Flat and built-up roofs:
This article describes flat and low-slope built-up roofing or BUR roofing materials, choices, installations, inspection, defects, built-up roofing repairs, and BUR product sources.
Our page top photo shows a worn-out multi-ply BUR roof. We were measuring the width between roofing seams to determine the number of plies that was probably installed.
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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. Carson Dunlop's sketch illustrates a 4-ply built-up roof installation.
[Click to enlarge any image]
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.
The Asphalt Roofing Manufacturers' Association (ARMA) describes BUR as follows:
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.
Shown below, a low-slope roof in Oxford in the U.K. This roof has a thick poured-on coating that has cracked badly.
The close-up photo of the same roof shown below shows some patching. BUR roofs rely on multiple plies of felt or equivalent material to give the roof membrane tensile strength. Just pouring on more asphalt won't do the job - it is not true that the bigger the blob the better the job.
Given the regularity and spacing of the cracks in the roof above I suspect that someone has poured a stunningly-thick coating of a bituminous roof compound onto this surface, filling it up to cover the raised seams of a metal roof below. Just speculatin'.
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 job sites.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Our 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.
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.
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.>
Carson Dunlop's sketch of blisters in built up roofing membranes, left, illustrates a problem than can occur in other membrane roof types as well, including MODIFIED BITUMEN ROOFING and EPDM or rubber membrane roofs MEMBRANE & SINGLE PLY ROOFS.
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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