This chapter describes how common materials used on slated and tiled roofs should be fixed. Any slight variations between the different materials will be dealt with in their specific chapters.

UNDERLAY (FELT)

General

It is important to understand that modern underlays are proprietary products, so the fixing instructions should be checked with the manufacturer’s information. However, in general, all underlay should be laid with a slight sag (nominally 10mm) to allow any water trapped in the roof to drain away to the eaves. It is important not to over-tighten the underlay as this can lead to water tracking across the battens and into the roof via nail holes. Likewise, the underlay should not be laid baggy as this can lead to excess flapping in high winds, and the ballooning effect can even lift tiles off the roof in some cases.

Although staples have become a popular way of fixing modern underlays, I would always recommend using large-headed galvanized clout nails, 19–25mm in length.

Eaves

As indicated previously, most slating and tiling underlays are not designed to be exposed to the elements and so it is important that felt-support trays (sidelap normally 150mm) are used to conduct water into the gutter, with the underlay set back out of harm’s way. Another popular method of treating the eaves is to use a membrane that has been designed for exposure, such as Type 5U (a flat roofing felt). When using 5U, it is not necessary to use a full 1m roll along the eaves. A strip of 330–500mm is normally quite sufficient and this can be cut on site from a full roll (i.e. into half or into thirds) or pre-cut rolls can be found at many roofing merchants. The leading edge of the 5U should extend approximately halfway into the gutter. Again, the leading edge of the underlay can then be set back. Where possible, the first strip of underlay across the roof should be all in one piece (that is, not jointed). The eaves are an area where water can gather if there are defects in the roof (such as broken or missing tiles), so avoid nailing in the bottom 200mm or so.

Felt-support trays at eaves with underlay set back.

To help in getting the underlay straight and level when rolling out, fix a small cluster of nails about halfway down the edge of the roll to act as a pivot, rather than use a line of fixings. Roll the underlay out across several rafters before straightening it and adding additional fixings.

Joints

When one roll stops part-way across the roof there are several ways of forming a joint to carry on the process. The important point is that no underlay should be left hanging about unsupported and that there is sufficient lap at the sides. To form a watertight joint, start by trimming off or folding back the surplus underlay and then start the next roll one full rafter back.

Horizontal Laps

The horizontal lap from one strip of underlay to the next depends on the pitch of the roof and on whether or not the roof is boarded, but the standard minimal headlap is normally 150mm. Most modern underlays actually come with the lap line already marked on, but, if this is not the case, it is a good idea to strike horizontal lines to maintain a consistent lap and to help in keeping the underlay straight.

At the Ridge

Much depends on the specification and the type of underlay used in determining how it finishes at the ridge. Most ventilated ridge details require the underlay to be set down at about 30 to 50mm on each side from the ridge to leave a clear air gap, while in standard, non-ventilated ridge details the common practice is to position one side level with the ridge and carry the other over the top by the same minimal lap (for example, 150mm) as has been used for the rest of the roof.

At the Hips

The hips can be finished in much the same way as the ridge, with a minimal 150mm lap, but this time being measured over the hip rafter rather than the ridge line. Alternatively, both sides of the underlay may be trimmed on to the hip rafter, fixed in place and then covered by a continuous strip (minimum 600mm wide).

At the Valley

For GRP valley liners the most common method is to fix a continuous, 1m strip of underlay down the valley, to give an even 500mm on each side. To roll underlay down a valley can be difficult, as well as slightly hazardous, and so to make the task safer and simpler measure the length you need (measure the middle of the valley and add about 3m) and cut it on the floor or scaffold. Then fold and crease it down the centre; this makes positioning and fixing much easier, especially when you are working in twos.

Technically speaking, all underlay should be cut back from exposure, but doing this at the base of the valley may leave it vulnerable to leaks. Thus in this case it is better to allow the continuous strip of underlay to come right down to extend over the fascia board and be trimmed halfway into the gutter.

The adjacent underlay is then cut or folded to the sides of the valley liner (once positioned) or, if specified, lapped on to it.

If the valley is lead-lined there should be no underlay at all beneath the lead. This is because the underlay could actually melt when the lead gets hot, thus trapping any water that gets in. The underlay should be lapped on to the sides of the lead as shown.

