Thin slabs of slate are perhaps the most common form of roof covering. They are cheap, easy to lay, and of good appearance. Slates may be obtained in practically any sizes, those best known being exhibited in the following table: -

Names.

Sizes.

Slates nailed 1 inch from Head. Lap,

3 inches.

Slates nailed near Centre. Lap, 3 ins.

Number per

Square.

Weight per

Square.

Nails required per Square.

Gauge.

Length of Doubling Course.

Gauge.

Length of Doubling Course.

Iron.

Copper.

Inches.

Inches.

Inches.

Inches.

Inches.

Cwts.

No.

Lbs.

Smalls ................

12

x

6

4

8

4 1/2

7 1/2

530

6 1/2

1060

6 1/2

Singles ...............

12

x

8

4

8

4 1/2

7 1/2

400

6

800

5

Doubles ..............

13

x

6

4 1/2

8 1/2

5

8

480

6

960

6

Ladies ................

l6

x

8

6

10

6 1/2

9 1/2

266

5 1/2

523

3 1/2

Viscountesses ...........

l8

x

10

7

11

7 1/2

10 1/2

192

6 1/2

384

2 3/4

Countesses..............

20

x

10

8

12

8 1/2

11 1/2

170

5 2/3

340

4

Marchionesses .........

22

x

11

9

13

9 1/2

12 1/2

138

5 3/4

276

3 1/4

Duchesses ............

24

x

12

10

14

10 1/2

13 1/2

115

5 3/4

230

3

Princesses .............

24

x

14

10

14

10 1/2

13 1/2

98

5 2/3

196

3

Empresses .............

26

x

16

11

15

11 1/2

14 1/2

79

6 1/4

158

3 1/2

Imperials ..............

30

x

24

..

..

13 1/2

16 1/2

36

8

72

3

Rags ..................

36

x

24

..

..

16 1/2

19 1/2

25

9

50

3 1/2

Queens ...............

36

x

24

..

..

16 1/2

19 1/2

25

9

50

3 1/2

Slates are also classified as "First," "Seconds," and "Thirds," according to their freedom from flaws and evenness in colour and thickness.

Before the slates can be laid, battens or boards must be fixed to the rafters to form a suitable nail-hold. The least expensive method of slating is to fix the slates to battens, usually about 2 by 1 inch, nailed horizontally across the rafters and spaced at suitable distance, according to the size of the slates.

Battens permit the passage of currents of air, which, if undesirable, may be prevented by the use of close boarding. When rooms are formed in a roof they are frequently unbearably hot in summer and excessively cold in winter, and it becomes essential to construct the roof in such a manner that they may be insulated as much as possible from external heat and cold. The simplest way to do this is to nail close boarding upon the rafters and cover it with sheets of Willesden paper, asphalted felt, or other insulating material. When slates are fixed in this manner the insulating materials are liable to decay, owing to the imperfect ventilation of the intervening space.

Proper ventilation may be provided by nailing horizontal battens on top of the insulating sheets. When a roof is in a comparatively inaccessible position, any slates that may happen to get broken are not repaired, and allow the rain water to lodge on top of the horizontal battens, where it causes decay both in the battens and in the insulating sheeting, so that finally the water penetrates to the ceilings below the roof. When ceilings have decorative paintings upon them it is a most important matter to exclude all moisture, and to do this battens are first nailed in the same direction of the rafters, and a second series of battens is nailed horizontally across them.

Fixing Slates

When slates are fixed on a roof the following technical terms are used in connection therewith: -

Head

The upper edge of a slate.

Tail

The lower edge of a slate.

Lap

The distance by which the tail of one slate overlaps the head of the slate in next course but one below (see Fig. 288).

Gauge

The distance apart of the nails which secure the slates to the boards or battens.

Margin

That part of each course of slates which is exposed to view.

Back

The upper surface of a slate.

Bed

The under surface of a slate.

Slates And Slating 553

Slates are fixed by means of nails, which may be inserted near either the heads or the centres of the slates, two nails being used for each slate.

Slates Nailed Near The Head

Before fixing slates the lap must be assumed, and the gauge must always be determined therefrom for the particular size of slate used.

The lap found most convenient in practice varies from 2 1/2 to 4 inches, but a 3-inch lap is most usual.

The following rule is used for determining the gauge of slating: -

Gauge = Length of slate - (lap + 1 inch) / 2.

The reason for this rule is apparent from Fig. 288, where "Ladies" have been used, so that the gauge is =16-(3+1) / 2 = 6 inches. It should be noticed that 1 inch is allowed above the nail-hole, which is not considered in the lap.

Slates And Slating 554

Fig. 288.

The gauge for various sizes of slates laid to a 3-inch lap is given in the Table at the beginning of this Chapter.

When battens are used they must, of course, be spaced centre to centre, to the gauge of the slate.

Slates Nailed Near The Centre

In this case the gauge is determined according to the following rule: -

Gauge = Length of slate - lap / 2.

The reason for this rule is apparent from Fig. 289, the margin being equal to the gauge in every case.

The slater usually calculates the position of the holes by adding the lap to the gauge, allowing 1/2 inch more for irregularities, and measuring from the tail of the slates thus: -

The distance of nail-hole from tail of slate = gauge + lap + 1/2 inch. The method of nailing slates near the centre is distinctly the better of the two methods of nailing, as it renders the slates less liable to strip in a high wind; while it is a trifle less expensive owing to the allowance of 1 inch over and above the usual lap in the case of slates nailed near the head. It is sometimes urged in favour of the method of nailing near the head that two courses of slates always cover each nail-hole, a statement which seems to infer that this method is more waterproof than the method of nailing near the centre; but no particular degree of leakiness is noticeable with the last-named method. In either method of nailing the holes should not be farther than 1 1/4 inch from the long side of the slate.

Slates And Slating 555

Fig. 289.

Torching-

When slates are laid on battens only the under sides of the slates are usually torched - that is, pointed with hair mortar composed of three parts of sand to one part of lime, with 9 lbs. of hair thoroughly mixed with every cubic yard.

Close And Open Slating

It is usual to lay the slates so that the longer edges are close against one another, as in Fig. 290, but in very sheltered situations considerable economy may be effected by spacing the slates in each course from 2 to 3 inches apart, as shown in Fig. 291. In this method of slating it is preferable to nail near the centres, so as to render them less likely to strip off in a high wind.