This timber cladding details post includes excerpts and details from our book Understanding Architectural Details – Residential.
Introduction to Timber Cladding Details
There are multiple, even infinite possibilities when it comes to timber cladding. It can give a beautiful appearance to a building both internally and externally. Here we will look at timber cladding details on the external walls of a building. First, lets look at some of the main principles of timber cladding.
Types of Timber Cladding
Timber cladding is a very versatile option for the external cladding of a building. You can select horizontal or vertical timber boards, but these can even be diagonal, or shingles and shakes. From here you can select different board sizes, with varied gaps between, profiles, and finishes all of which allow for a large variety of appearances and styles.
The most common form of timber cladding details that we see are the horizontal or vertical cladding but even these come with a number of variations.
Specification of Timber Cladding
When selecting the timber cladding materials it is important to look for environmentally sourced sustainable timbers. In the UK the main schemes to look for are FSC (Forest Stewardship Council) and PEFC (Programme for the Endorsement of Forest Certification).
Also consider other factors such as transport, manufacture and maintenance when selecting the timber cladding species.
The timber chosen for cladding must be durable and fit for purpose. British Standards have a class system for the durability of wood ranging from Class 1 – Very Durable to Class 5 – Not Durable. For timber cladding Class 3 or better can be used without preservative treatment.
Timbers can suffer from movement and dimensional changes which will vary by atmosphere, board thickness, species and so on.
Softwoods tend to be the most popular choice for timber cladding with a good variety available. Species such as European Larch, European redwood, Douglas Fir, Western Red Cedar are all options. Many of these are grown in the UK and are a good sustainable choice.
There are a variety of treatments, preservatives and surface protections available which go beyond the scope of this blog post. TRADA is a great place to investigate this further.
The Building Regulations include information for using combustible materials for cladding and these must be followed. There are different classes for products that control the surface spread of flame, and treatments accordingly.
Cavity barriers are required behind the cladding in locations such as around window openings, or compartment floors, to restrict the movement of smoke and flames behind the cavity. Again, it is worth checking the Buildings Regulations for exact requirements.
Design of Timber Cladding Details
Horizontal timber cladding is probably the most common, and perhaps simpler option. The boards are nailed to vertical battens on the substrate – this could be masonry, timber frame etc.
Vertical timber cladding can come in the form of tongue and groove, or board on board, along with open joint options too. Generally vertical timber cladding will require counter battens to enable moisture run off in the cavity. The most popular is board on board which is tolerant of timber movement.
Diagonal timber cladding angles the boards at 45 degrees and fixes them to vertical battens, or sometimes even diagonal battens which still allows for the run off of any moisture in the cavity.
Shingles are timber tapered ‘tiles’ which are fixed overlapping one another to give a similar appearance to roof tiles. They are quite a traditional form of timber cladding and particularly popular in North America, however they are also seen in many contemporary applications.
Movement and shrinkage needs to be considered when specifying timber cladding. Depending on the type of timber specified, sometimes a movement gap is preferred between the joints, which can also create a nice shadow line. If open joints are used it is worth checking with the breather membrane manufacturer the possible effects of sunlight on the membrane and whether degradation would occur.
Moisture is the biggest enemy of a timber cladding detail, so the design of timber cladding must ensure that suitable steps are taken to avoid moisture penetration or damage.
Timber cladding is designed as a rainscreen principle – which means that the cladding will have some moisture penetration and it is the system behind the cladding – the waterproof membrane, that will ultimately protect the structure from moisture. This secondary weather protection consists of the waterproof membrane, flashings and DPCs along with the vented cavity that allows any excess moisture to drain out of the cavity to the outside. The battens and counter battens provide the ventilation and clear drainage for the cavity. If there is masonry behind the cavity, a waterproof membrane is often not necessary. An insect mesh is fitted to the top and base of the cavity to ensure the cavity remains clear.
The timber cladding is also susceptible to water damage from splash zones – ie at the base of the building, so the cladding must be lifted by a minimum of 200 to 250mm.
Battens are usually placed at 600mm centres, and sized no less than 38x38mm. For diagonal boards the battens are brought a little closer at 400mm centres. Horizontal cladding boards are fixed to vertical battens which allow free draining down the cavity and air circulation. Vertical boards are fixed to horizontal battens which in turn are fixed to vertical battens to allow for free drainage and air circulation.