External timber framed walls

Also see:

6.2.10Protection from moisture

Timber structures and panels shall be adequately protected from the effects of moisture. Issues to be taken into account include:

  1. cavity construction
  2. DPCs.

Cavity construction

A drained and vented cavity should be provided to reduce the risk of rain penetrating the frame. Cavity widths, measured between the cladding and sheathing, should be in accordance with Table 2.

Table 2: Cavity widths

CladdingMinimum cavity width
Masonry50mm nominal
Render on backed lathing 25mm nominal
Vertical tile hanging without underlay No vertical cavity required when a breather membrane is fitted to the sheathing
Other cladding(1)15mm


1 See Chapter 6.9 ‘Curtain walling and cladding’.

In areas of very severe exposure to wind driven rain, wall construction should include a 50mm cavity between the sheathing and the cladding and:

  • a high performance breather membrane, or
  • masonry cladding which is rendered or clad with an impervious material.

Cavities should be:

  • vented to allow some limited, but not necessarily through, movement of air
  • kept clean, free of obstructions and capable of draining freely.

Where wall areas are divided by horizontal cavity barriers, openings should be provided to each section:

  • equivalent to open brick perpends every 1.2m
  • located to prevent the ingress of rain, and
  • below the lowest timber.

Proprietary perpend ventilators should be used to provide drainage of the cavity.

Horizontal battens, which obstruct the drained and vented cavity, should not be used to support cladding (except tile hanging).

Battens supporting lightweight cladding should be fixed to studs.

Masonry cladding should be constructed in accordance with Chapter 6.1 ‘External masonry walls’. Proprietary cladding should be fixed in accordance with the manufacturer’s recommendations and Chapter 6.9 ‘Curtain walling and cladding’.

Drained and vented cavities should not contain electricity cables other than meter tails.


DPCs and trays should be:

  • fitted at openings to prevent rain penetration
  • installed below the sole plates of ground floor walls and internal partitions.

In Northern Ireland, Scotland and the Isle of Man, and in areas of severe or very severe exposure to driving rain, masonry should form a rebate at the reveals of openings to avoid a straight through joint where the frame abuts the masonry.

Cavities should:

  • extend below DPC to allow drainage
  • be kept clear and be vented to allow limited, but not necessarily through, movement of air
  • be suitably drained to prevent water build-up.

The lowest timber should be a minimum of 75mm or 150mm above ground level, depending on the drainage arrangements.