Fibre-cement slates can normally be laid on roof pitches as low as 20 degrees (check conditions with individual manufacturers) right up to vertical cladding. The two most commonly used sizes in Britain are 600mm × 300mm and 500mm × 250mm, laid to a 100mm headlap. The gauge for fibre-cement slates is normally given in the manufacturer’s information (in this case, 250mm and 200mm, respectively), but, if you need to work it out, the same formula is applied as for natural slates and plain tiles (that is, length – lap, divided by 2). It is important to establish the amount of headlap the slates have been holed for because this affects the batten gauge. Nails should be 30mm copper clout, unless otherwise specified. Disc rivets are also copper and are usually supplied in boxes of 1,000.

It is important to remember that the minimum batten size for fibre-cement slates is 50mm × 25mm.

Ventilating the Batten Cavity

Because of their tight-fitting nature, some artificial slates are deemed impermeable, which means they offer little or no air through them. As such, in most specifications it is recommended that the batten cavity is ventilated using a counter batten not less than 25mm deep. This can be above or below the underlay, depending on the specification.

Storing and Loading out

A chemical reaction known as efflorescence, causing white staining, can occur if the slates get wet when stacked tightly together. The main cause of this on site is through condensation in the packs, which are invariably shrink-wrapped in plastic. It is good practice to restack the slates in such a way that plenty of air can circulate around them and then to cover them with a tarpaulin. If the slates are to be used within a short time, then making a series of slits in the plastic wrapping may be acceptable. With this problem in mind, it is preferable to plan deliveries in stages to coincide with the use of the slates.

Never load out more than you expect to fix in a day, especially if high winds or snow are expected. Most right-handed slaters like to have the slates stacked with the undersides of the slates facing them and the nail holes to the left so that the slates can be picked up and placed more quickly.

SETTING OUT

Batten Positions at the Eaves

There will need to be three thicknesses of slate at the eaves, and all the components that make this detail up should be available before the fixed points at the eaves can be established. The three thicknesses consist of the normal under-eaves and the first course arrangement found in natural slating, plus an additional ‘dummy’ eaves course. The under-eaves and dummy slates are cut from full slates. For 600mm × 300mm slates, the slates are cut across at gauge plus lap (for instance, 250mm + 100mm = 350mm) to provide an off-cut of 250mm for the dummy. The sole job of the dummy eaves is to provide support for the first line of rivets, which would simply drop out were it not there.

The positioning of the battens for the first course, under-eaves and dummy eaves will depend on the size of the slates and the specified headlap. Generally, the following rules apply:

Fibre-cement slates are extremely fragile if not fully supported, so there should be little or no kick on the eaves and sprockets should be avoided. Lifting the tails up above the natural line of the slating causes weak points and a loss of support for the disc rivets in the lower courses. If you cannot reduce the height of the fascia board then the under-eaves battens should be increased in height to provide more lift and support to the affected area (see the picture on page 121).

General Battening and Slate Positioning

It is important that the batten gauges are correct and fixed to a struck line. Irregular gauges may cause loss of headlap as well as fixing problems, such as holes being in the wrong place and heads gradually falling off the support batten. Deviations, which are often associated with not using a line, can be transferred to the tail alignment of the slates, if the top edge is used as a parallel reference. Striking a line along the battens (halfway up) is a good way to keep the heads and therefore the tails of the slates straight and can speed up the fixing process, especially for beginners and inexperienced roofers.

Setting Out across the Roof (Striking Perp Lines)

Accurate positioning of the slates is a vital part of fibre-cement slating so correct and careful setting out is important. When marking across the roof, slate widths plus a 3 to 5mm gap should be used. The wider of the two will give you a little more room to slide the rivets up and down and tends to make the slating a little easier, in my opinion. If your tape has imperial measurements on it, a convenient cheat is to mark 300mm slates out at 12in intervals, with the half slate at 6in. This makes setting out very easy because you start at either verge with a full slate (for instance, 50mm over the verge, mark off 300mm), and from there mark 6, 12, 18, 24 and so on, on the imperial scale. This gives a consistent gap a shade under 5mm, which is perfect.

