Roof Coverings

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Roof Coverings

Introduction - Materials

Thatch. A covering used in rural areas for centuries and also common in towns until the end of the medieval period. Still used today but only in rural areas. It is a fire risk and can attract vermin.

Natural slate. A hardwearing, traditional roof covering which can easily last 200 years. 100 years ago, most slate was quarried in Wales - much is now imported. The quality of slate varies from quarry to quarry. This is reflected in its price. Many speculative Edwardian and late Victorian houses were built with slate of average (or less) quality. In modern construction synthetic slates are also available. They are mostly made from fibrous cement.

Stone slate. Unlike natural slate this is often made from limestone or sandstone. It is generally not as durable as natural slate. Stone slates vary from region to region depending on the local geology. These thick stone slates are from the Black Mountains in south east Wales. Synthetic stone slate is also available today, mostly made from concrete.

Plain tiles have been used in this country for centuries -particularly in the 'lowlands' where natural clay was in plentiful supply. Plain tiles are still made from clay and are, once again, becoming popular. Concrete plain tiles have been available since the 1940s. Small nibs on the back of the tiles clip over the battens.

Pantiles were introduced to this country some 300 years ago. They are single lap tiles (unlike slates and plain tiles) and can be found in the south east and south west. Single and double romans were developed from pantiles. They offer slightly better weather protection than pantiles because of their closer fit. Popular until the 1940s but superseded by interlocking tiles.

The vast majority of modern roof coverings are concrete interlocking tiles. They are available in a huge range of colours and shapes. Traditional materials are making something of a comeback. These are traditional clay pantiles. Clay plain tiles and clay romans are also available.

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A simple modern roof

This section shows how a simple modern roof is covered with interlocking tiles. The first requirement for anyone studying roof construction is to understand some of the terminology. The wall plate. rafters and ceiling joists form the roof structure. At the eaves (the bottom of the roof) there is usually a fascia board (to hide the feet of the rafters and provide a fixing for the guttering). If the roof has overhanging eaves there may also be a soffit board to hide the gap between the fascia and the wall. At the end of the roof (the verge) there may be a barge board secured to a gable ladder. The latter is a timber framework which allows the roof to overhang at the verge.

Roofing feltAccording to British Standard 8000, 1990;.... "the underlay provides a barrier to minimise wind load generated under wind gusts acting on the slates and tiles. Contact should be avoided between the underlay and the underside of the slates or tiles to prevent the wind uplift load being transmitted to the slates or tiles. It also provides a barrier to prevent wind driven snow or dust from entering the roof space." There is no mention of protection against rainwater (its generally accepted function).

In modern construction there are a variety of felts on the market. These include traditional bitumen felts, polythenes, and modern breathing membranes. The latter are used in breathing roofs – these are still comparatively rare and therefore beyond the scope of this paper. The felt is held in position by the tiling battens and should have good laps and run over the fascia and into the gutter. The felt should sag slightly to allow water to drain under the battens.

Tiling battens support the tiles. Their spacing will depend on the nature and size of the covering. The battens are nailed in position with galvanised nails. The batten size is typically 38 x 25mm or 50 x 25mm. Getting the battens dead straight is important if the tiles are to lie correctly on the roof. Most battens are treated against rot and insect attack.

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Laying the tiles

Once the felt is in position the tiling can start. Modern interlocking tiles overlap by about 50-75mm. They interlock at the sides to provide a good watertight layer over the roof. There are a variety of patterns on the market, many of which have a distinctive profile (see page 1), but flat tiles are also available (see graphic below). It is normal practice to nail or clip all the perimeter tiles to prevent movement in high winds; the rest stay in place due to their side interlock and their weight. Unlike traditional tiling materials modern tiles can be laid to very shallow pitches, sometimes as low as 15 degrees or so depending on the characteristics of the particular tile.

There are a number of ways of finishing the verge. The traditional method requires an undercloak (see graphic above and left-hand photo below). This undercloak provides a slight overhang and supports for the mortar pointing which will fill the gap between undercloak and tile. This type of detail is quite slow to build and another option is to use special verge tiles. These are shown in the right-hand photo. Verge tiles can fit directly over the brickwork but this provides rather an ‘abrupt’ finish so it is more common to find verge tiles fixed over a barge board, but not necessarily as ornate as the one below.

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At the ridge or top of the roof it is normal practice to provide to provide some form of ridge tile. These are available in a range of shapes although the half-round one is probably the most common. Traditionally the ridge is bedded in mortar although nowadays dry fixing ridge systems are becoming popular. The latter require plastic clips and straps to hold the ridge tiles in position – their long term durability is not known. If the tiles have a pronounced profile any mortar pointing is likely to be quite thick. There is a danger that it will sag, leading to unsightly work and gaps in the pointing. This can be avoided by adding small tiles known as dentils. These can be seen in the left-hand photo.

Where roofs run into each other at right angles a valley is formed. Valleys can be formed in a number of materials including special valley tiles, lead, zinc and plastics. Nowadays, plastics are becoming very popular. The construction is shown on the right.

M

VentilationMost roof voids are insulated by laying a quilt across, or between, the ceiling joists. This means that the space above the quilt is very cold. To prevent condensation forming in the roof void, roofs need to be ventilated. Although some roofs have special vapour permeable roofing felts and are designed as breathing roofs most require the provision of ventilation to prevent condensation. This usually achieved by providing vents at eaves level either side of the roof. Improved ventilation can be provided by adding vents at the ridge. This ensures a good flow of air through the whole roof and even works when there is no wind blowing (through natural convection currents in the roof space)

Where roofs meet at right angles and form an external angle a hip is formed. Hips can be formed in lead but it is easier and cheaper to use ridge or hip tiles. The batten provides a temporary straight line. The tiles need to be well bedded to prevent uplift in high winds. Like ridge tiles there are a number of dry fixing systems.

