Roof decking and board spacing

What is under the shingles, by era

The decking, or sheathing, is the continuous surface nailed across the rafters or trusses that the underlayment and shingles fasten to. It carries the covering, ties the framing together, and provides the holding power for every nail on the roof. Its type in a Dallas-Fort Worth house is largely a function of when the house was built, and the era boundaries are approximate rather than sharp.

• Pre-war and early post-war: 1-inch solid or spaced board sheathing, usually 1x6 or 1x8 nominal, nailed across the rafters. Where the original covering was wood shingles or shakes, the boards were often deliberately spaced.
• Mid-century into the 1970s: plank sheathing gradually giving way to plywood. Mixed decking on one roof is common wherever additions or repairs happened, and plywood over the main house with planks over the original section tells you the building history.
• Roughly the 1970s onward: plywood, then increasingly oriented strand board.
• Recent construction: OSB, frequently radiant-barrier-faced OSB in North Texas, easy to identify from the attic by the foil face.

None of these is a defect. A plank-decked 1950s roof that has been shedding water for seventy years is a functioning roof. What changes with decking type is what happens at the next re-roof, and that is the practical content of this page.

Spaced sheathing and the composition shingle conversion

Spaced sheathing, also called skip sheathing, is board decking installed with deliberate gaps between the boards. It existed for a reason: wood shingles and shakes need to dry from the underside, and the gaps let them breathe. On a wood-shingled roof it is correct construction.

The problem arrives when that roof is converted to composition shingles, which is what happened to essentially all of them. Composition shingles need continuous support and continuous fastener holding power, and neither survives a gap. Fasteners landing in a gap hold nothing — a nail driven into open air between boards is not a fastener, so on a roof with regular gaps some meaningful fraction of every course is effectively unfastened, which shows up as tab loss in wind long before the shingle's rated speed. The shingle also has no continuous support over the gaps, so it flexes and eventually cracks along the void line, and the underlayment sags into the gaps and loses its function as a secondary barrier.

The remedy is not to nail harder. It is to overlay the spaced sheathing with a continuous panel deck before the new covering goes on. In Dallas-Fort Worth this shows up on genuinely old houses and on some 1960s and 1970s stock that originally carried wood shakes.

Solid plank decking has its own arc. Boards shrink, cup, split along fastener lines, and the joints widen over decades of North Texas heat and humidity swings. Widened gaps and split boards are the most common reason a tear-off on an older house turns into a partial re-deck — and the reason a re-roofing bid on a pre-1960 house should be read as an estimate with a decking allowance rather than a fixed price.

What an inspector can see, and what only tear-off reveals

Under the Standards of Practice the inspector must report as deficient deficiencies in installed framing members and decking. But the inspection happens from two vantage points only — the attic side and the roof surface — and neither sees the deck as a roofer sees it with the covering off.

From the attic, which is the real vantage point

• Decking type: plank, plywood or OSB, and whether more than one is present;
• Daylight visible through the deck at nail holes, gaps or deteriorated sheathing;
• Water staining on the deck underside, at rafters and trusses and around penetrations — and whether it reads as active or historic, crisp and dark against a dried tide line;
• Delamination of plywood plies, or swelling and frayed, cotton-candy edges on OSB from chronic wetting;
• Sagging between rafters or trusses, indicating a deck that has lost stiffness;
• Nail pattern — points telegraphing through, evidence of two generations of nails from a prior re-roof, and shiners that missed framing;

From the surface: deflection or soft areas underfoot, visible waviness where panels have failed, a springy deck. All of which require the inspector to be on the roof, which the Standards permit them to decline where they cannot safely reach or stay on it. Read the vantage-point line.

What cannot be determined is the true extent of deteriorated decking under an intact covering. It is only fully revealed at tear-off. This is the single most common source of a change order on a roofing job, and buyers deserve to be told plainly that a re-roof bid is an estimate until the old roof is off.

Layers: what the code actually says

The relevant model code provision is IRC R908.3, and it is more precise than the folklore.

First, a definition that does real work. R908.3 states that roof replacement requires removal of existing layers of roof coverings down to the roof deck. That is the difference between a replacement, which is a tear-off, and a re-cover, which is a layover — and it is the sentence that makes the deck inspectable, because tear-off is the only condition in which anyone sees it.