Upstands

At walls or chimneys, for instance, it is good practice to turn the underlay up because it adds to the water tightness of that particular detail. The underlay should be turned up high enough (normally 100–150mm depending on tile or slate type) to be extend under the flashing by no less than 50mm.

BATTENS

Marking up Gauges (in Sets)

One of the biggest mistakes that people make when marking up gauges is to do them one at a time. I have seen many trainees and inexperienced roofers scratching their heads and wondering why the top batten is running out by 30 or 40mm when they have checked the roof and all the battens and then just cannot find the error. What has normally happened is that they have marked one batten at a time and have made a number of tiny errors of 2 or 3mm each time they have transferred the tape. Marking gauges in sets saves time and is more accurate. For example, the maximum gauge for plain tiles on a pitched roof is normally 100mm. This means that there are ten courses per metre. If the roof is 5m long there will be around fifty courses. Marking one at a time means that there are fifty chances to get it slightly wrong. Theoretically, if one side of the roof is marked at 101mm and the other side at 99mm, then the roof could run out by 100mm, which is equal to one full course of tiles. Normally it is possible to fit about eight plain or nine tile courses on each strip of underlay, so this means that the same roof could be marked in six sets (multiples of 100mm). Suddenly there are fewer chances to make mistakes. Now, even if every set is out by 2mm (one high, one low) the battens will run out by only 10 to 12mm.

In reality, plain tiles are not normally a problem because the gauge is so simple, but I have used them as an example because the high number of courses makes it easier to illustrate the point about the risk of mistakes. The problem comes mainly in tiles and slates with gauges that can be adjusted, or if the gauge is an awkward one (for instance, 328mm rather than a neat 330mm). The fact is that all slate and tile gauges should be marked up in sets irrespective of the gauge; we shall cover this further in the product-specific sections.

Cutting and Fixing

General

Apart from the narrowest parts of a hip or valley, batten sections should be at least 1200mm long and span two rafters (three fixing points). When jointing battens, they should be neatly sawn and fixed on to the centre of the rafter with the nails angled slightly inwards. Always try and nail close to the central point where the batten crosses over the rafter, this avoids unwanted splits and missed fixings.

Joints

When working on trussed roofs, it is important that the joints be regularly staggered (that is, not all in a line) to brace the roof for added strength and to avoid splitting the truss. The rule is that no more than one in four joints should be on the same rafter for gauges over 200mm (for instance, interlocking tiles) and no more than three in twelve for gauges under that (for instance, medium or small slates and plain tiles).

If you are working on a traditional ‘cut’ roof the rafters are normally a good deal wider than the trusses and the whole structure is independently stronger, so there is no real necessity to stagger the joints. However, to be on the safe side I would still advise some staggering of the joints. The simplest way to do this is to move the line of the joints to another rafter for each new strip of underlay.

At Hips and Valleys

At the hips, the batten ends should be cut to an angle, be fully supported by the hip rafter or timber noggins and securely nailed. The treatment at valleys depends much on the formation, but the same rules apply in that the ends should be angled, supported and securely fixed. Ideally, all valley construction details should include noggins. These are supporting timbers set back parallel to the centre of the valley and which provide excellent fixing points for the batten ends. Unfortunately, this is one ‘luxury’ that does not arise nearly often enough now, so the only alternative for most roofers is either to install the noggins themselves or to ‘skew’ nail into the sides of the battens or valley boards.

At Verges

Where batten ends are cut at a mortar-bedded verge, the ends should typically finish 25–50mm back from the edge of the undercloak and be treated with a suitable wood preservative. This is not necessary for dry verge.

Fixing to Masonry

Occasionally, you may need to fix to walls, especially if you are doing some vertical plain tiling to gable ends. Normally, the best way to do this is not to try and batten straight to the wall, but to drill, plug and screw counter battens to the wall first. The maximum centres (that is, the spacing between corresponding edges) should be no more than 450mm. The battens can then be fixed as normal.

COUNTER BATTENS

Counter battens are timbers (normally 25mm to 50mm in depth) fixed in line with the rafters before the normal battens are installed. The main purpose of counter battens is to increase the flow of air in the roof and thereby reduce the risk of condensation. Counter battens tend to be specified mostly on roofs that are insulated at rafter level (over rafter insulation boards or between rafter insulation set down to rafter level). Whether the counter battens go on before or on top of the underlay depends largely on where the airflow needs to be directed, so you should always check the specification.

