Roof Decking: When Plywood Under Your Shingles Is the Real Problem

Shingles get all the attention, but they are only the weather layer. Everything above the rafters rests on the decking, the sheets of plywood or OSB that give a roof its shape and give the nails something to hold. When that layer goes soft, the best shingle on the market is fastened to mush, and nobody can confirm it until the old roof comes off.

What is actually under your shingles

On most North Georgia houses built since the eighties, the deck is four-by-eight sheets of OSB or plywood nailed across the rafters or trusses, with the underlayment and shingles on top. On older homes, the brick ranches in Marietta and Roswell and the farmhouses up toward Gainesville and Dahlonega, it is often plank decking: individual boards run across the rafters, sometimes tight, sometimes with gaps you can see daylight through from the attic.

Both kinds work fine when they are dry. Both fail the same way when they are not.

How decking fails

Water is the whole story, and it arrives from two directions. From above, a slow leak at a pipe boot, a chimney, a valley, or a skylight wets the same patch of wood every time it rains for years. From below, an attic that cannot breathe lets warm, humid house air condense on the cold underside of the deck all winter, and the wood never fully dries. Our ventilation guide explains why Georgia attics are so good at this.

Plywood responds by delaminating: the glued plies separate and the sheet goes spongy. OSB swells, the edges crumble, and the strands let go of one another. Either way the sheet loses its stiffness, sags between the rafters, and stops holding nails. A shingle nailed into rotten decking is held by friction and hope.

What you can see without tearing anything off

From the street, look along the roof plane in low light. Healthy decking reads as a flat field. Rotten or undersized decking shows as dishing between rafters, a wavy line along the eave, or a sag that follows one rafter bay. From inside, go into the attic on a bright day with a flashlight and look at the underside of the deck: dark staining, water trails running downhill from a penetration, white mineral deposits, fuzzy mold, or daylight where there should be none. Push a screwdriver into any stained area. If it sinks in, the wood is gone.

On the roof itself, a soft or springy spot underfoot is the classic sign, and it is also why you should not be the one walking it. Insulation hides the underside in most attics, so the absence of visible damage from below proves less than you would like.

Why nobody can promise you a number beforehand

Shingles and underlayment cover the deck completely. An inspector walking the roof will find the worst soft spots, and an attic check will find staining, but delamination in the middle of a sheet, rot under an old repair, and the condition of plank decking under a layer of felt cannot be confirmed from either side. Aerial imagery cannot see it at all. Any estimate that states exactly how many sheets you need before tear-off is a guess dressed up as a quote.

The honest moment is install morning, when the roof is stripped and the crew walks the bare deck board by board. That is when the real number appears, and the only question is whether you were told ahead of time how it would be priced.

Decking in an insurance claim

This is where expectations go wrong most often. A hail or wind claim pays to repair what the storm damaged. Decking that a tree limb punched through or a tornado lifted is storm damage and belongs in the scope. Decking that rotted over ten years under a leaking boot or a starved attic is wear and maintenance, and most policies exclude it. The carrier's estimate may include a small allowance for incidental sheets or nothing at all, and the rest is typically the homeowner's cost.

There is one more door. If your policy carries ordinance or law coverage and the existing deck does not meet current code, for example plank decking with gaps wider than the code and the shingle manufacturer allow, some or all of a required re-deck may be covered. Whether it applies depends entirely on your policy language, so ask your agent rather than assuming. When the crew finds damaged decking the estimate did not include, it gets photographed before anything covers it and submitted the way our supplements guide describes. Documented well, storm-related decking usually gets paid; rot usually does not, and a contractor who promises otherwise is setting you up.

Decking in a self-pay quote

On a roof you are paying for yourself, the decking line is the one that separates an honest proposal from a cheap one. Some bids include a set number of sheets, some include none and bill every sheet as an extra, and some say decking included and then define included on invoice day. In our market a replaced sheet, with material and labor, typically runs somewhere from around fifty to well over a hundred dollars depending on the material, the roof pitch, and the company, so a bad surprise on a big roof can run into the thousands.

Ask three questions of every bid: how many sheets are included, what each additional sheet costs, and whether you will be shown each one before it is covered. Our estimate guide covers the rest of the proposal line by line.

How we handle it

Our proposals print the per-sheet price before the job starts, so the number on the final invoice is arithmetic rather than negotiation. On install morning the crew inspects the stripped deck sheet by sheet. Every sheet that needs to go gets photographed where it sits, then photographed again once the new one is nailed down, and you see those photos before the underlayment covers anything. They become part of your job record, which you can follow in your RoofMap homeowner portal along with the rest of the work. If an attic check ahead of time tells us the whole deck is suspect, you hear that before you sign, not after.

Repair or replace

Not every bad sheet means a new roof. One or two soft sheets around a chimney or a pipe boot on a roof with years of life left are a straightforward repair: open that section, replace the decking, re-flash the penetration, and weave in new shingles. Widespread rot, a sagging field, or plank decking under a roof that is already at the end of its life is a replacement, and trying to patch it sheet by sheet costs more over time than doing it once.

If you have a wavy roofline, a stain in the attic, or a quote that is vague about decking, Robust Roofing will inspect for free, with photos and an attic look, and tell you what is actually under there. Call (470) 887-6637.