Second, R908.3.1 sets the conditions under which the existing covering must come off. A re-cover is not permitted:

• where the existing roof has two or more applications of any type of roof covering;
• where the existing roof or covering is water soaked, or has deteriorated to the point that it is not adequate as a base for additional roofing;
• over existing slate, clay, cement or asbestos-cement tile.

And it adds a requirement that cheap jobs ignore: flashings shall be reconstructed in accordance with approved manufacturer's installation instructions. Reconstructed. Not tarred over.

So the two-layer rule, correctly stated: the code does not say two layers are fine forever. It says a re-cover is prohibited where two or more applications already exist — you may generally go over one existing layer, so the third roof requires a full tear-off. And even the second layer is prohibited if what is underneath is water-soaked or deteriorated (City of Grand Prairie, Residential Re-Roof Guidelines).

The 25 percent rule is Florida's

You will hear this one across North Texas: once 25 percent of a roof is damaged or replaced within a year, the entire roof must be brought up to current code. Some versions attribute it to IRC R908.3.1 by section number, which makes it sound authoritative.

The rule is real. It is in the Florida Building Code. Florida Building Code — Existing Building §706.1.1 provides that not more than 25 percent of the total roof area or roof section of any existing building shall be repaired, replaced or recovered in any 12-month period unless the entire roofing system or roof section conforms to the requirements of that code.

What it is not is a model-code provision. The IRC R908.3.1 text contains no 25 percent provision — as set out above, it deals with re-cover conditions. And the North Central Texas Council of Governments 2024 IRC Regional Amendments, approved March 2025, contain no amendments to Chapter 9 at all, so the model roofing text applies across the region as written, without a regional 25 percent analogue.

The careful statement, and we will not overstate it: the rule is Florida's, the model code does not contain it, and the regional amendment package does not add it. Whether an individual city has adopted a local amendment is a separate question, checked ordinance by ordinance. If someone cites the rule to you in a claim conversation, ask which adopted code section they are quoting, by city and by edition. That question ends most of these conversations. (J.S. Held on Florida's 25 percent roofing rule; NCTCOG 2024 IRC Regional Amendments.)

Why a layered roof is a genuine finding

A second layer of shingles over an existing roof is legal in many circumstances and is not automatically a problem. It is a finding with consequences a buyer should price:

• Weight. A second layer roughly doubles the dead load of the covering on framing that may have been designed for one.
• Heat. A layover retains more heat, accelerating oxidation of the new layer — a penalty that is worse in a Texas heat regime than almost anywhere else.
• Concealment. The old layer hides decking condition and any prior leak damage. A layered roof is an inspection blind spot by construction, not by oversight.
• Fastening. Nails must be long enough to penetrate both layers and into the deck. Short fasteners are among the most common layover defects, and they are invisible.
• Flashing. The code requires flashings to be reconstructed. On cheap layovers they are frequently tarred over instead, converting a permanent detail into a maintenance product.
• Future cost. The next roof is a mandatory tear-off of two layers: more labour, more disposal.

And the limit that surprises people: the Standards state the inspector is not required to determine the number of layers. Inspectors often can and do infer layer count at an eave, a rake, or a lifted edge, and many will tell you. But it is not a required determination, and a report that does not state a layer count is not deficient for omitting it. If layer count matters to your decision — and on an older house it should — ask for it specifically.

What to do about it

If the decking is planks and the roof is performing, that is a monitoring item. Board decking on a pre-1960 North Texas house is original construction working as intended. Note it, disclose it, and carry a decking allowance into the budget for the next re-roof — do not treat it as a defect on a functioning roof.

Go into the attic yourself if you can do it safely. More is learned about a roof from underneath than from above: daylight, staining, delamination, sag between framing, and whether the nail points tell one story or two. If the attic access is smaller than 22 by 30 inches or the headroom is under 30 inches, the inspector is not required to enter — which means nobody has looked, and you should know that.

Read a re-roof bid as an estimate. On any house with plank decking or a known layover, ask the contractor in writing how replacement decking is priced, per sheet or per board, and what condition triggers it. A bid that does not address decking has not thought about your house.

Escalate when framing is involved. Active staining, daylight through the deck, or delamination is a before-closing item for a roofing contractor. A localised sag between framing members is different — if rafters, trusses, a ridge, or especially a modified or cut truss are implicated, that is a question for a Texas-licensed professional engineer, not a roofer.

Common questions

Published by Stonebriar Property Inspections. Written by Keith Boggs, TREC #9867 — a licensed Texas professional real estate inspector.