Slates must be so laid that wind and rain cannot pass directly l>etween them. To attain this object, the slates in alternate courses must be laid to break joint as shown in figs. 78 and 83, and the lower part or tail of the slates in any course above the two lowest must overlap the head of the course next but one below it. This overlapping is clearly shown in figs. 79 and 83, and is called the lap" to which the slates are laid. For steep roofs, and for low-pitched roofs where parsimony prevails, the lap is often no more than 2 inches; indeed jerry -builders reduce it to 1 or l inches. For good work, however, the lap should be 3 or 3 inches, and in exposed situations a 4-inch lap is often necemary.

Table IV. Elterwater Green Slates

Quality.

Price par Ton.

Computed to Cover

L

Best slate, 36 m. to 20 in. long, proportional widths, ...

Best slate, 20 in. to 12 in. long, proportional widths, ...

95/-

Sq. Yard 25 to 30

90/-

c

Second slate, 30 in. to 12 in. long, various widths, .. (Recommended for colour, which is equal to best).

65/-

11 to 25

P

Best Peggies, 14 in. to 7 in. long, ........

(Equal to first quality in texture and colour).

70/-

25 to 30

P

Second Peggies, 12 in. to 5 in. long, .. .. .. ...

40-

22 to 28

T

Thirds, 30 in. to 12 in. long, various widths,

(A strong slate suitable for mill work, farm out-buildings Arc).

40-

18 to 20

Before the slates are laid they must be holed for nailing. Each slate, unless it is of very small size, must have two holes, which may be either in the upper eorners of the slate as shown at a a, or near the middle of the sides as shown at bb. Objection is sometimes taken to the latter position on the grounds that moisture may be driven through the nail-holes, and that the nails may be injured by the nioi-ture. Neither ground of objection has much importance, and as the slates are certainly more securely held when nailed in the middle, and less liable to be broken and stripped by the wind, side-holing is becoming more common. The exact distance of the holes from the tail (or lower edge) of the slate is importaut. Slaters are apt to take it as the measure of the lap; thus, if slates 20 inches long are specified to be laid to a lap of 3 inches, the slater will, unless especially cautioned, drill the holes 11 inches from the tail, leaving 8j inches from the holes to the head; this, the slaters say, gives a lap of 3 inches, and a glance at the shaded slate and the dimensions in Fig. 83 might seem to bear out their view. Two little details, however, are ignored by the slater, the first being that slates vary a little in size, and the second that nails must be entirely above- the head of the slate below, and not exactly on the line of the head. These two little details are really of great importance, for, in order to make sure that the nails in one course will be quite clear of the heads of the longest slates in the course below, the slater allows from to of an inch from the head of the average slate to the holes in the slate above. I have known 20-inch slates holed as described, and yet laid with a lap of less than 2 indies, a lap quite insufficient. to render a low-pitched roof water-tight in an exposed situation, unless the slates are laid on boards and felt. Slates should therefore always be holed to show at least one inch more lap than is specified. The distance of the nail-holes from the tail of the slate may be found by the following simple rule, the several dimensions being stated in inches: -

Fig. 83  Plan of Slated Roof.

Fig. 83 -Plan of Slated Roof.

Length of slate + lap + 1 / 2 = distance of nail-holes from tail.

Slates are laid either on battens or on boarding; sometimes on battens nailed to boarding. For common work, battens alone (or, as they are often called, "laths") are used, as they are least costly. The laths are nailed to the rafters, and if these are not more than 12 or 13 inches apart, the laths need not be more than about 1 inch x inch or 1 inch x 7/8 inch. The laths must be spaced according to the size of the slates and the required lap: -

Length of slates - lap / 2 = distance of laths from center to centre

Thus, for slates 20 inches long, specified to be laid to a lap of 3 inches, the distance from centre to centre of the laths must be

20-3 / 2= 8 inches.

Slates laid on laths must he pointed or torched underneath with haired mortar.

The house, however, will be drier and of more uniform temperature if boarding covered with felt or waterproof paper be used instead of laths. The boards may be or 1 inch thick, tongued and grooved to prevent warping, and nailed to the rafters either horizontally or diagonally. As snow and rain will drive through the joints of the slates, it is necessary to cover the boards with some impervious material which will protect them from the moisture and from consequent decay. The material may be either Willesden waterproof paper (one-ply or two-ply), or some kind of felt. Felt is more frequently used. of this material there are two principal varieties - tarred felts (which are usually known as "sarking" or "roofing" felts), and felts prepared with resin instead of tar, and known as "inodorous" felts. The tarred felts are tougher and more durable, and leas liable to injury by vermin, and the smell is scarcely perceptible when the roof is complete. The thicker qualities of each kind should be selected. Felt is supplied in rolls usually 32 inches wide, and is laid in horizontal courses, each new course overlapping that below it about 1 or 2 inches, all being secured to the boarding with nails.

Sometimes the slates are nailed directly to the sheathing, or to horizontal laths nailed to it, but a better plan consists in nailing thereon laths or battens running from eaves to ridge either directly or diagonally, and in nailing to these the usual horizontal slating laths. The space thus formed between the sheathiug and the slates keeps the roof warmer, and helps to preserve the wood and slates from decay. The slater proceeds by nailing a tilting-fillet along the eaves, as shown at D in figs. 78 and 79. This is necessary, in order to give the slates that slight variation from the slope of the roof which is required to allow them to bed close to each other throughout their length. If the tilting-screed or its equivalent were omitted, the tail of each slate would stand clear of the slate below, and the wind and rain would find entrance, and might, indeed, strip off the slates.