Fire Damage to a Roof Assembly

Almost every disagreement about a fire-damaged roof comes from treating “fire damage” as a single condition. Physically it is four, and they can each be present without the others.

• ●Direct flame impingement. Combustion at the surface. It consumes material, and it is the easiest of the four to see and to scope, because what is gone is gone.
• ●Radiant heat. Energy arriving from a flame that never touched the roof — a neighbouring structure, a detached garage, a vehicle in the driveway, a chimney or flue fire venting past the covering. Radiant exposure can cook a whole slope into uselessness while leaving it looking, from thirty feet away, like a roof.
• ●Hot gas and smoke movement. Combustion products travelling through the attic and out through whatever openings exist. This is the agent that damages the roof from underneath, and it is the one that reaches parts of the building the fire never did, because smoke goes where air goes.
• ●Suppression water. A working fire takes a great deal of water, and all of it lands somewhere. Saturated sheathing, saturated insulation, swollen panel edges and a soaked ceiling assembly are fire-loss damage as surely as char is, and they keep doing harm for days after the fire is out.

Asphalt shingles

Asphalt is thermoplastic: it softens as it heats and it does not come back the way it was. The practical consequences run in a fairly reliable order as exposure increases. First the surface softens and the granules lose their embedment, so the slope sheds granules in sheets rather than in the scattered way an aged roof does. Granules are not decoration — they are the shield that keeps ultraviolet light off the asphalt — so a slope that has been stripped of them is on a shortened clock even if nothing else happened to it.

Then the shingle itself distorts. Tabs curl, lift and take a permanent wave; laminated shingles can separate at the laminating adhesive, which is what people mean by mat delamination — the two plies that were bonded into one shingle stop acting as one shingle. The glass mat inside does not burn, but the asphalt that carries the mat degrades around it, and the result is a covering that is brittle in some places and soft in others.

The failure that gets missed most often is the sealant strip. Modern shingles rely on a thermally activated adhesive bead to bond each course to the one below, and that bond is most of the wind resistance the roof has. Heat can liquefy it so it never re-bonds, or fuse courses into a mass that tears when it moves. A slope that has lost its seal is a slope that will start blowing off in ordinary weather, which is why heat-exposed shingles that pass a visual inspection frequently do not pass a hand check. The same bond is discussed from the installation side in how many nails per shingle, where the interaction between fastening and sealing decides wind performance.

Metal panels

Metal does not burn, which is where most of the misunderstanding starts. What metal does is conduct, distort and lose its finish. Coil coatings discolour, chalk and burn off, and once the coating is gone the substrate’s corrosion protection goes with it. Steel and aluminium both lose strength as they heat, aluminium considerably earlier than steel. Panels that have been heated and then quenched with a hose stream can end up permanently oil-canned or bowed, and a standing seam roof that has moved has almost certainly disturbed its clips, its seams, or both — the fastening system in a standing seam roof is engineered around controlled thermal movement, and a fire is not controlled thermal movement. See standing seam installation for why the clip-and-seam arrangement is so sensitive to it.

Tile, slate and low-slope membranes

Clay and concrete tile survive flame well and survive thermal shock badly. Hot masonry hit with cold water spalls, crazes and cracks, and those cracks are frequently invisible from the ground and sometimes invisible from the roof until the tile is lifted. Slate behaves similarly and is additionally fragile under the foot traffic a fireground generates. On low-slope roofs the distinction that matters is thermoplastic versus thermoset: TPO softens, shrinks and melts, while EPDM chars and embrittles rather than flowing. Underneath either, polyisocyanurate insulation chars and the adhesive or fastener pattern holding the assembly down is no longer doing what the uplift design assumed.

Roof covering is a consumable. Sheathing is not, and it is usually the sheathing that decides whether a roof is repaired, partly re-decked or taken to the framing.

