RCI Interface "Built-up Roof Inspections"

Background on this topic from Inspec

Why a built-up roof is read differently

A built-up roof (BUR) is an assembly, not a membrane. Alternating layers of bitumen and reinforcing felt are built up in place over insulation and a deck, then surfaced with aggregate, a mineral cap sheet or a coating. Because the waterproofing is constructed on the roof rather than delivered to it in rolls or panels, the conditions that matter during an inspection differ from those on a single-ply or modified bitumen roof. Adhesion between plies, the condition of the interply bitumen, and the way the assembly has moved over its service life all leave evidence on the surface.

Inspecting a built-up roof is therefore an exercise in reading symptoms. Most of what is worth recording is not a hole. It is a pattern that says something about what is happening between the plies, in the insulation, or in the deck below.

What to gather before walking the roof

The surface makes more sense when the history is known. Useful background includes original or re-roof construction documents, any warranty or specification on file, records of previous repairs and overlays, and the locations and dates of interior leaks reported by building staff. Knowing where someone has already patched, and where water has shown up inside, turns a general walk into a directed one.

Access and safety govern the method as much as the findings do. Hatch and ladder conditions, perimeter protection, skylights and other fragile surfaces, rooftop equipment that is energized, and the amount of daylight available all shape how much of the roof can be covered in one visit and whether destructive work can be done the same day.

Surface conditions worth recording

Walk the roof in a consistent pattern and note where the surfacing has gone. Aggregate migrates away from drains and off high points; a mineral cap sheet loses granules along traffic routes and at wind-scoured corners. Where the bitumen is left bare it weathers, oxidizes and eventually crazes into the pattern commonly called alligatoring. Alligatoring confined to the flood coat is a surfacing problem. Alligatoring that has opened into the felts is a membrane problem.

• Blisters — pockets of trapped air or moisture between plies, or between the membrane and the substrate. Size, distribution and firmness matter more than presence.
• Ridging — raised lines that often follow insulation board joints or deck flutes, indicating movement below the membrane.
• Felt slippage — plies that have crept downslope, visible as exposed felt edges and bunching at the low end of a slope.
• Splits — straight-line separations, frequently at changes in deck direction, deck type or structural support.
• Ponding — standing water and the debris, sediment and vegetation rings it leaves behind.
• Traffic damage — crushed surfacing and punctures along routes to rooftop equipment, usually concentrated at the service side of units.

Blisters, ridges and splits

Blisters form where a void between plies is subjected to repeated heating and cooling. Not every blister is a leak, and not every blistered roof needs replacement. What matters is whether the blister has broken, whether the surfacing over it has worn through, whether it sits where it will be stepped on, and how much of the roof area is affected. A firm, intact, well-surfaced blister may outlast the rest of the roof. A field of soft blisters with fractured tops is a different finding.

Ridging describes what is under the membrane. Ridges that align with insulation joints usually point to board movement or moisture in the insulation. Ridges over deck joints point to substrate or structural movement that a new membrane, installed the same way over the same substrate, would likely repeat. Splits are the end state of the same story: the membrane has been asked to accommodate movement it cannot absorb, and the location of the split tends to identify where that movement occurs.

Flashings, edges and penetrations

Base flashings, curbs, drains, pitch pockets, expansion joints and edge metal account for a large share of the leaks found on aging built-up roofs. Inspect for open laps, slipped or sagging base flashing, loose or corroded counterflashing, deteriorated sealant in pitch pans, drain strainers and clamping rings, and edge metal that has loosened at the fascia. Note the relationship between flashing height and the finished roof surface, particularly where overlays have raised the roof and shortened the effective flashing.

Walls above the roof belong in the same pass. Masonry, coping, through-wall flashing and sealant joints above the roof line can deliver water into the assembly in ways that look like a roof leak from inside the building.

Confirming what is below: test cuts and moisture surveys

Visual findings describe the top of the assembly. Test cuts and moisture surveys describe the rest of it. A test cut exposes the number of plies, the condition of the interply bitumen, the insulation type and thickness, the presence of a vapor retarder and the deck below. Nondestructive moisture detection maps where wet insulation is likely to be, so cuts can be placed to confirm what the survey suggests rather than sampling at random.

Read together, the two answer the question an owner is usually asking: is the wet material confined to a few areas that can be cut out and replaced, or is it distributed through the field? Cuts are destructive and are therefore planned in limited, representative locations, repaired the day they are taken, and recorded with photographs, measurements and a plan showing exactly where each one came from.

Related background on what sits below the membrane is collected in our educational resources, including the roof deck series — for example, cast-in-place structural concrete decks, which behave differently under a built-up roof than a steel or wood deck.

Documenting findings so the report can be used

Findings are only as useful as their locations. A roof plan with conditions mapped onto it, photographs keyed to that plan, and measured extents of each condition let the owner, the design professional and the bidding contractors all work from the same picture. Conditions that are likely to change, such as open splits or blisters already underfoot, are worth flagging for interim attention so the building is not waiting on the full scope.

From findings to a recommendation

A report should separate what was observed from what is recommended. Observations are the conditions, their locations and their extent. Recommendations follow from how much of the assembly is still serviceable, whether wet material is isolated, whether flashings can be reworked, what the structure and code allow, and what the owner intends to do with the building. Repair, phased replacement and full tear-off are all legitimate outcomes; the evidence decides which one.

Inspec provides commercial roof inspections and ongoing roof consulting, and where a roof has already failed and the cause is disputed, forensic roof investigation. We do not perform repair work, so the recommendation in the report is not tied to any scope of work we would go on to sell. To discuss a built-up roof on one of your buildings, contact us.

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