Wood and roof design

[edit] Introduction

Beyond the traditional pitched roof, a variety of timber roof styles showcase great design, from flat to curved, from shell to hyperbolic. But how practical are these variations in a country that gets more than its fair share of rain? What are key considerations when using different types of systems?

[edit] The traditional route

For most UK homes, the go-to design is a pitched roof, covered with clay, concrete or slate tiles. The pitched roof adorns homes across the UK for good reason. The sloped design directs rainwater into the guttering, providing effective drainage. This style of roof can be constructed as a cut roof – where timber rafters are cut on site - or a truss roof using trussed rafters (also known as roof trusses). Trussed rafters are an increasingly popular choice as they are precision cut and built offsite and delivered to site when they’re ready to be installed.

As the housebuilding industry continues to face pressure to build more homes, any method that reduces both build time and wastage is a bonus. Though the overall roof design and structure is carried out by the building designers (such as the architect and engineers), the trusses are designed by the trussed rafter designer.

Trussed rafters are made from timber so, as with all building products, they should be stored safely and kept dry to avoid the structure being compromised by excessive exposure to moisture.

The Trussed Rafter Association (TRA) recently published a buyers’ guide to trussed rafters to help those specifying roof trusses to make the best choice for their build. The TRA has also put together a spandrel panel guide. Like trussed rafters, spandrel panels are pre-fabricated. They can be used as a separating wall or as an alternative to traditional masonry gable ends.

In addition to a cut roof or trussed rafters, another way to create a pitched roof is by using roof panels. Also known as structural insulated panels (SIPs) or roof cassettes they are fabricated offsite and in the case of SIPs, are readily insulated. Always check that SIPs are waterproof and are protected from pests. The Wood Protection Association can advise on the best and safest treatments for pest-resistance. As with trussed rafters, it’s advisable that the building designer works closely with the SIPs manufacturer.

[edit] A contemporary alternative

Often seen as a cheaper alternative to pitched roofs and favoured for modern architecture, flat roofs first became popular in the UK after the second world war. Traditionally the flat roof is more common in warmer climates, however, with careful design and engineering, flat roofs are a viable option in the UK. Using timber joists across the shortest span of the roof, a waterproof membrane or sheeting can be used to ensure it is watertight.

In the UK, the National Federation of Roofing Contractors (NFRC) recommends a warm roof. This means applying an insulation layer between the timber and the waterproof covering. If the roof is inverted, the insulation should be placed on top of the waterproof covering.

One way of achieving a flat timber roof with effective insulation is with cross-laminated timber (CLT). An example is the innovative Redshank. Built to resist flooding, the entire superstructure was built from CLT and clad with cork panels, meaning there is no need for guttering as rain drains off the surface. Using CLT for the roof also means the interior could be left bare as the natural warmth of the wood was preferred over traditional plastering.

[edit] Getting creative

Despite their popularity, pitched and flat roofs are not the only options available. Shell roofs, vaulted ceilings, domes and hyperbolic paraboloids are ideal for statement designs.

Often reserved for bespoke projects such as self-build homes and commercial builds, this is where roof design can really shine. The flexibility of engineered timber, such as CLT and glulam, has allowed for eye-catching roof designs at M&S in Cheshire Oaks, Scottish Parliament and Bishop Edward King Chapel. Modified timber has also played its part and the entire structure for Sydney-based restaurant, Barangaroo House, is created from Accoya wood.

One piece of roof architecture that has been totting up the column inches is Europe’s first eco-mosque, Cambridge Central Mosque. The roof structure is made from CLT but it’s the intricate glulam vaulted structure that is the talking point. The elaborate design incorporates ‘The Breath of the Compassionate’, a historic Islamic eight-point star pattern, conjuring up the visual representation of breathing in and out.

[edit] Putting nature on top

With the pressure on to make the built environment a cleaner and greener place, green roofs are gaining popularity, particularly on commercial buildings. Eco homes have also been adopting this principle for a while and now these sustainability credentials are catching on for housing developments. Futurehome, a multi-award-winning 15-home project in Elephant Park in London, used a number of sustainable innovations, including green wildflower roof construction. This includes a roof level garden for residents to grow their own plants and vegetables. In high-density urban areas where space is scarce, using roofs as gardens can be an ideal solution.

Green roofs do need a lot of maintenance though. As with any garden, they need regular upkeep, and there is also the issue of drainage. In response to the upward trend in green roofs, the Green Roof Organisation was established in 2008 and published a Green Roof Code of Best Practice for the UK. In addition to drainage, other design considerations include root resistant materials and the type of vegetation used.

[edit] The sky is the limit

While the pitched roof continues to be the design of choice, timber is being embraced to enhance a roof’s design, functionality and sustainability. Engineered timber is literally bending the rules for roofscapes and offsite solutions such as trussed rafters look set to grow as the UK continues in its quest to build more new homes.

[edit] Related articles on Designing Buildings Wiki

• Cool roofs.
• Domestic roofs.
• Flat roof.
• Gable.
• Gambrel roof.
• Long span roof.
• Mansard roof.
• Metal roofing.
• Rafter.
• Roof slates.
• Roof tiles.
• Roofing defects.
• Sarking.
• Sheathing.
• Thatch roofing.
• Truss.
• Types of roof.