External cladding - the options

External cladding of buildings has been a common option for generations, it offers a decorative, weather resistant finish that almost anyone can carryout. Traditionally the finish has been timber planking however the availability of uPVC 'planking' now means that this low maintenance material is an alternative to timber.

External cladding planks should ideally be mounted horizontally so that rain etc flows down the face; even then, a certain amount of moisture will penetrate especially in windy conditions. Mounting the planks vertically will increase the amount of moisture penetration as water will flow down and through the joints, where planks are mounted vertically, the joints should be arranged away from the prevailing wind to minimise wind blown water ingress.

Timber cladding is mounted on timber battens fixed to the building, a damp proof (vapour barrier) membrane must be fitted between the battens and the building itself where the building structure is not weather-tight - i.e. timber frame or where insulation is to be fitted behind the cladding. Some uPVC cladding systems have their own method of fixing to the wall but generally even uPVC can be mounted on wooden battens.

A gap (normally at least 25mm (1 inch) for timber) for air circulation is required behind the cladding to ensure that moisture is not trapped. This needs to be considered where insulation is required behind the cladding, most insulation materials will need to go behind the vapour barrier membrane.

Various profiles (see right) are available for timber/uPVC cladding - the illustrations to the right and below show the top of the cladding to the right, and the front of the cladding to the top.

Square edge is rough sawn timber and each plank has a uniform thickness (normally 12mm (0.5 inch) to 18mm (0.75 inch)) across its width, the choice of widths typically being from 125mm (5 inch) to 225mm (9 inch). Overlapping the planks allows rain to run down the front.

Feather-edged timber is rough sawn and each plank is tapered across its width, various widths are available, typically ranging from 125mm (5 inch) to 225mm (9 inch). The tapering typically 18mm (0.75 inch) to 6mm (0.25 inch). Overlapping the planks allows rain to run down the front.

Ship-lap is planed timber (or moulded uPVC) with a shaped front face, various widths are available, typically ranging from 100mm (4 inch) to 200mm (8 inch). The profile is such that the bottom edge of each plank (left in above illustration), overlaps the top edge (right above) of the plank below, this causes any rain to run down the front face without finding its way behind the planks.

Ship-lap, tongued and grooved is very similar to ordinary Ship-lap except that the bottom edge is grooved while the top edge is shaped to fit into the groove of the plank above. An advantage over ordinary Ship-lap planking is that the top edge can be nailed and the nail head covered by the plank above.

Tongued and grooved is planed timber (or moulded uPVC) with a flat front face, various widths are available, typically ranging from 100mm (4 inch) to 200mm (8 inch). Having the groove fitted over the tongue of the plank below keeps rain from entering behind. By fixing through the tongue it is possible to conceal the fixings.

uPVC Cladding can offer similar front face appearance to timber cladding but is generally of thinner section as uPVC can be easily moulded, the example right shows typical uPVC ship-lap with tongued and grooved moulding. Because of the ease of moulding uPVC, some ranges offer different mouldings with the same front appearance:

• A solid uPVC moulding for low level use where some 'knocks and bumps' against the cladding can be expected.
  and
• A hollow uPVC moulding for use at higher levels where impact strength is generally not required - this hollow moulding has the advantage that it is lighter than the solid mould and is easier to handle during installation at heights.

uPVC can be easier to work with than timber because each plank is effectively identical, there are no knots or twists/warps which will be encountered with timber. uPVC is also low maintenance, a wash down once a year will keep it looking fresh for years to come.