A built up roof is a waterproof membrane assembled in place from alternating layers of reinforcing felt and hot bitumen, normally three or four of each, then surfaced with gravel or a mineral cap sheet. Roofers call it BUR, owners call it tar and gravel, and it is the oldest flat roof system still specified for new work. Its defining quality is redundancy: the waterproofing is several layers thick, so a single puncture is rarely a leak that afternoon.
That is also why it is still bought on buildings nobody can afford to have leaking. Where a roof carries rooftop units, service traffic and the people who maintain them, a membrane with margin in it behaves better than a thin one that is easier to install.
How the layers work
Two materials doing two jobs. The felt is reinforcement: it carries the tensile strength and holds the assembly together while the building moves. The bitumen is the waterproofing: it fills, bonds and seals, and it is what water actually meets. Neither works alone, which is why a repair has to rebuild both rather than spread one.
A typical assembly from the deck upward runs:
- Deck. Concrete, steel or timber, providing the structure.
- Vapour retarder, where the building’s interior humidity needs one.
- Insulation, usually in two layers with staggered joints, often tapered to create fall.
- Cover board, a rigid layer that gives the membrane a stable surface and protects the insulation.
- Plies, three or four layers of felt mopped in bitumen, each lapped over the one below.
- Surfacing, gravel set in a flood coat, a granulated cap sheet, or a reflective coating.
The two layers owners never hear about are the cover board and the tapered insulation, and both decide more about the roof’s life than the plies do. Roof insulation covers why.
Why there is gravel on it
Three reasons, and only one of them is obvious. The gravel shields the bitumen from sunlight, which is the thing that hardens asphalt and eventually cracks it. It protects the surface from hail and from foot traffic. And on a roof without mechanical fastening it provides ballast against wind uplift.
That makes bare patches a condition finding rather than an aesthetic one. Where gravel has washed away or been blown to one side, the bitumen underneath is aging several times faster than the rest of the roof, and that area will crack first. Gravel migration also tells you where water is running, which is useful information about the fall.
Where built up roofs actually fail
Almost never in the field. The failures cluster in the same places on every one of these roofs:
| Location | Failure | Why it happens there |
|---|---|---|
| Base flashings | Plies turned up too low, or unsealed top edge | Wind driven rain goes over the top |
| Drains and scuppers | Lead or clamping ring failed, bitumen cracked | Every drop on the roof passes here |
| Curbs under equipment | Plies broken by fixings or service traffic | Nobody told the technician where to walk |
| Expansion joints | Membrane asked to absorb building movement | It cannot, so it splits along the line |
| Blisters | Plies that never bonded, trapping air or moisture | Damp felt, cold work, or a dirty surface |
| Alligatored field | Bitumen hardened by sunlight after gravel loss | Surfacing was lost and not replaced |
The pattern is that the field of the roof is doing fine and everything the roofer had to think about is not. That is good news for an owner, because details are maintainable and the field is not.
Blisters, splits and what they mean
A blister is a pocket between plies where the bond failed and air or moisture expanded. Small, firm blisters away from traffic can be recorded and watched. A blister in a walking route will break, and once broken it is a hole. The decision is about location and growth rather than size.
A split is different and more serious. It means the bitumen has hardened enough that it can no longer absorb the daily movement of the deck underneath, and the movement has to go somewhere. Splits that run in long straight lines usually follow a structural joint, and repairing the line without relieving the movement produces the same split again next year. Roof recover against tear off is the relevant decision once splits are widespread.
Where the water goes once it is in
Built up roofs are heavy and thick, and the insulation under them holds water quietly for years. There is no surface sign. A roof can look perfectly sound with a third of its insulation saturated, losing most of its thermal value and carrying weight the deck was never designed for.
This is the single reason to buy a moisture survey before deciding anything expensive about a built up roof. The membrane’s appearance tells you about the membrane. It tells you nothing about the assembly, and the assembly is what decides between coating, recovering and tearing off.
Built up against modified bitumen
Modified bitumen is the direct successor: the same asphalt with polymer added for flexibility, made into rolls in a factory and bonded by torch, hot asphalt, cold adhesive or a self adhesive backing. It installs faster, with no kettle on site, and it keeps the multi ply redundancy.
Built up roofing holds its ground where the gravel surface is wanted, where coal tar pitch suits a dead flat deck, and on buildings where the existing roof is BUR and matching it is simplest. Modified bitumen roofing sets out the comparison in full, and types of flat roof membranes places both against single ply.
What to ask before you sign
- How many plies, stated as a number, and in which bitumen?
- Is there a cover board, and what is it?
- How is fall being achieved, and where does the water leave?
- What is the surfacing, and who replaces it after a repair?
- Where are the walkways going, and who tells the equipment technicians?
The last one gets left out of almost every specification and causes a disproportionate share of the damage. Rooftop units need servicing, and the people who service them will walk the shortest line unless somebody gives them a path.
