Do I Need Treated Timber For a Flat Roof?
When building a flat roof, selecting the proper timber products is of vital importance. Depending on the construction method chosen, certain timbers may need to be treated in order to comply with local building codes and stay durable over time.
When purchasing treated timber, always verify it meets AWPA U1-compliant and pressure-treated with preservatives before buying.
This can be determined by reading the quality stamp or end tag. Wood that has been treated with oil-based preservatives, such as copper naphthenate, coal-tar creosote and pentachlorophenol, is typically used for industrial, marine, agricultural and recreational applications.
These types of products are odourless, paintable and provide protection against fungal attack. Some roof membrane systems are incompatible with oil-borne treatments, so consult a roof membrane manufacturer about the best options for your project.
There are several methods to build a flat roof. One way is to lay joists at a slope using supporting tapered beams, which will create a fall externally and internally. Another option is to place strips of firrings on top of the joist run, which can also create a fall.
The joists should be fixed to a wall plate which is bedded in mortar on top of the inner skin and strapped down with galvanised mild steel holding down straps.
These should be spaced on edge at 400-600mm centres, supported by internal and external load bearing walls. The joists should be preservative-treated, and must be able to withstand the weight of the covering.
Depending on the waterproofing finish chosen, there are different flat roof falls that can be designed into the system. Building regulations suggest a fall of between 1:40 and 1:60 for a domestic flat roof, and this should be adjusted to suit the waterproofing solution chosen.
Joists
Wood that is exposed to the outside usually receives a protective coating of preservatives that are intended to guard it against rot and insects as well as extend its lifespan.
Pressure treatment is typically the preferred method, but in certain situations dry-treated timber can also be utilised – saving money while providing sufficient protection from weather elements. In such instances, by purchasing less of this product you may still benefit from its properties.
C16 graded timber that has been kiln dried is perfect for flooring, roof joists and garden buildings like sheds and greenhouses. While this grade may be lower than standard sawn timber, its superior properties make it stronger.
Wall Plates
Questioning whether treated timber is necessary for a flat roof is often raised, and the answer to that is no – this requirement doesn’t need to be fulfilled.
Wall plate placement will depend on your wall construction and which member it connects to; for instance, in a trussed roof wall it may connect directly with an end joist for example.
Masonry walls benefit from wall plates to safeguard perpendicular joints (perps), which may otherwise be susceptible to load-bearing pressure from above and distribute this burden across the wall plate and into its base plate.
A wall plate helps evenly disperse this weight across its entirety – from roof rafters through wall plate.
Consideration should also be given to the size and thickness of a wall plate. Many guidance documents advise using wall plates 38mm thick or thicker as recommended by standards bodies.
Firrings
If you’re thinking about installing a flat roof for your home, it is essential that you carefully consider its advantages and disadvantages.
While they absorb heat more effectively than pitched roofs, flat roofs may leak more frequently resulting in costly repairs later.
Flat roofs may be constructed of timber, steel or concrete. A waterproof membrane such as EPDM or modified bitumen may be applied as the finishing touch to complete this look.
Building a flat roof requires meticulous consideration to ensure all loads, such as dead loads and wind loads, are safely transmitted to the ground. To do this properly requires compliant structural design with Building Regulations. You may need to search for flat roofers in your local area to help
A flat roof can be given the illusion of falling in one of two ways; either with joists laid to falls, or by using firings (tapered strips mounted above joists).
Firrings are the more commonly used approach and create a slight slope both internally and externally.
Battens
Flat roofs are common features in commercial buildings, often serving as relaxation spaces for employees or providing an installation platform for solar panels. Furthermore, they’re easy to repair and require less materials than pitched roofs.
Battens are an integral component of any flat roof, so it is vital they are installed correctly. Any timber used should be treated with preservative to protect it against weather damage.
Hardwood timbers make the best batten material. Hardwoods can withstand higher wind loads than softwoods and endure greater stresses than them.
Roof battens are a crucial element of any roof system and should only be installed by experienced professionals. Search for companies accredited with organisations like Confederation of Roofing Contractors, Federation of Master Builders or TrustMark.
Read our other article: Do You Need Scaffolding For a Flat Roof?