TSCHITICAL INFOffi.'IATION ON BUILDING llATlffilALS

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F03 USE IN THE DESIGN OF LOVZ-COST HOUSING

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THE NATIONAL BUHSAU OF STANDARDS

UNITED STATES DEPAHTIISNT OF GOIGIESGS

V/ASHINGTON, D. C.

NovetnDer 28, JL_9p6_

liA'TEH IALS USED IN BITuG-IINOUS BUILT-UP HOOFING

Host bituminous built-up roofing consists of an impervious membrane of appreciable thickness assembled on the job from layers of

bituminous saturated felt ivNich are cemented to each other and to the

roof deck with a bit'uminous cement, usually asphalt or coal-tar pitch,

applied hot by m.opping*

Built-up roofs m.ay be either sirrfaced or unsurfaced. The bitumen

used in unsumfaced built-up roofs is invariably asphalt, while both

asphalt and coal-tar pitch are used for mineral-sumfaced roofs. Be- cause of the difficulty of retaining mineral surfacing materials on steep slopes, unsurfaced coverings are used on roofs of steeper pitch.

Built-up roofs which are subject to m.ucli foot traffic may be finished

with prom.enade tile embedded in a coatiiag of cement mortar laid over the final mopping of bitimien.

Coal-Tar Pitch ; Coal-tar pitch is a product of bituminous coal.

In the process of m.anufact'om ing illuminating gas or coke from such

coal, a black viscous liquid known as coal-tar is obtained which,

when redistilled to drive off the more volatile constituents, leaves

a residue of coal-tar pitch. The quantity of pitch obtained and its

characteristics are governed by the type of tar used and the method of

production. In general, however, it may be stated that coal-tar pitch,

because of its high resistance to the disintegrating action of water,

makes an excellent roofing and waterproofing agent. It is particularly adaptable for use in the construction of the so-called '’deadlevel" bituminous built-up roofs on vjhich water may collect and stand.

(See Federal Specification R-P-381,^ "Pitch; Coal-Tar (for) MineralSumfaced Built-Up Roofing, Waterproofing and Dampproof ing" , Type I,

Coal-tar pitch for built-up roofing.)

Federal Specifications listed in this paper may be obtained

from the Superintendent of Documents, Government Printing Office,

V/ashington, D, C., (Price 5 cents each).

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Aschalt ; Asphalts for roofing purposes may be either fluxed native

asphalts, (Trinidad or Bermudez or Lake asphalts), or those derived from

petrdleurl (artificial or petroleum asphalts). Most of t]ie asphalt used

for rocffing' purposes in this country is obtained as a by-product in the

manufacture of gasoline, -fuel oil, lubricating oil, etc,, from crude

petroletLm, Like coal-tar pitch, these are also distillation residues,

variously treated, their characteristics depending on the nature of the

original oil and the process by which th^ey are produced, iisphalt is

also an excellent roofing and waterproofing material, probably less

resistant to moisture and sunlight than coal-tar pitch, but also less

susceptible to temperature changes and, therefore, better suited than

coal-tar pitch for the construction of built-up roofing on steeper slope

(See Federal Specification SS-A-6Sb,^ ’’Asphalt; (for) Built-Up Hoofing,

V/aterproof ing and Bampproofing”, Type I, Asphalt for surfaced built-up

roofing. Type II, Asphalt for unsurfaced built-up roofing.)

Felt: Felts used in the construction of bituminous built-up roofing may be roughly classified as rag or asbestos felts. The rag felts

are made on a paper making machine from varioiis fiber stocks, the most

common source of which is cotton rags. In recent years, however, it

has been the general practice to substitute such materials as jute and

manila bagging, old newspapers, and sawdust for a part of the rag stock.

'The raw felt is impregnated by running the sheets thron^h a tank

containing asphalt or coal-tar pitch maintained at a high temperat’ure

.

Unsaturated rag felt for the construction of built-up roofing should

weigh not less than 5.2 pounds per 100 square feet and sat'.irated rag

felt between 13 and 15 pounds per 100 square feet. For flashings, the raw felt should weigh net less than 10,4 pounds per 100 square feet

and the sat’orated felt between 27-|- and 30i- pounds, (See Federal

Specification HII-F-191, ’’Felt; Asphalt-Saturated (for) Flashings,

Hoofing and ’Taterproof ing” , Type I, Asphalt-saturated felt for minerals'orfaced built-up roofing and for miembrane waterproofing, and Type II,

Asphalt-saturated felt for flashings; also Federal Specification

HH-F-201, ’’Felt; Coal-Tar Saturated (for) Roofing and ¥/aterproof ing, ”

)

Asbestos felts are usually made from asbestos fiber mixed with

various sizing materials; and, at times, cotton rag stock, w'ood pulp,

hair, or other material is added, Tliey have less strength than the

rag felts and do not absorb as much saturant, (Federal Specifications

for bituminous saturated asbestos felts have not been issued.)

^Federal Specifications listed in this paper may be obtained

from the Superintendent of Documents, Government Printing Office,

Washington, D. 0,, (Price 5 cents each).
Siirfacing Materia ls for Bituminous Built-Up Hoofing; Sur fa c ing

materials are used to permit the application of heavy surface coatings

of bitumen which, otherwise, if exposed directly to the sun’s rays,

would tend to flow and disintegrate more rapidly.

Slag and Gravel t Crushed blast-furnace slag or water-worn gravel,

dry and free from sand, clay and other foreign substances, and' from

one-quarter to five-eighths inch in size, are the most commonly used

surfacing materials.

Promenad e Tile; Vitrified shale tile, slate and alberene stone

are used as promenade tile. They are usually embedded in a coating of

Portland cement mortar of not less than three-vquarters of an inch in

thickness, laid over a five-ply bituininous built-up roof. Joints between

the tile should be grouted with portland cement mortar. Expansion joints

extending through the tile and cement mortar are necessary between the

tile and all flashings, and throu/jhout the roof sujrface, in no case spaced more than tv,»enty feet apart,

’

Prepared Roofing ; iiineral-surfaced asphalt prepared roofing is

sometimes used as a cap sheet for comparatively steep built-up roofs,

'This cap sheet usually consists of a single thickness of roofing felt

impregnated with an asphaltic saturant, coated on one side, approximately one-half of the width, with an asphaltic coating in which are embedded mineral granules.

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 Navy bepartment

Specification No. 7Yg, "Hoofing, Siding and Sheet ''"etal Work; Dampproofing and Membrane Waterproofing", obtainable free, upon request,

from the Bureau of Yards and Docks, Navy Department, Washington, D. C,