PREFORMED VALLEY LINERS

General

Individual manufacturers have their own fixing instructions, which you must follow to ensure that you are fitting the products in line with the warranties. The general procedure though is very similar. Ideally, the valley section should be boarded out by the joiner so that the boards (19 to 25mm deep) are dropped in between and at the same level of the rafters. Alternatively, the valley can be boarded out on top of the rafters by using 6mm plywood (or similar). The second method is less labour-intensive, cheaper and can be done by roofers and so tends to be the more common of the two. These valleys tend to be about 400mm wide (200mm each side), so, to ensure support for both the valley liner and the batten ends, the plywood boards should be at least 225mm wide on each side.

Most GRP valleys benefit from having the fascia boards (or over fascia vent strips) notched out to allow them to retain their shape as they come through to the gutter. Notching should be done before the guttering is fixed to ensure that it is not set too high. If you are unable to take a notch out of the fascia then the valley will flatten, be under stress and the tiles or slates will kick at the eaves. The only alternative to notching is to draw the edge of the valley back from where it begins to break and install a lead saddle from code 4 lead or better. Where two valleys meet at the top (that is, at the ridge) then they should be cut as tightly together as possible and finished with a lead saddle.

Bedded and Non-Bedded Valleys

GRP valley liners are normally installed on battens fixed to the outer edges and which run parallel to the centre of the valley. The battens on the main roof should then be cut (at an angle) to the sides of the valley battens and secured to either a noggin, nearby rafter or one of the valley battens.

Dry Valleys

In recent years several dry valley systems have come on to the market. Dry valleys have a raised section running full length down the middle of the liner. The slates and tiles will be cut to this and fixing kits, including clips and instructions, are normally supplied as standard. These valleys tend to be fixed direct to the boarding (in-set boards or plywood) with the battens from the main roof overlapping the edges of the liner.

HIP IRONS

The leading edge of the hip iron should be set approximately 50mm over the fascia board so that it comes in line with the bottom edges of the tiles or slates. The leading corners of the first hip tile will then be cut to this same angle to ensure a correct finish. One common mistake is to set the hip iron back to avoid having to cut the end hip tile. This is poor practice and can result in a leak where the fascia boards join below the hip iron.

UNDERCLOAK

Bargeboards and, in some cases, gable end brickwork are prone to deviation in their length, just like the fascia boards at the eaves, and this can transfer itself to the verge detail when installing the undercloak. For this reason it is advisable to lay undercloak to a line, especially on longer rafters. If you are using fibre-cement strips of uniform width, measure the required overhang and mark the width of the undercloak on the top and the bottom batten (for instance, 50mm over 150mm). Strike a line between the points and fix the inside edge of the undercloak to this.

If you are using verge clips, then you fix the top and bottom ones, suspend a line between them and use the line as a guide for the outer edge of the undercloak. This is especially useful when using undercloak of non-uniform width or when the undercloak needs bedding.

If you are not using verge clips, attach short lengths of batten fixed to the top and bottom of the verge and saw notches (or tack in nails) at the required overhang. Use the notches or nails to suspend the line between the two points.

Bedded Undercloaks

Before bedding an undercloak it is important to make sure that the brickwork or stonework at the gable end is correct. It should be cut neatly to a line about 10mm below the rafter line for a slate or fibre-cement strip undercloak, or about 25mm for plain tile undercloak. If the masonry has not been cut neatly to a line and comes level with or above the rafter line in places, this will result in a wavy and sometimes cracked undercloak and, in turn, noticeable deviations in the verge slates or tiles.

To install the undercloak, first determine the overhang (see the sections on setting out). If the roof has already been battened, ease up the batten ends carefully so that the undercloak may be inserted beneath them. If verge clips are to be used, then they should be fixed to the batten ends with the outer edges to the line, before they are eased up. Bed the wall so that it is about 25mm higher than the rafter line. Slide the undercloak into place so that the outer edge is level with the line and tap down with a straight edge until the desired level is reached. Normally, a bedded undercloak goes under the underlay, but in some cases may this be specified the other way around. The important thing in either case is to provide a good key for the mortar, so, if the underlay intrudes into the mortar, then trim it back. Finally, tap down the batten ends.

