TECHNICAL INFORMATION ON BUILDING MATERI ALS

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FOR USE IN THE DESIGN OF LOW-COST HOUSING

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C,

November 24, _19_3

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TYPES OF DECKS ADAPTED TO BITUMINOUS

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T ULT-UP ROOFING

Bituminous built-up roofing is particularly adaptable for flat

roofs s\ich as are usually found on hotels, apartment houses, industrial

buildings, etc., but has also been used to considerable extent on smaller structures where low pitch roofs are desired. It may be applied to roof decks of wood, poured concrete or gypsum, precast con- crete or gypsum blocks, book tile and steel, and when properly constructed should give long service with but little attention. Insulation may

be incorporated with built-up roofing on any of the above mentioned

deck types. The Underwriters* Laboratories, Inc., established by the

National Board of Fire Underwriters, has issued specifications for the

construction of built-up roofing under Classes A, 3, and C; Class A

being effective against severe fire exposure, Class B against moderate

fire exposure, and Class C against light fire exposure.

Following are some general comments applicable to each type of

roof deck:

Wood Decks : Roof decks of wood should have a nominal thickness

of 7/8 TT with all end joints bearing on, and securely nailed to rafters.

Rafters should be so spaced as to prevent any perceptible deflection

under a load of one hundred and fifty pounds. Wide cracks and knot

holes must be covered with metal nailed to the sheathing. Use must

be made of seasoned Lumber to avoid undue strain which would tend to

tear the membrane roofing. Before applying felts, the deck must be

smooth, dry, and clean.

Concrete and Gypsum : Roof decks of these materials, whether

poured or constructed of precast blocks, should be finished smooth

before applying the roof. Depressions should be filled with mortar

and high spots leveled.

120336-C 1
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Book Tile : Decks of this type should he surfaced with a smooth

coating of Portland cement mortar, not less than one-half inch thick,

and should he allowed to set and dry before the roof is applied.

Steel Decks : Steel decks invariably carry an approved insulation, Unless primed when manufactured they should he given a priming coat of asphalt primer followed hy a mop coat of roofing asphalt,

into which, the insulation is embedded while the asphalt is hot.

Some means of fastening the insulation to the steel deck must he

provided on slopes of more than two inches to the foot. The builtup roofing is then applied over the insulation as over other surfaces.

Some specifications for asphalt and asbestos felt roofs recommend

a layer of asphalt-saturated asbestos felt cemented to the steel deck

under the insulation, where humid conditions exist.

Insulation : Any of the before mentioned types of roof decks

may be covered with an approved insulation. The type selected must

retain roofing nails, and be thoroughly dry. Installation of the

roofing should immediately follow application of the insulation and,

in no case, should more insulation be applied than can be protected

from sudden weather changes.

For a condensed summary of the Federal Specifications covering

materials used in the construction of bituminous built-up roofs;

also for methods of laying various types of such roofs (not covered

by Federal specifications), reference may be made to Davy Department Specification Ho. 7Yg, "Roofing, Siding and Sheet Metal Work;

Dampproofing and Membrane Waterproofing", obtainable free, upon

request, from the Bureau of Yards and Docks, Navy Department,

Washington, D. C.
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