290 FLOORS 



of dry gravel or sand, has been used on the Ashland division of 

 the C. & N. W. R. R., and cost about 4 cents per square foot exclusive 

 of the cost of the old timbers. This floor has proved to be quite satis- 

 factory. 



FLOORS ABOVE GROUND. The type of floor used for the 

 upper stories of mill buildings will depend upon the character of the 

 structure and the use to which the floor is to be put. In fireproof build- 

 ings the floors should preferably be constructed of fireproof materials, 

 although there is comparatively little risk from fire under ordinary 

 conditions with a heavy plank floor. Where the load on the floor is 

 very heavy some form of trough or buckled plate floor is very often 

 used. 



Timber Upper Floors. Where steel floor beams are used the 

 floor is often made by placing 2" x 6" or 2" x 8" planks on edge and 

 spiking them together, the wearing surface being made of hard wood 

 boards. Where there is much danger from fire this floor can be fire- 

 proofed by plastering it below with wire lath and hard plaster and by 

 putting a layer of cement or lime mortar between the plank and the 

 wearing surface. The upper surface is also sometimes finished with a 

 wearing coat of cement or asphalt. 



The standard floor recommended by the Boston Manufacturer's 

 Mutual Fire Insurance Co., for mill buildings constructed of heavy 

 timbers, calls for a layer of spruce plank, generally 3 inches thick, laid 

 to cover two floor beam spaces and breaking joints every 3 feet ; on this 

 are laid 3 thicknesses of rosin sized paper, each layer being mopped 

 with tar. The top floor is ij/s-in. hard wood, preferably maple. The 

 main beams are spaced 8 to 10 feet. "The floor is smoother if laid 

 across the line of the plank in the under floor, but traveling loads are 

 better distributed when moved in and out of the store house if the top 

 floor is laid parallel to the lower plank." 



Brick Arch Floor. The brick arch floor shown in Fig. 140 was 

 formerly much used in fireproof buildings and is still used to some ex- 

 tent in mills and factories. The arch is commonly made of a single 



