22 BULLETIX 801, U. S. DEPAET3IEXT OF AGKICULTUEE. 
EXTERIOR WALLS. 
The exterior wall of the fire-resistiye warehouse should be of 
noncombustible material. It need not, however, be of such material 
and design as to offer as long-sustained resistance to fire as the di- 
vision wall unless that particular exterior wall has a very hazardous 
exterior exposure, in which case it should conform to requirements 
for a division fire wall. The material suitable for the usual ex- 
terior wall is not as limited as is the case for the division fire wall. 
Brick, concrete (with or without steel reinforcement), stone, and 
sand-lime brick are all accej^table, and usually are to be given pref- 
erence in the order named. Where there is no exposure hazard, 
hollow cement blocks and hollow clay wall tile may be used for panel 
walls of two-story buildings, but even in this case the adoption of 
this material for a fire-resistive warehouse is not desirable. 1 
In discussing exterior walls, brickwork is the basis for stating 
thickness, just as it is for the division fire wall, but in this case no 
increased thickness is required when plain concrete is substituted 
for brickwork. (In this connection it is well to note the require- 
ments for " panel walls " set forth later.) 
The exterior wall of the usual bearing type which supports floors 
and roof may be i inches less in thickness than was specified for a 
division fire wall, provided the minimum thickness be 12 inches for 
the first story, where the height of the story does not exceed 18 feet, 
and that where there is more than one story the wall for the top 
1 Brick, cement blocks, and stone may be laid in lime mortar, except that for 
the portion of the wall below and 2 feet above the ground level, the cement 
mortar, previously mentioned, should be nsecl. The lime mortar should con- 
tain one part lime putty, or hydrated lime, to not more than four parts sand, 
the exact proportion being adapted to the character of the sand used. (See 
U. S. Bureau of Standards Circ. 30; Lime; Its Properties and Uses, 1911.) 
The replacement of one-fourth of the lime with a like amount of Portland 
cement will give excellent results without a great increase in cost. In this 
ease the mortar may be mixed in advance and the cement added as the mortar 
is used. These mortars are equally suitable for the cement blocks. 
Walls of brick — either clay, clay and shale, or sand-lime — and of concrete 
with and without reinforcement may be used to support the roof and floor, 
this construction being a "bearing wall." In case of walls of cement blocks 
or clny tile it is best that the floor and roof slab be supported independently 
by columns and beams and that the wall be built independently so as to be 
tied to the floors and columns in such manner that the wall will support only 
its own weight, thus forming a " curtain wall." In case the building is a 
number of stories in height and the tile or blocks are used it will be better 
and more economical to use a " panel wall " built between the columns and 
beams in such manner that the inclosing wall for any floor is supported by 
that floor and not by the wall for the story below. 
