What to remember: Roofs

Roofs are a horribly under-used resource. The typical pitched roofs of domestic houses only ever seem to be used for storage, while the flat roofs of commercial buildings are used unimaginatively for plant, or else completely ignored.

As density increases, thought should be given to maximising the potential offered by the roof. With very little additional work the specifier can help turn all or part of a roof into a terrace, garden area or additional living space.

Room in the roof

All pitched roofs can be specified to ensure the volume within can be turned into useful space. Small changes to the structural design and general arrangement will allow the space to be used for accommodation. If this isn’t part of the original design, the specifier can ensure the basics are in place, thus enabling the space to be turned into accommodation at a later date. This approach is used in much of Europe but has largely been ignored in the UK until now, despite the added benefits of this approach.

A structure capable of supporting living accommodation should be specified and tanks, pipes and wiring should be located so they won’t compromise the location of rooms. A space allowance for a stair should also be considered.

Roof terrace

Alternatively, specifying part or all of the roof as a terrace can be simply achieved. The first consideration must always be simple and safe access – preferably the main stair should continue up to the roof level. Ensure that there is safe access for maintenance, equipment and personnel.

Roof terraces must be well guarded. There must be a complete barrier around the accessible area, at least 1.1m high. If the barrier is fixed to a parapet, check this doesn’t act as a step up. According to BS 6180 – the code of practice for barriers in and about buildings – the height should be measured from the parapet top. The guarding should not be open, as this could encourage climbing or allow objects to fall through.

The specifier should consider the promenade deck with care. This may be paving, timber, tiles or even a metal grille. It should not present a trip hazard and must provide an even and ideally threshold-free area. If the door threshold is designed with adequate drainage below, it can be level.

The surface must not present a hazard. It must drain freely to avoid ponding and the build-up of ice. The retention of moisture also encourages moss and lichen to grow. However, falls of more than one in 60 will be a hazard in their own right when people walk over the surface. Ensure that the top surface is virtually level but allows water to drain through. There should always be a minimum installed fall of one in 80 to the waterproofing membrane to ensure adequate drainage. Ensure that the surface provides the best slip resistance possible. Man-made materials should be tested for slip resistance using the pendulum testing method.

Any insulation that is subject to moisture will have poorer performance. Most will lose at least 20% of the thermal performance due to contact with rainwater. A breatheable vapour barrier can be installed on top of the insulation to minimise this effect. Insulation, waterproofing membrane and structure must not be considered in isolation but as one complete roof assembly. The top finish should be easily removable for access. Ensure that the roof drainage system is also easily accessible.

To make any roof terrace area convenient to use, consider lighting – including emergency lighting, power and water supplies.

Roof garden

Successful specifications for roof gardens rely on a very good quality waterproofing layer that is well protected with a relatively deep layer of soil. This ensures a durable roof and allows the planting to mature. To avoid excessive weight, plants can be grown in containers and finishes can either hide or enhance their appearance. Other issues to consider include light pollution, overlooking and privacy.

If the roof has the benefit of uninterrupted sunshine, design in shelter and consider the choice of plants to take advantage of this. Remember to consider the effects of prevailing or channelled winds.

Water and energy collection

Using the roof for recreation can be combined with using it as a natural resources collector, which is now becoming a project requirement. Solar collectors should be positioned to give a generally clear southern aspect but up to 30° either side of south will give a good return. Rainwater collection is becoming more common – this typically involves connecting rainwater drainage pipes to a suitable collection tank.

Check points

• If you intend to make use of the roof make it safe.
• Access should be simple and easy.
• Practical issues such as water and electrical supply must be included.
• Help with costs by collecting energy and water.