These valley sections sit on the felted roof. The tiles are laid, cut on the angle to suit the valley and then pointed in cement mortar. Their lifeis unknown but is unlikely to match that of lead where 100 years or more can be expected.

Eaves to eaves is the minimum acceptable to the Building Regulations. However, the apex is not always ventilated successfully in this method and it only works when the wind is blowing.

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Traditional Roof CoveringsModern interlocking tiles are single lap coverings. In other words each tile laps part of the tile below. They work because the sides of the tiles also overlap (ie the interlock). Clay double romans and pantiles, both popular during the Victorian period and right up until the Second World War work in a similar way although because they overlap rather than interlock they are not always as effective.

Plain tilesIn the midlands and south east, where there were plentiful supplies of clay, the traditional roof covering was plain tiles. These can still be found today (in concrete as well as clay) and can immediately be recognised by their small size. These tiles are laid in a very different way from those described above. Consider the left-hand photo below. This shows plain tiles laid to a single lap. You should be able to work out that the tiles, laid like this, are completely ineffective. Tiles can only be laid to a single lap if they overlap at the sides. Where there is no overlap they must be laid with double lap (right-hand photo). This means that every tile laps not just the one below, but the one below that as well. This is expensive construction, not just because of the costs of tiles and battens, but also in terms of labour.

These clay double romans are the forerunners of modern interlocking tiles. They first appeared in the mid 19th century and were a development of pantiles (see page 1). They are single lap tiles and generally much cheaper than the forms of construction shown below.

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To be effective plain tiles need to be laid to a pitch of at least 45 degrees or so. In addition every 5th

course of tiles (as well as the perimeter tiles) must be nailed to prevent uplift in high winds. Alloy or copper nails are preferred.

Despite their cost, plain tiles are often used in up-market housing developments and in commercial buildings where they can provide a very effective and attractive roof covering. Details of eaves and verge finish are shown below.

Natural slate

In the UK slate has traditionally been available from parts of Scotland, the Lake District (now Cumbria), Cornwall and, of course, Wales. With the coming of the railways in the early 19th century, slate became a popular material throughout the country. Nowadays, slate tends to be used on prestigious buildings or in areas where the planners demand it. This is because it is relatively expensive when compared to other roofing materials - at least in the short term. Slate is a dense, fine-grained, rock, usually formed by the metamorphism of shale or clay. This process consolidates the rock and in so doing forms cleavage planes along which the slate can be split into sheets of varying thickness. Slate is a excellent material. It is non combustible, impermeable to water, highly resistant to frost attack and chemical attack. Its colour does not fade and it can generally have a very long life; 250 years is not unknown. Failure is usually caused by defects in fixings or structure rather than failure in the slate itself.

Slate is available in a range of sizes. Like plain tiles slate is laid with a double lap, but every slate needs to be nailed as there are no nibs. Nails can either be fixed at the top of the slates or in their centre using alloy or copper nails. The slates can be holed on site or pre-drilled at the quarry. Slates can be laid to quite shallow pitches, as shallow as 20 degrees or so.

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The graphic below shows the stages in nailing head nailed slate. Notice that every other course must start with a larger slate to break or stagger the joints. The principles of laying are just the same as plain tiles. You should be able to see that each slate laps two others – hence the term double lap. At the eaves and verge the construction is very similar to plain tiles. Valleys and ridges are usually formed in lead.

Flashings

Where a roof abuts a wall or chimney flashings are required to seal the joint. Some flashing details are complex and require skilled labour, others are more straightforward. Flashings are usually formed in lead although zinc and copper are not uncommon. Sometimes flashings in mortar can be found –these are rarely successful long term as differential movement causes cracking and provides a path for rain ingress.

The graphic below show simple flashing where a wall abuts a roof. This type of flashing can be used for slate, plain tiles or interlocking tiles. The flashing comprises two components, a series of soakers which sit on each tile and a stepped cover flashing dressed into the wall.

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This chimney flashing is a little more complicated. The side flashing is similar to the one on the previous page. However, at the top and bottom of the stack additional flashings are required, an apron at the bottom and a back gutter at the top.

Conclusion

This brief paper has explained the principles of modern interlocking tiles, traditional plain tiles and natural slates. The principles, and good practice, have not changed for hundreds of years although nowadays there is a huge range of peripheral roofing products including valley, ridge, verge and eaves systems. If you understand the difference between a single and double lap roof and if are aware of the construction around the roof perimeter (eaves and verge) you have a sound foundationfor further study.

Questions – all the answers are in the text

1. What is, a) a pantile, b) a double roman?

2. What is the function of roofing felt and how should it be fixed?

3. Explain the difference between single and double lap coverings

4. How can roofs be ventilated? Which method is preferable? Why?

5. How can interlocking tile roofs be finished at the verge?

6. In a modern roof how are hips and valleys formed?

7. How is slate fixed in position and why are larger slates required at the verge on alternate courses?

8. What is a soaker?

9. What size (typically) are roof battens and how are they fixed?

10. Define a) headlap, b) gauge, c) margin.

Side flashings can be a continuous strip or soakers

Front apron

Back gutter with coverflashing over