Striking perpendicular lines for the slates.

Transfer the marks to a marking batten, if you are comfortable doing that. With this particular material, my preference is to strike every slate, but you can do every third, if you prefer. Some roofers do not see the need to strike these slates at all, but it is normally beneficial to have at least some reference points. It is all down to personal preference and, provided the slating is straight and comes in properly at the verges, there is no hard and fast rule.

It is always better to try the roof through to see whether you can adjust your verge overhangs to fit the slates, but, if you cannot adjust the overhangs, all you need to do is to trim to width the slates one back from the finishing verge. Once you have decided on your overhang, mark the full and the half slates (or slates-and-a-half ) widths on the starting verge and then apply the marks (slate widths + 3 to 5mm) to the bottom and the top course. Then strike lines between the corresponding points to ensure correct alignment.

CUTTING TECHNIQUES

Under-eaves slates, halves, slates-and-a-half and tops will all have to be cut unless the manufacturer can offer them preformed. It is highly desirable if you can get them, but check what premium (if any) you are paying for this service.

Slates can be cut by scribing and snapping over a straight edge or by ‘nibbling’ through with hand-held cutters.

MORTAR WORK

Mortar does not usually adhere well to fibre-cement slates, so wherever possible, dry fix should be the preferred option. However, if mortar work is required then it is recommended that a bonding agent such as a PVA liquid adhesive (readily available from all DIY stores) is added to the mix.

It is also advisable to apply the bonding agent to the surfaces of the slates (and ridge tiles if applicable), and to ensure this is set before bedding takes place. I have also seen wire mess glued or nailed to the slates to provide a key for mortar in some cases.

FIXING DETAILS

Basic Laying Procedure

All standard width slates should be twice nailed and riveted once at the tail. The head of the slate should sit halfway on to the battens. The nail holes should sit above the head of the course below by about 10mm, to provide a continuous fixing point just above the centre of the battens.

Fibre-cement slates are laid thus:

  1. Insert rivets between slates (below the nails) during fixing.
  2. Hook the slate above over the rivet.
  3. Knock the rivet down.
  4. Move the slate into position and nail it.

Accurate and consistent positioning of the rivet is important for speed of fixing. Ideally, it should be set down from the head of the slate it rests on at a distance of 25mm less than the headlap (for example, 75mm down for a 100mm headlap). If you can achieve this consistently, then the slates will be at the correct height each time and so any adjustments should be minor. It is now possible to find specially designed rivet guides, which set the rivet at the perfect height every time and, although I have not used one myself, there is evidence to suggest that they can save on fixing times, especially on larger roofs.

Eaves

Begin by nailing the dummy eaves in place. You may need to re-hole these to ensure a secure fixing because the existing nail holes are quite near the top of the slate and are inclined to break off now and again. Fix the under-eaves to the marks, placing a rivet between each one at approximately 25mm up from the tail. Note how the nail holes in the under-eaves are set down from the head by 100mm (that is, a distance equal to the headlap). This prevents the tail of the slate from tipping up and putting downwards pressure on the tail of the slate and therefore on the rivets.

Verges

Wet-fix verges are constructed in the standard way by nailing or bedding undercloak to the wall or bargeboard at the desired overhang (preferably 38 to 50mm), and then bedding and pointing the slate on to it. The slates may actually be cut in half lengthways and used as undercloak (laid the good side down) to form an effective finish.

Verges are normally formed by using standard width slates and slates-and-a-half on every other course. New nail holes and rivet holes will have to be formed in the verge slates. Holes should be formed using a 3mm masonry drill bit to the positions shown (rivet holes in solid black).

If you cannot get slates-and-a-half then you may need to buy double slates and cut them down to size. Where this applies, the cut edge always goes on the inside. The picture (above) shows where the two main nail holes need to be positioned, but an additional one will also have to be formed (in situ) because all slates-and-a-half should be fixed with three nails. Normally, the best place for the third hole is about 100mm in from the verge.