Nailed Undercloaks

Plain tiles are not normally suitable for nailing as undercloaks and so the following applies to slates and fibre-cement strips only. For an undercloak to be nailed there has to be something to nail it to, and this is normally an end rafter finished with a bargeboard. The bargeboard should be level with or even a fraction lower than the end rafter so that the undercloak is properly supported and does not tilt inwards towards the roof when laid. Slates should be nailed at least twice per piece, and fibre-cement strips should be nailed at each end, plus two or more other places towards the centre. There needs just to be sufficient fixings to hold it in place until the batten ends are nailed through. Undercloak should be fixed into the end rafter (rather than the bargeboard) with largeheaded clout nails.

Cutting and Positioning Pieces (Fibre-Cement Strips only)

At the eaves there is normally a tilting fillet, which fills the gap between the bargeboard and the top of the fascia. The machined edge of the undercloak should start from here, and the top should be cut off at the apex by nibbling with slate cutters or by scribing and snapping. The piece at the eaves should then be installed by putting a machined edge to the junction and cutting the leading edge halfway on to the fascia board. Drawing the edge back from the edge of the fascia by a few millimetres means that the line of the fascia board is not affected by the thickness of the undercloak (approximately 6mm) and so the slate or tile can come through at the verge without lifting. Ideally, the undercloak should extend over the fascia board to meet the edge of the slate or tile, but this requires that the fascia board is notched out to the width of the undercloak and to a depth of around 25mm, which is not always practicable, especially if the guttering has already been fixed into place. Small sections are quite fragile and may crack if you try to nail straight through, so it is better to form the holes first.

MORTAR (MIXING AND USING)

Use and Consistency

Mortar for roofing work should be used within two hours of mixing; adding water to ‘knock-up’ the mix should be avoided because this can affect the strength. Its consistency should be much firmer than that of bricklaying mortar (almost like very soft clay), with the possible exception of the mortar for pointing, which is normally mixed slightly softer for ease of use.

It is important to get the consistency of bedding mortar correct; it needs to be firm enough to ‘stand up’ rather than slump once positioned, but it should not be so dry that it is unworkable. A good firm mortar helps with compaction when tiles are pressed on to it, which, in turn, helps the mortar bed to withstand the vibration caused by nailing or clipping. The fact that the bed is quite solid also makes pointing-up much easier.

Bedding and pointing should be carried out as soon as possible and certainly in the same working day to ensure that the two parts bond together properly.

When bedding and pointing ridge or hip tiles in hot conditions it is advisable to soak the tiles in clean water for a couple of minutes first. This is especially important when using clay materials, which can draw the water out of the mortar mix quite rapidly. If the mortar dries out too quickly then it will shrink, crack and not achieve full strength. It is important to ensure that the mix is used within two hours or thrown away. Do not add extra water to a mix later on as this will reduce the strength.

CLIPS

General

Clips are specified when nailing alone is not deemed sufficient to keep the slates and tiles securely on the roof. Some product ranges have to be clipped in all specifications, but normally additional clipping is required when the roof reaches a certain pitch or is located in an area with particularly high wind speeds. In some cases, the better-safe-than-sorry philosophy comes into play, so many clients opt for additional clipping even when the minimal fixing specification does not call for it.

Verge Clips

Verges form one of the most vulnerable parts of the roof, and tiles and slates can easily be lost from these areas in high winds. The purpose of a verge clip is, of course, to prevent this and, if installed correctly, it is a job they perform very well. All verge clips have two nail holes and should therefore be nailed twice with the inside edge of the clip lined up with the outer edge undercloak. Fix one verge clip per course per side, with the exception of the top course, which does not need one. Some manufacturers provide the nails (often ribbed) in a fixing kit, depending on the type of slate or tile on the roof; but, where this is not the case, the nails should be 20mm galvanized, large-headed clout or similar.

Tile Clips

These are used to prevent the tails from lifting in high winds and other extreme pressure conditions such as vortices from large aircraft. Most tile clips work by hooking around the interlock and then being nailed into a batten. This ensures that the tail of the tile that is clipped benefits from a much greater resistance to wind uplift.

Some tile clips also tie two tiles together, while others are a clip and a nail in one. There are, in fact, a number of variations and so it is important to check with the manufacturer or supplier that you have the right clips and know how to fix them.

Eaves Clips

This is simply a clip specifically made for the eaves to perform the same job as a tile clip. Again, there are many variations so check that you have the right ones for the tiles.