Roof Decking & Plywood Replacement in West Dallas: What's Actually Under Your Shingles

Roof decking is the layer of wood fastened across your rafters that everything else attaches to. Underlayment rolls out over it, shingles nail into it, and if it has gone soft, none of the material above it will perform the way it should — regardless of what you paid for the shingles.

It is also the one component of a roofing job that genuinely cannot be quoted accurately in advance, because nobody can see it until the old roof comes off. That single fact explains most of the disputes homeowners have with roofing contractors: the "surprise" charge on the final invoice is very often decking, and whether it feels like a surprise depends entirely on how the estimate was written. This guide covers what your decking probably is, how it fails, what replacement costs in Dallas, and how to read that line on a bid.

What Roof Decking Is

Decking — also called sheathing — is the structural skin of the roof. Rafters or trusses form the frame; decking spans across them to create a continuous surface. Everything visible from the street sits on top of it.

Its job is not primarily waterproofing. That is the underlayment's and the shingles' role. Decking exists to provide a nailable substrate and to transfer loads — wind uplift, the weight of the roofing material, a roofer walking across it, and hail impact — back into the framing. When decking softens, it stops holding fasteners, and a shingle roof is only as attached as its nails.

You can see your decking from inside the attic. Look up: the underside of the roof surface between the rafters is the decking. What you see there is a far better indicator of your roof's real condition than anything visible from the driveway.

OSB vs. Plywood vs. Plank Sheathing

Type	Typical era	Thickness	Behavior when wet
OSB (oriented strand board)	1980s – present	7/16"	Swells at edges, loses strength, does not recover
Plywood	1950s – present	1/2" – 5/8"	More tolerant; can dry out if caught early
1× plank / board	Pre-1960s	3/4" nominal	Very durable; individual boards replaceable
Skip sheathing	Older tile/shake roofs	1× with gaps	Needs overlay before shingles

The practical difference between OSB and plywood shows up specifically when water gets in. OSB is manufactured from wood strands bonded with resin, and when it gets wet it swells — particularly along the edges — and does not return to its original dimension when it dries. That produces the visible ridging along sheet seams you sometimes see telegraphing through a shingle surface. Plywood, being cross-laminated veneers, handles a wetting event better and can often dry without permanent loss of strength if the leak is caught quickly.

Neither survives sustained moisture. The difference is one of margin, not immunity.

How Decking Fails in North Texas

Four mechanisms account for nearly all the failed decking we cut out on Dallas roofs.

Slow leaks at flashing. The most common by a wide margin. A cracked vent boot or failed chimney flashing lets in a small amount of water at every rain. It never reaches the ceiling because the insulation absorbs it, so the homeowner never knows. Meanwhile the decking directly beneath the leak stays damp for years and rots in a roughly circular patch. By the time it appears on a ceiling, the decking damage typically extends well beyond the stain.

Condensation from poor ventilation. An attic that cannot exhaust moisture condenses it on the underside of the deck. This produces damage over a broad area rather than a single patch — often the whole north-facing plane, which stays cooler. The signature is uniform dark staining across many rafter bays, sometimes with white efflorescence or visible mold.

Hail and impact. DFW's hail exposure damages decking indirectly. Large hail can fracture the deck beneath the impact point, especially on thinner 7/16" OSB, and it accelerates failure at any spot already weakened. More often hail compromises the shingles and flashing, opening the path for the slow leaks above. Our storm damage page covers the inspection process after a hail event.

Age and repeated nailing. Every re-roof drives a new set of nails into the same decking. A deck that has been through three or four roof cycles is perforated with thousands of old holes, and around each one the wood is slightly compromised. This is one of the arguments against roofing over an existing layer rather than tearing off — it defers the moment anyone actually looks at the substrate.

Warning Signs You Can Check Yourself

From inside the attic, with a flashlight, on a dry day:

• Dark staining on the underside of the decking — the single most reliable indicator. Map where it is; it usually sits directly below a penetration or valley.
• Delamination — plywood veneers separating, or OSB strands lifting and looking furry rather than smooth.
• Daylight through the deck, anywhere other than intentional vents.
• Rusted or backed-out nail points. Nail tips protruding through the deck should be clean; rust means moisture has been present.
• Mold or a persistent musty smell in the attic.
• Sagging between rafters — sight along the underside of the deck; it should be flat between framing members.

From outside, at ground level:

• A roofline that dips or waves rather than running straight along the ridge and planes.
• Visible ridging along regular seams, suggesting swollen OSB edges.
• Areas where the shingle surface looks slightly sunken compared to the surrounding plane.

A roof that feels spongy underfoot has failed decking, full stop. If a roofer tells you the deck felt soft while walking it, that is a substantive finding, not a sales tactic — though it should always be backed up with photographs of the specific areas.

What Decking Replacement Costs

Decking replacement in Dallas generally runs $70 to $160 per 4×8 sheet installed — roughly $2.20 to $5.00 per square foot. Each sheet covers 32 square feet, so one sheet is just under a third of a roofing square.

