Fire-Retardant Plywood Roof Sheathing From the 1980s

Why this matters

In a lot of townhouses built in the 1980s, the strip of roof sheathing on either side of each party wall is not ordinary plywood. It is fire-retardant-treated plywood (FRT plywood), and some of the treatments used then broke down under attic heat until the panels lost much of their strength. From the attic it looks like dark, scorched-looking wood. From the roof it looks like nothing at all, until a roofer, a solar installer or a satellite tech steps on it and a foot goes through.

Before anything else: if you suspect FRT degradation, nobody walks that strip. On the roof, stay on the parts of the deck you have reason to trust and work from roof ladders or planks that span to framing, tied off under your fall-protection plan (for residential construction, 29 CFR 1926.501(b)(13) requires protection at 6 feet or more). In the attic, stand and kneel only on framing members, never on the panels, and do not test a panel by pushing on it from below with your weight behind the push.

Why it is there, and why it is only there

Townhouses are separated by fire-rated party walls (the building code calls them fire walls or dwelling-unit separation walls). The model codes have long required that wall to stop fire passing over the roof, either by extending it above the roofline as a parapet or by an exception: roof sheathing that is noncombustible or fire-retardant-treated for 4 feet on each side of the wall, among other conditions. The current IRC carries that exception in its townhouse separation provisions (section R302.2 and its subsections; the subsection numbers have moved between editions), as adopted by your jurisdiction.

Builders took the exception because a parapet costs flashing, framing and a maintenance problem forever. So in a row of townhouses, the FRT is a band 4 feet wide on each side of every party wall, running from eave to ridge, and the rest of the deck is ordinary plywood or OSB. Some builders also used FRT lumber in the trusses along that wall, and a few used FRT sheathing across whole roofs. The band is the common case, not the only one.

This sits mainly in attached housing built roughly from the early 1980s to around 1990, concentrated where that housing boomed, with the Mid-Atlantic the best-known area. Detached houses rarely have it, because there is no party wall to protect.

What went wrong, mechanically

Several of the fire-retardant formulations of that era were acidic, phosphate-based chemistry. Under the temperatures a dark asphalt roof drives into an attic on a summer afternoon, and over years of repeated heat cycles, those chemicals attacked the wood fiber itself (a process the industry called thermal degradation, with acid hydrolysis of the cellulose). The wood did not rot. It got weaker, then brittle, and in advanced cases it crumbled.

Two consequences follow from the mechanism:

• It is worst where the panel is hottest. South- and west-facing roof planes, panels directly under dark shingles, and attics with poor ventilation degrade first. The north plane of the same roof can look fine.
• Strength loss runs ahead of appearance. A panel can lose a large part of its strength before it changes color, so a panel that looks only slightly dark is not cleared by looking at it.

The industry responded in the early 1990s with high-temperature test standards that FRT products are now evaluated under (ASTM D5516 for plywood and ASTM D5664 for lumber, with ASTM D6305 and D6841 used to derive design adjustments), and products made to them are not this problem. The degraded panels are the ones already on 1980s roofs.

What it looks like

From the attic, which is the only reliable place to look:

• Color: darker than the neighboring untreated sheathing, ranging from tan-brown to a charcoal look, often banded so the 4-foot strip contrasts clearly with the deck beside it.
• Surface: cross-grain cracking, checking and splitting on the underside face; the face veneer may flake or powder when brushed.
• Shape: sag between rafters or truss top chords. Look along the roof line from the attic access with a light raked across the underside; deflected panels show as shadows between framing.
• Fasteners: nails pulled through or loose, because the wood around them has lost its grip.
• Marking: FRT panels usually carry a treater's or certification stamp. You may see it on the underside where it was not covered. A stamp tells you the panel is FRT, not that it is degraded.

From the roof, a soft or spongy feel underfoot near the party wall line, a dip in the roof plane parallel to the wall, or shingle nail pops in a band are cues, but by the time a roofer feels it underfoot he is already standing on it. That is why the attic look comes first.

     unit A           party          unit B
                      wall
 --- plywood ---+--FRT--+--FRT--+--- plywood ---
                |<-4ft->|<-4ft->|
                        |
        attic A         |         attic B
                        |
 each unit owns the band on its own side;
 the band runs eave to ridge on both planes

What else turns a deck dark, and how to tell

Three other things darken roof sheathing, and each has a different pattern:

• Attic mold or mildew follows moisture, not the party wall: it is heaviest on the north plane, near bath fan outlets and at the eaves where cold sheathing condenses, and it wipes or shows a fuzzy or speckled surface. FRT darkening follows the band edge in a straight line and is worst on the hot planes.
• Old leak staining has a tide line and starts under a penetration, a valley or a flashing, then runs downslope.
• Soot or fire damage is surface char with a smell and a source.

