Hot-Mopped Built-Up Roof Application

A built-up roof (BUR) is a low-slope membrane made of several layers (plies) of reinforcing felt bonded together with hot bitumen and finished with a surfacing such as gravel, a cap sheet or a coating. In hot-mopped application the bitumen is heated in a kettle or tanker, carried to the roof and spread by mop or mechanical applicator just ahead of each roll of felt.

Bitumen

Two bitumen families are used:

• Asphalt, graded by softening point and suitable slope. Canadian guidance lists Type 1 (minimum 60 °C / 140 °F) for the flattest roofs, Type 2 (75 °C / 167 °F) and Type 3 (90 °C / 194 °F) for steeper slopes and flashings.
• Coal-tar pitch, used only on very low slopes (below 1:25, or 4 percent) and never mixed with asphalt in the same membrane.

Temperature control

The key concept is the equiviscous temperature (EVT), the temperature at which the bitumen has the right viscosity to flow, spread at the correct rate and bond. Canadian guidance says the application temperature should stay within 15 °C (27 °F) of the EVT at the point of application, checked with a thermometer, and that bitumen must never reach its flash point. Kettles are run slightly above EVT to allow for heat loss in transport.

Laying the plies

1. Start at the lowest point of the roof, laying felts so water flows over the laps, not against them.
2. Mop no more than about 900 mm (3 ft) ahead of the roll.
3. Apply at the specified rate; Canadian guidance gives about 1.0 kg/m² (20 lb per square) for organic felts and 1.2 kg/m² (25 lb per square) for glass-fibre felts.
4. Broom organic felts into the bitumen to expel trapped air; glass-fibre felts generally need no brooming.
5. Shingle the plies so that the lap sets the number of layers: half-lapping gives two plies, two-thirds three plies and three-quarters four plies.

Canadian guidance discourages phased application, where some plies are left for a later day, because dirt and moisture trapped between plies cause poor adhesion and blisters.

Surfacing and flashings

The finished membrane is protected with a flood coat and aggregate, a mineral-surfaced cap sheet or a reflective coating. Base and cap flashings at walls and curbs are built from reinforced membrane plies. Kettle operation, hoisting and carrying hot bitumen carry serious burn and fire risks and require trained crews.