46 BULLETIN 801, U. S. DEPARTMENT OF AGRICULTURE. 
It also tends to prevent the warping which, is particularly damaging 
to roofing. The planking should be securely nailed to timbers and all 
the joints should be close. Over this should be laid one layer of 
waterproof paper covered with a durable wearing floor about seven- 
eighths of an inch thick. There should be provided, however, an 
open joint of about one-half inch at the junction of the plank edge 
and wall to permit the plank to swell in damp weather ; disregard of 
this precaution result* in buckling on the floor and. occasionally, in 
displacement of the wall. For floors, this space should be closed by 
a light strip of hardwood loosely nailed at the top of the floor and 
carefully fitted to the wall, as this serves in case of fire to retard the 
passage of flame from one story to that above. Instead of this strip, 
or in addition to it, the masonry wall should provide a ledge, 2 
inches or more wide, just under and close to the floor plank. This 
prevents the quick spread of fire from one floor to another. 
The first floor should rest directly on the earth and should be con- 
structed as described for fire-resistive construction. "Where this en- 
tails prohibitive cost, as may be the case on a rolling site, the first 
floor may be constructed as described for other floors. In such case 
it is desirable that the timbers supporting this floor rest directly on 
masonry piers. These floor timbers should not be nearer than 18 
inches to the ground and the space under the floor should be venti- 
lated (through the exterior wall) if rapid decay is to be avoided. 1 
Openings seldom should be permitted in floors and when unusual 
conditions make such openings necessary the opening should be in- 
closed by fire walls with all wall openings fully protected by roomy 
vestibules and double fire doors automatic in operation. Such open- 
ings should be avoided by placing stairs, elevators.' hoists, and bale 
chute well removed from all wall openings and so as to connect plat- 
forms. 
ROOFING. 
Roofing should conform to the same requisites set forth for mate- 
rial used in fire-resistive construction, although there is slightly more 
latitude here inasmuch as the material should be applied with nails. 
1 Distinction between slow-burning and the usual light joisted construction has 
been drawn by fixing certain minimum requirements for sectional area and 
dimension of planking and timbers. It is generally conceded that roof planking 
should not be less than 2* inches and floor planking should not be less than 3 
inches thick, with a permissible reduction in thickness of three-eighths of an 
inch taken off in dressing. Timbers should have a minimum sectional area of 
64 square inches (equivalent to 8 inches square for columns) and should not 
be less than 6 inches in any dimension. (This does not refer to light wall 
timbers continuously supported by a ledge, these timbers usually being 4 inches 
by 8 inches.) In any event, the size of the timber should be such that charring 
to a depth of one-half inch on all exposed sides would not reduce the strength of 
•the timber more than 20 per cent and in no case to the point of failure. All 
timber and planking should be dressed on all exposed surfaces. It is desirable 
