Repairs: roofs and rainwater goods

Flat roofs, beloved of the housing booms of the 1960s and 1970s, are fraught with problems due to design, materials and workmanship. They rarely have an extended lifespan when compared with a pitched roof.

Most flat roofs are covered in a roofing felt – usually a cheaper felt at that. The flat roofing industry has also been plagued by contractors carrying out shoddy work - this undermines the performance that could be achieved by properly detailed flat roof coverings.

A typical cheaper domestic roofing felt (not shed grade felt) with a fibreglass mat base will last 10-15 years. A synthetic membrane may last longer depending on its situation, the quality of the workmanship and so on, but will be rather more expensive.

Well-constructed mastic asphalt coverings on flat roofs can perform very well and are one of the options to consider for roof gardens, for example, where turf is overlaid. However, it will probably cost more than most felt systems. There are also modern roofing membranes designed to accommodate this type of detail.

Potentially a large number of roofs in the UK housing stock will be failing or about to fail at any given time. Rather than simply replacing or recovering them like-for-like, there is an opportunity to make some improvements. Options include:

• improve the roof fall and reduce water ponding up on the roof (many flat roofs have a completely inadequate amount of fall and do not shed water properly, causing differential thermal movement patterns, stressing the covering and accelerating failures) ;
• improve the lifespan and type of covering material;
• improve the roof insulation possibly in conjunction with improving the roof fall.

Always:

• advise the client about the risks inherent with flat roofs and their generally limited life expectancy;
• consider the quality of any guarantees offered by contractors, what surety they can offer, and whether there are special terms (some manufacturers only warrant the product if it has been inspected by one of their representatives);
• check that the workmanship comes with a guarantee over and above the guarantee for the material;
• record your advice to the client in writing (the budget frequently controls the specification choice, so your advice may not always be adopted).

Roofing felt

Roofing felt is the most common covering. It is usually the cheapest solution and is found on thousands or even millions of domestic and commercial smaller buildings throughout the UK.

Before effecting a repair, carefully consider the reasons for failure. The most common and straightforward is the felt perishing once it has reached the end of its life (at 10 years or so). Look carefully at the various specifications and grades of felt before advising a client.

Mastic asphalt

Mastic asphalt is a blend of rock asphalt and bitumen. Detailing standards are set out in BS 6925. In Building Elements: Roofs and Roofing: Performance, diagnosis, maintenance, repair and the avoidance of defects, HW Harrison suggests an achievable lifespan of up to 60 years if well-maintained, and indeed the asphalt-covered flat roof and roof garden covering the former staff flats built for Cunard in the heart of Southampton lasted some 60 years or so before needing any attention.

Lead and copper

Sheet lead also offers potential high performance as a roof covering, and it may be a mandatory requirement to use it on a listed building or in a conservation area.

Sheet lead is heavy – especially when used in the heavier/thicker gauges, which may be needed for exposed situations. It is also expensive – not just for the material.

This is not a material/finish to leave to chance. If you need to prepare a specification then read Rolled Lead Sheet – The Complete Manual (June 2007) published by the Lead Sheet Association. The manual replaces the old 3-volume version, and although it is not cheap it offers very sound advice with numerous diagrams, enabling you to specify with confidence in a variety of situations.

If setting up very simple jobs, data sheets from the manufacturer are also likely to be helpful, and simple lead detailing should be within the capacity of most building firms. However, although many builders and roofers carrry out lead work, comparatively few firms can address the more complicated and detailed jobs with confidence.

The first port of call in these situations may be to approach your local Cathedral Works Organisation (a quick search on the internet should find one close to your project), or the local authority’s conservation officers to see which firms are active and competent in the local area.

Poor attention to detailing and the use of the wrong lead gauge have led to huge numbers of unnecessary failures and water penetration problems. Even comparatively minor omissions can sometimes direct significant amounts of water to places it should not go (i.e. indoors, and often visible dripping down a light pendant or onto an electrical switch).

When supervising works, check detail items such as:

• whether the ends of any damp-proof trays are turned the right way and not full of mortar slovens and drips, and that the weeps are not blocked;
• whether the lengths of lead are short enough to allow for any thermal movement and prevent fissuring and cracking to the lead (which can occur quite quickly, if not).

