01 May 1999
kettles. Occasionally a kettle may be placed on the roof because of extreme height or transportation
difficulty. Melted bitumen is transferred to luggers and applied using mops or bitumen spreaders. In
cold weather, luggers, pump lines, and mop carts may be insulated to reduce heat loss.
(3) Familiarity with the System and Site. Built-up roofing is well known by most commercial
roofing contractors. Since fumes are generated while handling the bitumen, the kettle should be
placed where the fumes will be least objectionable. Burns from the hot bitumen are of concern.
Workers should wear non-synthetic fiber work clothes with long sleeves, gloves, and work boots.
Protective pedestrian fencing may be needed since the kettles remain hot for many hours after use
and both the hot bitumen and heating fuel are dangerous.
(4) Life Expectancy. Well built BUR systems typically last 15 years or more provided periodic
inspection and maintenance is performed. Poorly built, unmaintained systems usually are problematic
within a few years. Penetrations and flashings are especially vulnerable. BUR systems are cost
effective on both an initial and life cycle basis.
4-2. BUILDING ELEMENTS.
a. Slope. A minimum design slope of 2% (1/4 in./ft) is required. Aggregate surfaced BUR
systems using either coal tar pitch or Type I asphalt have been used in ponded situations, but that is
(1) Low-Sloped Built-Up Roofs. Coal tar pitch should not be used on roofs with slopes
greater than 2% (1/4 in./ft). Aggregate imbedded in a flood coat of bitumen is considered more water
resistant than capsheets or bare, lightly coated surfaces.
(2) Steep Built-Up Roofs. At slopes above 4% (1/2 in./ft) wood nailers are needed to prevent
slippage. The felts are backnailed to prevent slippage. Aggregate surfaced roofs are limited to a
slope of 25% (3:12) while smooth surfaced membranes (properly backnailed) have been used at
steeper slopes. Working with hot bitumen at slopes above 8% (1:12) is dangerous and torch-applied
modified bitumens or cold applied membranes are preferred.
b. Structural Considerations.
(1) Roof Loads. BUR systems are not especially heavy--smooth surfaced roofs weigh
approximately 5-10 kg/m (1-2 psf); aggregate roofs weigh 29-34 kg/m (6-7 psf). Roofing equipment
may be very heavy and loads must be distributed by appropriate means (i.e., installing plywood
walkways) to avoid deck damage.
(2) Drippage. Non-nailable plank decks may require grout or tape to prevent bitumen
drippage into the interior.
(3) Deck Venting. Moisture bearing lightweight insulating fills require provisions for venting to
the building's interior as well as for a nailed moisture resistant base sheet. Special fasteners are
required for those low-density fills.
Expansion Joints, Seismic Joints, and Area Dividers (figures 4-1 and 4-2).
(1) Raised Joints. Joints should be located at high points where practicable and placed on
curbs above the water line. This requires coordination of slope layout and drainage plans.
(2) Expansion Joint Locations. Expansion joints should be provided only at each expansion
joint in the structure. Structures need expansion joints at intervals not over 60 m (200 ft) in length or
width. Joints should also be used at changes in deck direction or membrane materials. If a structure
does not contain enough expansion joints it is inappropriate to solve the problem by just adding
expansion joints to the roofing system.