THIRTY-FIFTH ANNUAL CONVENTION. 269 



made into almost any shape, and is durable. The concrete 

 roof of the Iowa silo constructed at the Iowa Experiment Sta- 

 tion, was made by placing a layer of expanded 

 metal known as "Hy-Rib" over the centering placed in 

 the silo and then plastering on top with two coats of cement 

 mortar amounting to about two inches. Then after this had 

 set, the centering was removed from its stumps and the steel 

 plastered on the under side. The cornice was built up with 

 a trowel. The rafters were supported by stumps extending V/ 2 

 inches over the top of the wall and were tied together by collar 

 beams so as to prevent any outward thrust on the wall. Where 

 a single roof is to be constructed, the plastering on the expanded 

 metal may be the cheaper method of construction as a large 

 amount of the forming is dispensed with. Where several roofs 

 are to be constructed and where the forms may be used several 

 times, more perfect forms and less reinforcement may be used 

 economically. It is to be noted in building a concrete roof, a 

 band of steel should be placed at the bottom of the cone to care 

 for the outward thrust of the roof walls. 



Following is the itemized cost of the roof described and as 

 erected on the Iowa Silo at Ames. In building the second roof, 

 no doubt the labor cost could be reduced very much : 



Cost of the Concrete Roof on the Iowa Silo at Ames, Iowa. 



Steel reinforcement — special expanded metal $18.00 



Centering — native lumber 5.00 



Labor in erecting centering — 20 hrs. at 20c 4.00 



Labor in putting steel in place — 75 hrs. at 20c 3.00 



Cement— 10 bbls. at $1.50 : 15.00 



Sand and gravel — 5 yds. at 50c 2.50 



Sash, hinges, etc 2.50 



Labor in putting concrete in place — 63 hrs. at 25c 12.60 



Superintendent— 3 days at $3.50 10.50 



total $64.jl0 



