BUILT UP ROOFS - CONTENTS: BUR roof materials, basic design, inspection, leak detection, diagnosis. Tar and gravel roofing. Definition of BUR or built-up roofing. Flat and low slope built up roofing inspection and repair
Built-Up roofs or BUR or "tar and gravel" roofs (if tar and gravel top coat were applied) are constructed traditionally using multiple layers of heavy-weight roofing felt (roof plies) cemented together by hot mopped asphalt. Cold applied asphalt methods are also used for some BUR roofs.
[Click to enlarge any image]
Carson Dunlop's sketch (left) illustrates a 4-ply built-up roof installation.
Built up roofing is usually used on flat or low-slope roofs (less than 3" rise in 12" of run). We have found an occasional BUR on steeper slopes, including a tar and gravel steep slope roof whose gravel kept washing down into the building's gutter.
Built-up Roofing (or BUR) is the most popular choice of roofing used on commercial, industrial and institutional buildings. BUR is used on flat or low-sloped roofs and consists of multiple layers of bitumen and ply sheets. Components of a BUR system include the roof deck, a vapor retarder, insulation, membrane and surfacing material. The components are assembled at the job site to actually form the built-up roof. At the heart of this roofing system is the roofing membrane, which consists of roofing bitumen and multiple reinforcing plies of roofing felt.
Roofing bitumen is the primary adhesion/waterproofing agent used between roofing plies. Bitumen arrives at the job site in solid form, but is heated and applied as a liquid. Roofing bitumens may be either a product of petroleum refining (asphalts) or a product of the coal-cooking process (coal tar pitch).
Multiple reinforcing “plies” are asphalt-coated roofing sheets or felts installed in three or more layers to strengthen and stabilize the BUR membrane. These multiple reinforcing felts also make the membrane more pliable and resilient, protect the bitumen from water degradation, and serve as a fire-retarding element in the membrane system.
BUR roofing membranes can be protected from solar radiation by embedding gravel in the bitumen, applying a surface coating or applying a granular-surfaced “cap” sheet. Light-colored surfacing materials can be used to reflect heat from the building. In addition, surfacing agents can provide additional fire protection.
Built-up roofing (BUR) systems dominated the commercial
and residential low-slope roofing markets until the
1980s, when single-ply membranes became widely accepted.
BUR roofs consist of layers of asphalt-impregnated
felt bonded with hot asphalt, or in some parts of the
country, hot coal tar. The average life span of a hotmopped
BUR roof is 15 to 20 years, although this can be
extended by applying an aluminum coating every three to
five years to reduce UV degradation and alligatoring.
BUR roofs can have either a smooth coated surface
or a stone surface created by spreading crushed stone
or gravel into a thick flood coat of hot asphalt or tar.
Aggregate-faced roofs are typically more durable due to
the heavier flood coat and the protection offered by the
stone from UV radiation, hail, and other environmental
wear and tear. However, the stone coating makes leaks
harder to find and repair.
Proper detailing of metal flashings at openings, parapet
walls, and roof edges for BUR roofs is critical, and these areas need
regular inspection and maintenance. The most likely place
for leaks is flashings, particularly metal edge flashings due
to their thermal movement. Asphaltic or rubber flashings
may also become brittle and crack.
Pros and Cons of Built-Up Roofs
BUR roofs are reliable if properly installed,
and their multiple layers provide some protection
against small installation errors. However, the long set-up
time makes BUR expensive for small residential jobs. Also
the heavy equipment, odors, and potential spills associated
with a hot-mop job are not welcome on many residential
Thanks to Carson Dunlop for providing the sketches (below) of these common low slope or flat roof defects. We illustrate blisters in built-up roofing membranes (these can occur in modified bitumen roofs and EPDM roofs too), alligatoring or cracking (sketch and photo just below) on flat roofs, particularly where tarred surfaces or un-coated modified bitumen roof surfaces are exposed without gravel or mineral granule protection.
Our right-hand photo shows our client inspecting built-up roofing installed on a low-sloped roof with surrounding parapet walls. The roof was worn, badly cracked, having been "repaired" with multiple coatings of tar - it needed to be replaced.
Ponding, a problem on flat roofs likely to lead to leaks, is illustrated just below.
Any roof needs to drain properly to survive. The flooded BUR (photo, above right) in New York is likely to leak soon as it does not drain, leaving ponding over much of the roof surface for days after rainfall.
Especially in a freeze-thaw climate, even the smallest imperfection in a flat or low-slope roofing seam will become a leak on roofs that flood.
A "rule of thumb" used by some roofers is to estimate that the typical life expectancy of a built-up roof will be 5 x the number of plies installed. It is not uncommon for a well-installed built-up roof covered with tar and gravel (for protection from sunlight) and properly drained to last for 40 years.
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