BUILDING MATERIALS. 253 



in all cases be used. Zinc nails are very apt to be weakened by rust, 

 which causes the slates to fall from their place. 



6. Tiles. For roofing tiles do very well in some cases. They should 

 be made of the best clay, and moulded with great care. Under general 

 circumstances, however, they are not nearly so economical a covering 

 in roofing as slates. Once slates are properly fixed on, they remain 

 many years without requiring any outlay in repairs; but tiles very 

 soon require attending to, and cause a considerable outlay in repairs. 

 It takes eight hundred three-inch plain tiles to cover a square, and six 

 hundred four-inch of roofing, with plain and pan tiles, cost from 2 to 

 3 per square. 



Both tiles and slates are estimated by the square of one hundred super- 

 ficial feet. The tiles are generally made ten and a half inches long, six 

 and a quarter broad, and five-eighths of an inch thick. 



7. Mortar. After slaking common lime with water, it is then made 

 into a paste which is termed mortar. This is always of the best quality 

 when it is not allowed to lie too long after being made. A sufficient 

 quantity of water should be poured over the burned stones, so as to re- 

 duce them thoroughly to a fine powder. Before the powder is made into 

 a paste it is mixed with a proper quantity of sharp sand. Some varieties 

 of lime require more sand than others. There is a very injudicious 

 habit amongst masons of adding too much sand to the lime, thus weak- 

 ening it very much, and making it so that it will not adhere for any 

 time to the stones or bricks. The proportions in common use amongst 

 masons are 



Parts. 



Lime, 1 



Sand, 6 



Water, 1 



The proportion of sand necessary must be guided by the character of the 

 limestone. Some limestones contain a portion of sandy matter. Most 

 of the superior kinds of lime in the north of England are best made 

 when mixed with only four parts of sand to one of lime. The sand must 

 be of a fine quality, especially when used for fine work ; but for common 

 rubble-work a coarser sand will suffice. Common lime in a powder 

 costs about Is. per bushel; and when mixed up with hair for the purpose 

 of plastering, it costs 1, 4s. per load, and in towns Is. per hodful. 



With builders sand is classed into three kinds namely, fine, middling, 

 and coarse. It is either procured in pits, from the sea-shore, or from the 

 sides of rivers. Good pit-sand is by far the best for building purposes. 

 Sea-shore sand is seldom used, from the salt which it contains. Eiver- 

 sand is generally preferred for plastering purposes, the grains being finer 



