There are many, many potential problems to consider when specifying a roof. Barbour Index and Scott Brownrigg run through the 10 main points to consider

1. General points

With roofs, the basic considerations seem often to be the ones that are overlooked. For example, ensure the finishing material is appropriate for the performance, life and aesthetic required. Given half a chance, moisture will find its way through a roof. Ensure that all the expected conditions are catered for, including those during the construction sequence. Always design for sensible, safe maintenance.

2. Planning consent

Ensure that you have consent from the planning authority before commencing a roof design, particularly if you are in a conservation area.

3. Weather conditions

Never allow ponding to occur, even if it is temporary and the manufacturer says it is okay. Specify a minimum installed fall of at least one in 60 and, in particular, consider joints in the finish – especially joints that cross the fall line.

Always consider how successful the design is likely to be under a wide range of conditions, including driving rain, snow, ice, high humidity both inside and out, high temperatures both inside and out, excessive solar gain and ultraviolet light. Roofs have to cope with years of exposure, and many won’t be maintained properly, if at all.

4. Maintenance

Allow for regular access to the roof and gutters, which need frequent attention especially if they are near trees. Any build-up of leaves that fall into the gutter system in autumn needs removal.

5. Safety

Roofs are top of the list when it comes to accidents, so careful considerations of questions of safety are vital.

  • If possible, design the roof to be prefabricated or built completely on the ground so as to minimise the amount of work to be undertaken at height.
  • Ensure no complex procedures take place at height – especially in confined areas.
  • Ensure that adequate guarding and protection are written into the documentation. Any changes to this can then be highlighted as a design issue and under CDM Regulations the author of the change is the designer and liable if any problem ensues.
  • Wherever possible, avoid the use of construction processes that have a fire risk as many clients prohibit their use. Many systems are now available that avoid any hot processes.

6. Roofing types

Ensure that you do not specify a particular manufacturer or type of roof before you have thoroughly researched its track record.

  • Ask to see a current, independent test certificate including, but not limited to, British Board of Agrément certification.
  • Ask to see built examples of types similar to that being considered. Ideally they will be at least 10 years old.
  • Ensure that the method of construction and technical performance of the roof are appropriate for your design.

7. Roofing materials

There is a wide choice of coverings: metals, timber shingles, plastic membranes – including PVCu and thermoplastic polyolefin (TPO) – and clear materials, including polycarbonate and glass. If the roof has a shallow pitch you can also have a “green” roof by adding a special moss called sedum, which comes in a purpose-made tray that is fixed above the metal roof finish (see pages 62-63 for more on green roofs).

  • Metal coverings all follow the same principles. First, the metal is formed into open-ended trays which are lapped along the fall of the roof. The edges of each tray are then bent over each other to make the roof watertight. This is laid directly on top of the insulation structure and vapour control layer in a warm roof, or onto the slip membrane and deck if the insulation is at ceiling level.
  • Generally, metal roof finishes have a very long life – typically 40 years – and if laid correctly and maintained have a very good record of reliability and weather resistance.
  • All metals will absorb oxygen and tarnish unless you regularly apply a protective lacquer. Depending on your choice, this means a copper roof can stay golden or patinate to a bright green. It you choose to leave it to nature, the rain run-off will stain green anything it drips onto and be poisonous to plants.
  • Zinc has very similar properties to copper, except it will age to a dullish grey.
  • Lead has covered church roofs for centuries so it obviously has a good track record.
  • Very high-profile projects may warrant the use of stainless steel or titanium. Both are expensive but provide exceptional longevity and unique aesthetics.
  • There are also types of plastic and composite that may be considered. ETFE foil is useful for creating an exciting, transparent roof but care over detailing and maintenance must be exercised.
  • Glass can be used to great effect, and if low-emissivity and solar-control coatings are specified these can offer relatively high performance.

8. Photovoltaic panels

Photovoltaic panels can be considered, although the initial cost, and the fact they produce a rather small amount of electricity unfortunately still limits their feasibility. Solar water-heating panels are much more cost-effective than photovoltaic panels, and can be integrated into the roof.

9. Roof geometry

Careful consideration of the roof geometry can result in better interior ventilation and internal daylight gain.

10. Weathering

Consider the weathering of the roof as many are affected by the build-up of staining, lichen or algae, which may have a detrimental effect and can make the roof look poor. If the materials are well-considered and detailed for the location, however, many roofs can look good and last for years of good service.