Wood exposed to fire pyrolyses and forms char. Char is not damaged wood — it is a residue with no structural value at all, and the only material that counts is the sound section beneath it. That is why the depth of char matters more than its area: a wide, shallow scorch across a deck can be less serious than a small, deep burn-through beside a rafter. It is also why char depth is measured rather than estimated from photographs, and why the measurement belongs to somebody with the training to interpret it.

The two common deck materials do not behave the same way. Oriented strand board is strands and adhesive; heat attacks the adhesive, and water finishes the job by swelling the strands, particularly at panel edges, so a fire-and-water-exposed OSB deck can be soft and out of plane well beyond the charred area. Plywood is veneers and glue lines, and it tends to delaminate ply by ply, which can leave a panel that looks acceptable from above sitting on a bottom ply that has let go. Older plank or skip sheathing is solid lumber and holds section better, but it also gives fire a path along the boards and leaves you with a deck that may not be a legal substrate for a modern covering when you re-cover it.

Fasteners are the quiet part. Deck nails lose withdrawal capacity when the wood around them chars or when the panel swells and shrinks, and deck attachment is exactly what the roof’s wind resistance is built on. A deck that is structurally adequate for gravity load can still be inadequately attached for uplift after a fire, and re-nailing the field is routine work on a fire repair for that reason.

Because everything above happens under the covering, out of sight, or in a way the eye reads as normal weathering. Five specific gaps between what a roof looks like and what it is:

• ●The damage is on the underside. A deck can be charred to a significant depth from the attic side with only heat discolouration showing on top. The attic is the primary inspection surface on a fire loss, and it is the one a ground-level inspection cannot reach.
• ●The covering is unsealed, not missing. Shingles that no longer bond look identical to shingles that do until somebody lifts one.
• ●The frame failed at the joints, not the members. Backed-out truss plates and cooked connectors leave straight, clean-looking members that are no longer connected the way the design assumed.
• ●Water did the second half. Suppression water saturates insulation and sheathing and keeps working long after the fire is out — swelling, fastener loosening, and mould growth in an unventilated, still-warm attic.
• ●The residue is a condition in its own right. Smoke deposits into porous decking and insulation and stays there. A roof that is structurally sound can still sit above an attic that has to be dealt with — see smoke odor in decking and attic insulation.

The reverse error is just as common and costs owners more. A roof with a dramatic hole in it may be largely repairable, because a fire department ventilation cut is a clean, deliberate opening in otherwise sound material. Visual drama and structural significance are close to unrelated on a fire loss.

Fire losses have more professionals in them than storm losses do, and crossing the boundaries between them is how bad decisions get made. The fire department owns the scene until it releases it, and its incident report is the primary record of what burned and what tactics were used. A fire investigator determines origin and cause, working to the methodology set out in NFPA 921, the National Fire Protection Association’s guide for fire and explosion investigations. A structural engineer determines whether members and connections are adequate and writes the repair details — and is the only person on this list qualified to say a damaged frame is sound. A restoration contractor and, under them, the roofer, execute. A roofer’s competence is the condition of the roof and what it costs to put right; not causation, and not coverage.

This article stops at the building. The residue, the exposure and the claim each have specialists who cover them in more depth than a roofing reference should attempt.

• ●SmokeDamage.org on how smoke testing works — what tape lifts, wipe samples and air sampling actually measure, and how a laboratory report is read. Relevant here because the question “is this slope’s attic contaminated?” is answered by sampling, not by looking.
• ●SmokeHazard.com on indoor smoke exposure — the health-side library on what is in combustion residue, fine particulate and volatile organic compounds, and which occupants are most at risk. Anyone spending hours in a burned attic is in an exposure situation, and this is the material that explains why.
• ●SmokeMitigation.com — laboratory smoke testing and certified mitigation as a service, including on-site sampling under chain of custody and interpretation by industrial hygienists. The commercial counterpart to the reference material above.
• ●National Fire Protection Association — the standards-developing organisation behind NFPA 921, the guide fire investigators use to determine origin and cause.