Modified bitumen and built-up roofs

What these systems are and where they fit

A built-up roof (BUR) is the classic tar-and-gravel roof: alternating layers of asphalt and reinforcing felt, finished with gravel or a cap sheet. Modified bitumen is its modern relative. The asphalt is blended with polymers so it stretches and resists cracking, and it's made in factory rolls that are applied in two or three plies.

Both fit low-slope roofs that see a lot of traffic or have many penetrations: restaurants and retail with crowded rooftops, older offices, schools, churches and apartment buildings. They're also the natural choice when a building already has an asphalt roof and the owner wants like-for-like.

Torch-down, self-adhered and cold-applied

Modified bitumen can be installed several ways, and the right one depends on the building:

• Torch-applied: the underside of the sheet is heated with a torch and fused to the layer below. Proven and durable, but it involves open flame, so it needs fire-safety controls and isn't right for every building
• Self-adhered: peel-and-stick sheets with no flame and little odor. A good fit for occupied buildings, schools and places where a torch isn't acceptable
• Cold-applied: sheets set in adhesive, with no flame and no hot asphalt kettle
• Cap sheet: the top ply has a granule surface for UV protection, available in reflective white

Pros and trade-offs

Compared with single-ply membranes such as TPO and PVC:

• Redundancy: several plies mean one small puncture is less likely to become a leak
• Stands up to foot traffic and dropped tools better than a thin single sheet
• Familiar and widely repairable
• Heavier, and slower to install, than single-ply
• Dark surfaces run hot unless a reflective cap sheet or coating is used
• Granules wear off over time, and ponding water ages asphalt faster than it ages most membranes

What's included

Our proposals for modified bitumen and built-up work spell out:

• Tear-off, or a recover where the existing roof is dry and code allows another layer
• Insulation and tapered insulation to fix ponding
• The base sheet, the number of plies and the cap sheet, with the application method named
• A reflective cap sheet or coating as an alternate
• New flashing at walls, curbs and penetrations, plus edge metal, drains and scuppers

Lifespan and warranty

A modified bitumen roof commonly lasts 15 to 25 years, and a well-built multi-ply built-up roof 20 to 30. Drainage is the biggest factor: asphalt roofs that hold water wear out early.

Our own workmanship on commercial roofs is covered by a 20-year workmanship warranty, in writing. The manufacturer's warranty on the materials is separate, and we register it for you at closeout.

Manufacturer material and system warranties are available, subject to the manufacturer's requirements. We tell you which ones your project qualifies for.

Repair, maintenance and restoration

Typical repairs are split seams, blisters, cracked flashings at walls and worn areas where granules are gone. When the roof is aging but dry, a coating system can seal it, make it reflective and extend its life without a tear-off. When moisture is trapped in the insulation, those sections have to be replaced first. We find out which with an inspection and, when needed, a moisture scan through our testing partner.