Copper is used much less on flat roofs than other sheet materials. The techniques are broadly similar to lead work (for example, traditional joints can normally be formed in a 'roll' both in lead work or sheet copper).

Considering the appropriate tolerances for gauge, thermal movement, exposure, and climate is essential. Again, select a suitably competent and experienced contractor.

Copper can be formed as a standing seam where the edges of the copper sheet are turned up and a section of copper strip folded over to close the seam and make it watertight. This system is often employed on sheet aluminium roofing as well. Standing seams are generally only employed on pitches over about 60 degrees; below that, it would be usual to use a traditional wooden roll technique to form the laps of the sheet material.

Milled sheet lead needs to be of an appropriate gauge for the level of exposure and spans of the sheet lead to be used. Where sheet copper is used it needs to be annealed to BS 2870, which advises various grades within the standard.

Sheet metal materials can generally be nailed or use 'boss' fixings, which are capped over with some similar metal sweated in place to seal the fixing.

Care needs to be taken to prevent galvanic corrosion occurring, i.e. using 2 different metals with different electrical potential means that corrosion begins because the different electrolytic potentials of the 2 materials interact to form a conductor as soon as it rains.

When fixing copper, for example, nail fixings should be of copper or copper alloy. Screws need to be brass. Steel screws might be used to fix lead rolls, but if there is the slightest possibility of other metals coming into contact it would be better not to use them. A minimum measure would certainly be to countersink screw fixings to ensure none of the metal fixings touch the covering.

With both sheet copper and sheet lead it would be usual to employ a felt underlay. This provides a slip-plane to prevent thermal stressed movement affecting the decking and substrate and, in the case of copper, it would also provide an electrical insulation layer to prevent the covering touching any steel fixing nails. It also tends to reduce wind and rain noise.

Special fixing techniques are generally required where low-density roof decks are used (e.g. wood-wool slab) although this is less commonly used in modern construction than perhaps 30 years ago.

A particular advantage of a copper roof is that it can be combined to form part of the lightning conductor system for a building, although the values would require careful calculation (BS EN 62305: Protection against lightning), and suitable detailing is needed to reduce the risk of galvanic corrosion where other materials may be combined.

Sheet metal coverings weather differently. Lead tends to provide a consistent pale grey colour, but ensure that a suitable patination oil is used. Without it, green staining can result (particularly where the sheet lead is in contact with cement mortar): this tends to look somewhat unsightly and patchy.

Copper roofs are well known for their green, weathered colour, which is a natural process and a feature of this material. Emphasise this aspect to clients; they may not appreciate the potential of the copper sheet to weather down in this way.

Aluminium sheet covered roofs are unlikely to weather significantly, apart from the surface of the material matting down and losing its shine. It is likely to present a very clean finish suitable for modern building styles.

The Federation of Traditional Metal Roofing Contractors (FTMRC) has prepared a Good Practice Manual for sheet metals (excluding lead), published in 2012 as: The UK Guide to Good Practice in Fully Supported Metal Roofing and Cladding.

Asbestos cement and fibre cement profiled sheets

This type of covering material has been used in vast quantities, particularly on speculatively built industrial and warehousing developments. The material was the mainstay of many developments from about the 1930s through to the 1950s and 1960s.

Although asbestos cement sheeting has many advantages – including cheapness, and ease of fixing and cutting – nevertheless, the vast acreages of estates built using this material are now likely to be nearing the end of their lifespan, if they have not been replaced already.

Health and safety risks are associated with asbestos contained in old roofing sheets, but there are other issues affecting the specification of replacements if a like-for-like modern fibre cement sheet is not to be used. Check the weight of the sheeting. Will it need regular maintenance (and therefore access)? Are safety harness fixings required? Is the roof sub-frame strong enough?

PCM sheet roofs

Plastic-coated metal (PCM) sheet roofing came into significant use during the late 1970s. Typically, this is based on a profiled steel sheet, but other materials are also in use – aluminium probably being the most common.

The plastic coating on a sheet metal covering typically lasts 10–15 years before it starts to flake away, and regular maintenance is normally required to maintain the condition of the material.

The manufacturer’s warranty is often based on (and indeed stipulates) annual maintenance, which somewhat dilutes the impact of using a cheap covering.