Repairs: roofs and rainwater goods

Slate

Once perhaps the most common covering of pitched roofs, slate has fallen into decline in favour of concrete tiles. It still has much to recommend it despite being (usually) a more expensive option than the concrete version.

Do not neglect the aesthetics of re-roofing a period property with its original style of coverings (assuming of course it was slate), though it can be hard to persuade the building owner of the merits of using a more expensive material.

Slate is generally a much lighter covering than concrete tiles. This means that it is often possible to avoid roof frame repairs and reinforcement when reroofing in slate. However, in all instances of reroofing work it is important to check the integrity of the frame and fixings prior to any work.

If you do specify repairs to a slate roof, remember that it may be possible to reuse some of the slates by turning them round. If re-covering in a different material, check that any value in the old slate stock is set off against the cost of the works.

Be wary of the contractor's '98% breakages' - if fairly sound slates are removed, the slate itself might not be significantly affected by wear and tear (in the case of nail sickness, for example). Breakages have a nasty habit of being stacked out of sight and may never make it to the skip.

The typical slate size is the 'two-by-one', that is to say, 2 ft x 1 ft (600mm x 300mm). The best quality slate, and by far the most expensive, is the Welsh Blue.

Slates come in different thicknesses too, which also affects the price, the weight and the expected lifespan of the covering. Better quality slate is more impervious to frost, and is likely to offer much better value in the long run.

Consider the wearing quality carefully before making any recommendations to the client. Study the supplier's brochure carefully: a reliable and experienced roofer can probably offer advice as well.

Slate roofs - hip details

• Usually follow the cheapest option! Covered in ridge/hip tiles with lowest hip slate secured by a 'hip iron' bracket screwed into the hip board.
• In better class work it could be a mitred slate hip over a sheet lead soaker beneath (min code 4 lead) or one of the following:

Where the roof meets a gable or parapet abutment - usually detailed as a soaker and cover flashing:

Slate roof detailing:

• slates normally have double lap (can be as little as 1/3 at sides);
• laid to BS 5534; and
• to BS 8104 driving rain index - governs the lap needed to prevent driven pain penetration.

Alternatives to slate

Alternatives to natural slate include:

• Asbestos cement slate – Less often encountered now, these generally had a lifespan of about 40 years. As the slates aged they were prone to curl up at the edges, resembling a dried out sandwich.
• The Eternit™ and other fibre–cement slate composites – These normally require a copper rivet and washer to be fixed through a hole at the bottom of the slate to resist wind lift, which may otherwise snap the slate. The fibre-cement composite slate does not have a laminated structure in the same way as a natural slate and is comparatively weak. So this type of product may not be best suited to an aggressive weather aspect, such as buildings on a seafront.
• Pressed slate composites – Made of resin and slate dust (fairly realistic).

Many of the problems associated with synthetic slates (particularly curling at the edges and sometimes bowing or bending, and general lack of flatness) are generally attributed to a moisture build-up under the slate. With roofing felts being obligatory, there is likely to be much less through-flow of air under the slate; consequently, this issue may continue for the foreseeable future.

If this is an issue, consider counter-battening the roof to create more of an air space beneath, i.e. lining battens over the rafters and then applying the main roofing battens along the line of the roof in the traditional way. This may reduce the problem to some extent, but is an additional cost that is likely to be resisted, except on the most expensive schemes where composite slates are unlikely to be used anyway!

Slate alternatives do, however, offer a solution where weight needs to be limited, and where cost is an issue, or where the use of a concrete tile is to be avoided for cosmetic or other reasons.

Concrete tiles

Concrete tiles are probably the most commonly used roof covering material of the last 30 years or so, mainly due to the lower cost. But as they weigh considerably more than a slate covering, the roof frame has to be strong to bear this weight.

When reroofing using concrete tiles, check that any additional bracing timbers, new purlins and struts, for example, have been installed properly first.

Concrete tiles are prone to their own particular problems – usually those associated with concrete products in general, such as attack from chlorides and acid.

A marine climate or exposed position could lead to wind-driven sea salt contamination and thus to chloride attack. Areas prone to acid rainfall might also suffer from problems because of the dissolving of the top layer of the tile and rapid washing out of the fine aggregate material from the top.

This might not initially affect the performance of the tile itself, but large amounts of aggregate (loosened as the cement binder is dissolved by the acid rain) washing into the gutters, downpipes and (particularly) soak away chambers can lead to a quick deterioration in the efficiency of the surface water drainage system. Re-calcified fines from the tiles are extremely difficult to shift from blocked underground drain systems, especially in clayware land drain pipes, which may have to be totally replaced.

The worst case the author has seen involved concrete tiles dating from c1930, which were very spongy, having deteriorated quite significantly. This was on a development of bungalows situated on a hillside, built between the First and Second World Wars.

Concrete tiles offer a great range of design, shape, size and weight, and can generally be produced to emulate any of the styles of clay tile. So, although the tiles with interlocking edges are perhaps the most common, a range of coloured and sized plain tiles (i.e. flat ones) is available, providing various options to help fit in with prevailing local architectural details.

Clay tiles

Most people would probably agree that clay tiles offer a significant aesthetic advantage over concrete tiles but usually at some increased cost.

Clay tiles, in particular, can be prone to frost damage. Consider this if you are specifying a tile, especially where cheaper tiles are involved. Tiles imported from warmer climes may not always achieve the performance standards needed in the UK.

Where concrete or clay tiles are used in an exposed area prone to high wind loads they can be prone to chatter as the wind ripples through the lower lapped edges of the tiles. Double lapping the tiles may reduce this tendency to some extent, but in very exposed areas it is possible (and advisable) to clip the lower edge of the tiles in a similar fashion to the composite slates described above.

Clay tiles (and clay bricks too) can be prone to effluorescence.

Novel products

Various forms of photovoltaic slate are coming to the market. Although you will probably not be approached for repairs-related advice in the near future, clients may ask about these products when reroofing or major renovations are being considered. Establish whether the insurers are happy to cover the risks; the author has experienced problems on commercial projects due to insurers' concerns about electrical arcing at conductor connections, and with earthing.

For social housing schemes, in particular, roof covering materials must be selected to demonstrate a sustainable approach. In the past concrete tiles have been popular because of their low cost and comparatively low environmental rating (EcoHomes), but other options are becoming available – from tiles made from recycled materials such as tyres to ‘green roofs’ planted with sedum. Each has a range of benefits and drawbacks.