44 BULLETIN 801, U. S. DEPARTMENT OF AGRICULTURE. 
Walls longer than 100 feet and not tied by cross Trails should be 
braced at intervals of 100 feet by buttresses similar to those in Plate 
XXIII, figure 2. The top of the wall should extend above the roof 
to form a parapet 3 feet high if there is an exposure ; this arrange- 
ment is shown in the photograph just referred to. If there is no se- 
rious exposure, the wall may terminate as a parapet only 18 inches 
above the roof, or the wall may be finished with a corbel support- 
ing a metal gutter as shown in Plate XXIII, figure 2. In the latter 
case it is important that the division fire wall provide the corbeled 
wing projection shown. It is preferable that there be no overhang- 
ing cornice as this is a source of fire hazard. 
Walls not terminating with a parapet should have suitable 
wrought iron timber anchors built in and secured to the framing, 
in order to resist the lifting action of wind. The size and design of 
the anchor depends on the arrangement of framing. 
Openings in the wall should conform to the same size and arrange- 
ment as stated for fire-resistive construction. Doors may be of thick 
and substantial wooden construction if there is no serious exposure. 
Windows should be metal glazed with wire-glass. 1 
FRAMING. 
The usual arrangement of roof and floor timbers is such that the 
end of the timber is supported by the division wall. The end of any 
timber supported by a wall should rest freely upon a ledge, such as 
an offset or corbel, and not be built solidly into the wall. The free 
support permits the timber to fall without damage to the wall as 
previously illustrated. This is one of the most important details 
of slow-burning construction, and failure to observe this requirement 
leads to serious damage to the building and also rapid spread of fire. 
The drawing, Plate XXYT, shows such construction in figure 3 where 
the floor timbers are supported by corbels and offsets or by pilasters 
and the roof timbers are supported by corbels. These timber-ends 
often may rest directly upon the brickwork provided sufficient bear- 
ing area is supplied for the comparatively light load generally sus- 
tained. Timbers parallel to and supported continuously by the wall, 
1 A condition which requires that doors be constructed as a fire door is occa- 
sioned by the use of wood platforms. This protection should also be used in 
the case of doors in one story placed under those above and not separated by 
a reinforced con< .form. Wooden doors are used only where no severe 
exterior exposir o anticipated. A wooden door is satisfactory if all of the 
provisions here ted are fulfilled, but the fire-resistive door is much better. 
In any event, -3 should fit close to the wall and threshold. If the exterior 
exposure is very severe, the wall and all openings should be constructed as in 
a fire wall. 
