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Fixing the top course of double lap slates - By Chris Thomas

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Whilst it can be proven that the centre nail fixing of double lap slates is a very efficient method of fixing slates to resist the most severe wind conditions, it is impossible to centre nail the top row of slates. The inappropriate detailing of the top slate fixing is the most common problem found on a slate roof.
In previous articles we have looked at the effect wind suction has on the exposed section of the slate and how the top of the slate stops the slate rotating about the nail fixings in the middle. But with top slates, they are cut short to weather the laps in the lower slate course. Therefore the nail fixings which are the same location relative to the bottom of the slate, are now very close to the top of the top slate. This makes the slates very vulnerable to rotation, as there is little or no resistance to the wind suction.

Picture 1

Uplift rotation
The reason the slates are so vulnerable relates to the fact that the head of the lower slate rests on the same batten that the top slate is nailed too. Therefore the top slate is laying in contact with the head of the slate below and the nails are passing through the top slate, down the top edge of the lower slate before penetrating the timber batten. This means that below the top slate on the down side of the nail is the lower slate and on the up side of the nail is fresh air.

Picture 2

Therefore if the wind sucks the top slate up, the slate can rotate about the head of the lower slate/the nail fixings, until the upper part of the slate comes into contact with the top of the batten.
This situation is far from ideal, as in reality the weight of a ridge tile or top edge flashing could cause the same rotation of the top slate by their self-weight. Also the wind uplift force is likely to use the slate as a lever to lift the ridge tile up and crack the mortar bedding.
To overcome this situation, one of four details should be adopted: -
Double top batten
The double top batten detail requires a second thicker batten to be nailed against the head of the top batten. The top slate is cut some 50-75mm above the nail holes and nailed into the smaller top batten. The head of the slate rests on the larger second batten, preventing the slate from rotating. The additional height of the second batten should be equal or slightly greater than the nominal thickness of the top course of slates. .

Picture 3

It is common to see a 32x25mm tile batten turned on edge for the second batten, or a standard batten with an additional 6mm lath nailed to it, with a 6mm thick natural slate. If the second batten is smaller than the combined thickness of the batten plus the slate thickness, the top slate will still be able to rotate
Shouldered slates
The shouldered slate detail involves setting the top batten 25-30mm lower than the normal batten gauge, such that the second course of slates down from the ridge totally covers the batten. The top course of the slates should be shouldered to expose the full depth of the battens. This is to allow the top course of slates to be nailed to the top batten with two nails closer to the middle, without compromising the head of the lower course of slates resting on the top batten.

Picture 4

The top of the top slates in now resting fully on the top of the lower course of slates and restricted from rotating.
Slate hooks
The slate hook detail involves using slate hooks nailed into the top batten as well as double nailing the top slates. To ensure that the slate hooks sit correctly and do not shunt the side laps of the lower course of slates, one edge of each slate should be trimmed back by the thickness of, and for the length of, the slate hook.

Picture 5

Tail rivets
The use of copper tail rivets requires a rivet hole to be punched in the leading edge of the slate 50mm up and coinciding with the joint between the lower slates. Having located the copper disc rivet in the lower joint and through the hole in the top slate. The top slate can be nailed in the normal way and the rivet should be folded to hold the leading edge of the top slate down.

Picture 6

Of the three details the double batten detail is the simplest and most secure, but does require additional timber battens, or a suitable sized packing lath. The shouldered slate detail requires cutting of the top corners of the lower slates and fresh nail holes in the top slate closer to the centre of the slate. This detail requires less material and more site work. The slate hook fixing detail is the least secure and the hooks will be visible. This detail will require additional slate hooks and trimming of the lower slates. However slate hooks can be very affective if the slates are less rigid, as with some imported slate, reclaimed slate or some resin slates. The use of tail rivets for the top slates is a common detail with Fibre Cement slates, where all the slates are held down with tail rivets, making their visibility less of an issue.

Depending upon the double lap slate material used, the choice is yours.

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