The standard procedure for installing a standard verge detail is:

  1. The verge normally starts with half an under-eave or a slate-and-a-half under-eave cut down to size; remember to insert the rivet.
  2. Next, locate the first verge slate. Position a rivet in the hole just below the nail holes and hold it between your fingers to prevent it from falling out while you hook the slate over the rivet at the eaves.
  3. Once the slate in is place, knock the bottom rivet down and nail the slate, insert another rivet about 75mm below the nail hole and lay two or three more slates to prepare for the next course.
  4. From here it is just a case of fixing slates and slates-and-a-half in alternate courses and slating away from the verge as necessary; the important thing to remember is to install all the rivets in the right holes, although drilling all the holes first does help in this respect because what you need to do becomes much more obvious.

Slates-and-a-half or doubles are expensive, so some jobs are done with half slates on the verges. If you score and snap full slates exactly down the middle you will find that you can use both halves at the verges (cut edges to the inside). Bear in mind that most manufacturers recommend slates-and-a-half so, if you choose to go with half slates, you are advised to check with the manufacturer to see how this may or may not affect your warranty.

Short Courses

At the Ridge and Top Edges

Because these types of slate are fixed gauge, short courses are common and usually unavoidable. The last full course should be trimmed halfway on to the top batten and a topper course cut to maintain the slate margins and provide sufficient cover for the ridge tiles.

At Eaves

Depending on regional practice or personal preference, the short course can also be positioned at the eaves. To form a short course in this way, the first fixed points at the eaves are set as normal, but the rest of the gauges are set down from the top edge. This results in a reduced gauge between the top edge of the first full course and the second course. The short course is then created by allowing the slate on the second course to overlap the one below by an increased headlap and is fixed by repositioning the nail holes. The benefits of this are that there is no cutting (apart from a standard topper) and the short course is arguably less obtrusive than at the top edge.

Abutments

Slates should be cut as closely as possible to the sides of chimneys and other abutments to provide as much lap and support as possible to the flashings. The cut edge of the slate should always be placed next to the wall and slates-and-a-half or doubles used to avoid small pieces where necessary. The usual flashing detail is a step flashing over soakers, the same as for natural slates and plain tiles.

Ridges

Proprietary fibre-cement ridge coverings are fixed dry on to the slates with wood screws and made watertight with sealing washers (both supplied with the ridge tiles) and mastic. The ridge tiles lap over one another like a cap ridge and are sealed between the joints with a bead of mastic. The fixings are then located into the battens through site-drilled holes (usually one on each side of the cap and one on each side at mid-distance).

Hips

Hips are finished in the same way as the ridge tiles but with a shallower unit designed to minimize the sweep at the three-way intersection. The intersection should be covered with a lead saddle, which can be held in place by passing through the corresponding drive screws.

Open Valleys

Fibre-cement slates should never be cut to a point in the valleys, and doubles may be used to avoid this problem. The double slates are supplied blank without nail holes or rivets to give maximum flexibility in a variety of situations. This does mean, of course, that there will be some holing for nails and rivets to be done. Positioning new nail holes is fairly straightforward. At least two nail holes should be made at gauge + headlap + 10mm (as measured up from the tail of the slate), with special consideration being given to the hole nearest the valley to ensure that it is over a secure fixing point.

Accurately positioning new rivet holes is rather trickier. You will need one rivet hole near the tail of each cut slate (normally 25mm up, and half the width of the tail across) and a second one at gauge + 25mm up, and half the distance of the tail width across for the course above.

If all this appears complicated do not worry, there is no need to be too obsessed with perfect positioning. The important thing is to make sure that there is a rivet hole in approximately the right place for the tail of the next course, and that you insert a rivet into it before you nail the slate down. The next hole can then be lined up with the rivet in both directions as shown.

Check after each course that each tail is secured with a rivet and that there are at least two nails in each slate.