CEMP-ET
TI 809-53
01 May 1999
CHAPTER 4
BUILT-UP ROOFING SYSTEMS
REFERENCE CEGS 07510
4-1. OVERVIEW OF BUILT-UP ROOFING SYSTEMS.
a. Components. Two basic components, felts and bitumen, are laminated in the field (built-up)
into a waterproofing membrane. The bitumen can be either asphalt (ASTM D312) or coal tar pitch
(ASTM D450), which serves both as the system adhesive and as the waterproofing agent. Reinforcing
felts of glass fiber (ASTM D2178), organic (ASTM D226), or non-woven (ASTM D5726) synthetic fiber
mats provide strength and stability to the membrane. Surfacing (ASTM D1227, D1863, D2823, D2824,
D4479, D6083) provides UV resistance and fire protection. Because the built-up roof membrane is
field assembled, field quality control is very important to its success.
b. General Concept. A bituminous built-up membrane is more brittle than elastic when cold. It
relies on firm anchorage to distribute stress uniformly across its surface. Suitable substrates for BUR
include well anchored, heat resistant thermal insulation boards and stable, nailable and non-nailable
decks. On nailable decks such as wood, OSB, plywood and nailable planks a base ply is
mechanically fastened to allow for seasonal changes in deck dimensions. Remaining layers are
adhered with hot bitumen. Over thermal insulation and non-nailable decks the base ply is adhered
with hot bitumen. Key steps in installation include heating the bitumen to adequate fluidity, spreading
the hot bitumen uniformly, and embedding the reinforcing felts while the bitumen is still hot. Application
instructions are provided in industry literature and should be verified by field quality assurance.
(1) Climate/Weather.
(a) Cold and wet weather are serious detriments to the installation of hot built-up roofing.
Entrapped moisture within the layers of a BUR can result in blistering. Roofing materials need to be
kept dry before, during, and after delivery to the site. The substrate must be unfrozen, dry and warm.
A conductive cold substrate can cool the bitumen enough to prevent proper attachment.
(b) Rolls of felt may have to be placed in heated storage prior to use. Hot bitumen gels
within minutes so once the membrane is installed it is immediately weather resistant. Membrane
tensile strength should exceed 35 kN/m (200 lbf/in.) at 18oC (0oF) for colder climates. Elongation
should exceed 0.7%.
(c) In hot climates higher melt point bitumens, backnailing, and reflective surfacings may
be needed to prevent slippage. Higher melt point asphalts should be used (Type III or IV/ASTM
D312). Light colored membranes reduce heat load and slippage potential.
(d) Membrane strength of 26 kN/m (150 lbf/in.) at 18oC (0oF) is considered adequate for
moderate climates. In colder, northern climates a minimum strength of 35 kN/m (200 lbf/in.) is
recommended.
(e) Too thick an application of bitumen can result in slippage and too cool can result in
delamination.
(f) Finished built-up membranes surfaced with aggregate
when
placed
over
incompressible solid substrates are relatively resistant to hail and ice.
(g) Nightly tie-offs need to be installed so that moisture does not get under finished work.
(2) Logistics. BUR systems are material intensive. Bitumen and felts are hoisted to the roof
and placed where needed. Bitumen may be delivered to the job site in heated tankers, or heated in
4-1
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