The straight-edged, wall-parallel, 4-foot band is the tell. If the dark area does not respect the wall line, it is probably something else, and it still gets recorded and routed if the wood is soft.

Worked example: an interior unit reroof

A roofer is bidding a tear-off and reroof on an interior unit in a 1985 row. The unit is 20 feet wide. Each roof plane measures 16 feet along the slope from eave to ridge, front and back.

First, the look. Before anyone goes up, he asks the owner for attic access. With a light raked across the underside, the bands beside both party walls are darker than the rest of the deck, the south-facing rear plane noticeably more so, and the rear plane's band shows cross-grain cracking and visible sag between trusses at 24 inches on center. The front plane's bands are darker but flat. He photographs both, notes "appears to be fire-retardant-treated plywood, darkened, cracked and deflected on rear plane; darkened, no visible deflection on front plane", and does not probe it.

Then the quantity. An interior unit has two party walls, and it owns the 4-foot band on its own side of each, so 8 feet of its 20-foot width is FRT:

• One plane: 8 feet wide times 16 feet of slope is 128 square feet.
• Two planes: 2 times 128 is 256 square feet, which at 32 square feet a sheet is 8 sheets of sheathing.
• The whole unit's deck is 20 times 16 times 2, which is 640 square feet, so the bands are 256 of 640, or 40 percent of the roof.

An end unit in the same row has one party wall: 4 feet times 16 times 2 is 128 square feet, 4 sheets, or 20 percent of a deck the same size.

The call. He does not price the bands as "replace if bad on tear-off." The rear plane's bands visibly failed, and the front plane's look darker with no visible sag, which under the mechanism above is not a pass. He writes the bid with all 256 square feet of band sheathing as a replacement line, replaced with currently rated FRT or another assembly that meets the party-wall requirement as adopted where the house is, and recommends a structural engineer look at the truss top chords along both walls before tear-off, because if the builder used FRT lumber too, the sheathing is not the whole problem.

On the day. The crew does not walk the bands at all. Tear-off on the band strips is done from roof ladders hooked over the ridge and planks bearing on trusses, with each worker tied off to an anchor installed in the trusted field of the deck, away from the bands.

What changes the answer:

• The neighbor's side. The band on the other side of each party wall belongs to the neighboring unit. In a condominium the roof deck may be a common element the association owns and insures, in which case nobody bids one unit's bands without the association. Ask who owns the roof before you price it.
• Code for the replacement. Replacing FRT with ordinary plywood removes the fire protection the exception relied on. Whether a parapet, rated FRT or a gypsum-backed assembly is acceptable is for the local building official, as adopted where the house is.
• Truss involvement. If the engineer finds FRT trusses degraded, the job is now structural repair with an engineer's design, not a reroof.

The failure mode

The expensive version is not the roofer who misses it on a bid. It is the tech who is on the roof for something small and short: a satellite dish, a vent boot, a solar site survey. He has no reason to go into the attic, walks the ridge to the chimney, and crosses the party-wall band twice. The day it goes, it goes as a fall through the deck into an attic full of truss webs. Any trade that works on attached 1980s roofs should add one question to its intake: is this a townhouse, and is there access to look at the underside before we go up?

Who settles it

Recognition is all a tech does here. A structural engineer decides how much strength is left and what the framing needs; a roofer replaces the sheathing; the building official decides what assembly is acceptable at the wall. Walk in with the attic photographs, the build year, which planes and which bands, and whether the stamp was visible. See related: Recognize, Record, Route - the Rule for Legacy Materials, and Dating a House by What You Find in It.

References

• See related: How to Photograph and Record a Suspect Material; Telling a Customer About an Old Material Without Scaring or Understating
• International Residential Code, townhouse separation provisions (section R302.2 and subsections), as adopted by your jurisdiction
• ASTM D5516 and ASTM D5664 (elevated-temperature testing of FRT plywood and lumber); ASTM D6305 and D6841 (design adjustments)
• OSHA, Fall Protection in Construction, 29 CFR 1926.501(b)(13)