One more question belongs on that list if the building is occupied: how is the hot work being managed. A traditional built up roof needs a kettle, hot bitumen and the smell that comes with both, and on a school, a hospital or a restaurant that is a scheduling problem as much as a roofing one. Cold adhesive and self adhesive systems exist precisely to avoid it, and they cost more. Deciding which you want before the bids arrive stops the question being settled by whoever quoted lowest.
Where to go next
If you have a gravel roof and no idea of its condition, a moisture survey and a details inspection answer every question that follows. If the roof is sound but the gravel has gone and the bitumen is crazing, surfacing or coating is the cheap intervention and it has a deadline. And if you are specifying new work, read commercial roofing systems alongside this page, because the choice between BUR, bitumen and single ply is usually decided by traffic and redundancy rather than by price per square.
Common questions
What is a built up roof?
It is a flat roof membrane made on site from alternating layers of reinforcing felt and hot bitumen, usually three or four of each, finished with gravel or a mineral cap sheet. Because it is built in layers rather than laid as one sheet, roofers call it built up.
- Layers of felt bonded with bitumen, built up in place
- Three or four plies is the common specification
- Surfaced with gravel, granules or a reflective coating
- Also called tar and gravel, or BUR
- The oldest flat roof system still specified today
The felt carries the tensile strength and the bitumen does the waterproofing, which is the reverse of how most people assume it works. That division explains the system's behaviour: it is strong, heavy and very hard to puncture through, and it degrades by the bitumen hardening rather than by the reinforcement wearing out. A hundred year old technology is still specified because on a flat deck that takes traffic, redundancy beats elegance.
How many plies should a built up roof have?
Three or four in most commercial specifications, and the number is the specification rather than a detail. More plies means more redundancy, more weight and more cost. Fewer means a thinner membrane with less margin, and a roof that a single puncture can take through to the insulation.
- Two ply is a minimum and rarely the right answer
- Four ply is the traditional commercial specification
- Each ply is lapped so no two edges stack up
- The ply count should be in the contract as a number
- A repair must match the original count
Redundancy is the whole argument for this system. A screw dropped on a single ply membrane can be a hole through the only waterproofing there is. The same screw on a four ply roof has three more layers to get through. That is why built up roofing persists on roofs with rooftop equipment, service access and regular traffic, and why cutting the ply count to save money removes the reason you chose it.
What is the difference between asphalt and coal tar pitch?
They are two different bitumens with different behaviour. Asphalt is the common choice, works on slopes and is widely available. Coal tar pitch self heals in warm weather and tolerates standing water far better, but it flows on a slope and has largely been displaced by health and handling concerns.
- Asphalt suits sloped and ordinary low slope decks
- Coal tar pitch tolerates ponding and reseals itself
- Pitch cannot be used where the fall is significant
- The two must not be mixed in one assembly
- Many older pitch roofs are still in service
If you have an older building with a dead flat deck that has never leaked despite holding water, there is a reasonable chance it is a coal tar pitch roof, and that is worth establishing before anybody repairs it. Asphalt products do not bond reliably to pitch, so a well meant repair in the wrong material can create the failure. Ask for the existing bitumen to be identified before a scope is written.
How long does a built up roof last?
A properly built and maintained built up roof is a long lived system, typically among the longest serving low slope assemblies in commercial use. What shortens it is almost never the plies. It is standing water, lost surfacing, failed details and the traffic that comes with rooftop equipment.
- Service life depends more on drainage than on the membrane
- Lost gravel exposes bitumen to sunlight and ages it fast
- Details and flashings reach the end first
- Foot traffic without walkways concentrates the damage
- Wet insulation underneath shortens everything above it
The useful way to think about a built up roof is that the membrane rarely sets the date. Drainage, surfacing and details do, and all three are maintainable. That is why these roofs reward a maintenance program more than most: keeping water moving, gravel in place and flashings capped buys years at a fraction of what the years cost any other way. A roof nobody has visited since it went on will reach the end early, however many plies it has.
Can a built up roof be coated instead of replaced?
Often yes, and it is one of the better candidates for coating. A built up roof that is sound underneath, drains properly and has intact details can be cleaned, prepared and coated to restore its surface and shield the bitumen from sunlight. Wet insulation underneath rules it out.
- The membrane has to be sound, not merely in place
- Gravel surfacing must be removed or embedded first
- A moisture survey decides this, not a visual check
- Details and flashings are repaired before any coating
- A coating resets the surface, not the insulation
The honest framing is that coating buys time on a roof that is still working. It does nothing for water already in the assembly, and spread over wet insulation it seals the moisture in and accelerates what is happening underneath. So the sequence matters: survey first, repair the details, deal with the wet areas, then coat. A proposal that reverses that order is selling a product rather than solving the roof.