covering spacing, fastening, and ventilation. It’s aimed at builders but any homeowner can follow it.
Do Not Forget Ventilation
About that ventilation. Fresh decking over a stuffy attic ends up in the same sorry shape as the old decking, just on a delay. No ridge vent? Soffit vents buried under insulation? Fix it while the roof’s open. Adding airflow during a reroof costs a fraction of what it does afterward.
Questions to Ask Before You Sign
The takeaway’s pretty simple. When you collect bids, ask how decking gets handled. Is inspection included? What’s the arrangement for replacing sheets if they’re needed? Will they photograph the bad areas for you? A roof is only as good as the wood holding it up, and your one chance to get that right comes on the single day every twenty years when the shingles are off.