Scope	Typical added cost	Notes
2 – 4 sheets	$150 – $650	Very common; localized leak damage
8 – 12 sheets	$600 – $1,900	Usually one plane or a broad leak area
Full deck, 20-square roof	$3,000 – $8,000	Uncommon; ventilation or long-term moisture failure
Individual plank boards	$40 – $110 each	Older homes; boards replaced singly

Two things drive the per-sheet price. Labor to remove a damaged sheet without disturbing surrounding material takes real time, and the crew is working around framing that may not be on modern 24-inch centers. Material cost for OSB and plywood has also been notably volatile in recent years, which is why some contractors quote decking as a pass-through at current market price plus labor rather than a fixed figure.

The Decking Allowance on Your Estimate

This is the practical takeaway. Because decking condition cannot be known before tear-off, every honest roofing estimate handles it as a conditional line item. There are three common approaches:

1. Per-sheet unit price, billed as used. The estimate states a price per sheet and you pay for the count actually replaced. This is the most transparent method and the one we prefer — you see the price in advance and pay only for real damage.
2. Included allowance. The bid includes a set number of sheets, with additional sheets at a stated rate. Reasonable, provided the included count and the overage rate are both written down.
3. No mention at all. This is the one to push back on. Either the contractor intends to bill it as a change order at a price you have not agreed to, or they intend to roof over whatever they find.

Whichever approach, ask for one thing in writing: photographs of every sheet replaced, in place, before the new decking goes down. This is trivial for an honest crew — everyone has a phone — and it removes the entire category of dispute about whether the work was needed. Any contractor unwilling to document it is telling you something.

If you are gathering roof replacement quotes, comparing the decking terms is often more revealing than comparing the headline price. Two bids $1,500 apart can invert once decking is billed.

Older West Dallas Homes and Plank Decking

Much of the housing stock around West Dallas and the older parts of Oak Cliff predates plywood sheathing and has 1× plank decking — solid boards, typically 3/4-inch nominal, laid across the rafters with small gaps between them.

Plank decking has real advantages. The boards are thicker and denser than modern OSB, they hold nails well, they have often survived seventy or eighty years in serviceable condition, and individual damaged boards can be swapped without disturbing a whole sheet.

There are two complications worth knowing about before a re-roof.

Gaps between boards. Modern shingle manufacturers specify a continuous nailable substrate. Where plank gaps are wide, nails can land in a gap and hold nothing. The standard remedy is to overlay the planks with a layer of thin plywood or OSB, which adds cost to the job but produces a proper substrate. A crew that nails straight onto wide-gapped planks is setting up for fastener pull-through.

Lost drying capacity. Those same gaps historically let the deck breathe and dry. Old asphalt-saturated felt underlayment was fairly vapor-permeable and worked with that. Modern synthetic underlayments are much less permeable — better at shedding water, worse at letting the deck dry upward. Re-roofing an old plank deck with synthetic underlayment while leaving the attic's ventilation unchanged can start a moisture problem in a house that never had one. If your home has plank decking, ventilation should be assessed as part of the re-roof scope rather than assumed adequate because it always was.

Frequently Asked Questions

What is roof decking?

Roof decking, also called sheathing, is the structural layer of wood fastened across the rafters or trusses that everything else attaches to. Underlayment goes on the decking, shingles nail into it. It is typically 7/16-inch OSB or 1/2-inch plywood on homes built since roughly the 1970s, and 1-by-6 or 1-by-8 solid planks on older houses. You cannot see it from outside — it is visible only from inside the attic.

How much does it cost to replace roof decking?

Decking replacement in the Dallas area generally runs $70 to $160 per 4x8 sheet installed, which works out to roughly $2.20 to $5.00 per square foot. Most re-roofs need only a few sheets replaced. A full deck replacement on a 20-square roof can add $3,000 to $8,000 to the job, which is why the allowance on your estimate matters.

How do I know if my roof decking is bad?

The clearest signs are visible from the attic: dark water staining on the underside, delaminating or spongy plywood, daylight visible at seams, rusted nail points, or mold. From outside, look for a roofline that sags or dips between rafters, or a roof surface that feels soft underfoot. Decking damage is usually discovered at tear-off, because until the shingles come off it is largely hidden.

Should decking be replaced during a re-roof?

Only the damaged sheets, in most cases. Sound decking does not need replacing just because the shingles do. What matters is that the contractor inspects the whole deck once it is exposed and replaces what has failed, rather than nailing new shingles over soft wood. A reputable estimate includes a per-sheet decking allowance precisely because nobody can know the count in advance.

Can you put a new roof over bad decking?

It can be done and it should not be. Shingles fastened into soft or delaminated decking do not hold — the nails pull through under wind load, which voids most manufacturer warranties and typically leads to blow-offs in the first serious storm. If a bid is notably cheap and contains no decking allowance at all, this is one of the corners most likely being cut.