Sand is usually sold by the load, which varies in different localities from 18 to 27 cubic feet.

The volume of the mortar when mixed is generally about equal to that of the sand before screening.

Improved Wall Plasters.

Owing to the difficulty of obtaining an economical and satisfactory quality of walls and ceilings by the use of the ordinary lime mortar, other and more reliable plastering materials have been invented, and are now being extensively employed, especially on the largest and most costly structures, and are giving general satisfaction.

Among the best known of these improved plasters are the Acme and Climax cement plasters, Adamant, Windsor cement dry plaster, and Rock wall plaster. The Acme and Climax cements are natural products found in certain parts of Kansas and Texas, and simply calcined. The others are composed principally of plaster of Paris with certain chemicals added. All appear to produce about the same results. The Windsor dry plaster, Adamant, and Rock plaster are mixed with the proper proportion of sand by the manufacturers, and only require being "wet up" before using. All of these materials are sold by weight. They should be used strictly in accordance with the directions furnished by the manufacturers. Among the advantages gained by the use of these plasters are : Uniformity in strength and quality; extra hardness and toughness; freedom from pitting; saving in time required in making and drying the plaster; minimum danger from frost; less weight and moisture in the building; and greater resistance to the action of fire and water.

The pitch of a slated roof should be about one in height to four in length. The usual lap is about three inches, but it is sometimes four inches. Each slate should be fastened by two 4d or 3d slate-nails, either of galvanized iron, copper, or zinc. On roofs of gas-houses the nails should be of copper or yellow-metal.

A square of slate is one hundred superficial feet, allowances being made for the trouble of cutting the slates at the hips, eaves, round chimneys, etc. The sides and bottom edges of the slates should be trimmed, and the nail-holes punched as near the head as possible. They should be sorted in sizes, when they are not all of one size, and the smallest placed near the ridge. The thickness of slates varies from three-sixteenths to five-sixteenths of an inch, and their weight from 2.6 to 4.53 pounds per square foot.

Elastic Cement.-In first-class work, the top course of slate on ridge, and the slate for two to four feet from all gutters, and one foot each way from all valleys and hips, should be bedded in elastic ement.

Roofing-Paper. — Roof-boards should be covered with one or two thicknesses of tarred felt roofing-paper, before the slate are laid. No dry or rosin-sized felt should be used on roofs.

Flashings. By "flashings" are meant pieces of tin, zinc, or copper, laid over slate, and up against walls, chimneys, copings,

etc.

Counter-flashings are of lead or zinc, and are laid between the courses in brick, and turned down over the flashings. In flashing against stone-work, grooves or reglets often have to be cut to receive the counter-flashings.

Close and Open Valleys.— A close valley is where the slate are mitred and flashed in each course, and laid in cement. In such valleys no metal can be seen. Close valleys should only be used for pitches above forty-five degrees.

An open valley is where the valley is formed of sheets of copper or zinc fifteen or sixteen inches wide, and the slate laid over these.

Rule for computing the Number of Slates in a

Square.

Subtract three inches, or the amount of head-cover, from the length of the slate, multiply the remainder by the width, and divide by two. This will give the number of square inches covered per slate; divide 14,400 (the number of square inches in a square) by the number so found, and the result will be the number of slates required.

The following table gives the number of slates per square for the usual sizes, allowing three inches for head-cover: —

NUMBER OF SLATES PER SQUARE.

The weight of slate per cubic foot is about 174 pounds, or, per square foot of various thicknesses, as follows:

The weight of slating laid per square foot of surface covered will, of course, depend on the size used. The weight of 10 by 18 state, three-sixteenths of an inch thick, for example, per square foot of roof, would be 5.86 pounds.

An experienced roofer will lay, on an average, two squares of slate in ten hours.

Ordinary roofing-paper weighs about fifteen pounds per square, and averages about fifty pounds in a roll.

At the present time [1884] the additional cost of laying slate in clastic cement varies from thirteen to fifteen per cent.

Comparative Cost of Different Sizes of RoofingSlate.

The following table shows the prices for No. 1 Monson (Maine) roofing-slates delivered on wharf in Boston, May 20, 1885. It will be seen that the medium sizes, such as 16 × 10, 16 × 8, 18 × 10, cost the most; and, as the sizes increase or diminish from these, the price decreases. The price of Brownville (Maine) slates are in all cases $1 per square more than the Monson slates.

The price of Bangor (Pennsylvania) slates in Boston, at the same date, is very nearly the same as for Monson slates, except for 16 x 8's, which are $1 a square less.

Red slates cost from $12 to $12.50 per square.

The average width of a shingle is four inches: hence, when shingles are laid four inches to the weather, each shingle averages sixteen square inches, and 900 are required for a square of roofing. If 4 inches to the weather, 800 will cover a square.

This is for common gable-roofs. In hip-roofs, where the shingles are cut more or less to fit the roof, add five per cent to above figures. A carpenter will carry up and lay on the roof from fifteen hundred to two thousand shingles per day, or two squares to two squares and a half of plain gable-roofing.

One thousand shingles laid four inches to the weather will require five pounds of shingle-nails to fasten them on. Six pounds of fourpenny nails will lay one thousand split pine shingles.