6.2 External timber framed walls

Also see:

6.16.9

6.2.10 Protection 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. drainage and ventilation
  3. DPCs and cavity trays.

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(1) where a breather membrane is provided and fixed to the sheathingDependant on batten support layout and spacing(1)
Other cladding(1)15mm

Notes

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 (see Clause 6.2.13), 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.

Masonry cladding should be constructed in accordance with Chapter 6.1 ‘External masonry walls’. Lightweight render cladding should be constructed in accordance with Chapter 6.11 ‘Render’. Vertical tile hanging and slating should be constructed in accordance with Chapter 6.9 ‘Curtain walling and cladding’. Proprietary cladding should be fixed in accordance with the manufacturer’s recommendations and Chapter 6.9 ‘Curtain walling and cladding’.

External wall cavities should not contain electricity cables other than meter tails.

Drainage and Ventilation#

Drainage and ventilation should be provided to the cavity between the timber frame wall and external cladding.

Cavities between masonry cladding (including brickwork, rendered blockwork and stone) and timber frame walls should be vented. Cavity vents should be:

  • equivalent to open brick perpend joints ever 1.2m
  • located to prevent the ingress of rain
  • located to drain moisture from the cavity.

Proprietary perpend ventilators should be used. Perpend ventilators should be installed to the base of external wall cavities below the lowest timber sole plate and above finished ground level; if continuous cavity trays are installed at sole plate level, perpend ventilators should be installed above and below the tray to provide drainage and ventilation to all areas of the timber frame. Where wall areas are divided by horizontal cavity barriers and/or cavity trays, each individual cavity should be vented. Weepholes should be provided at cavity trays over openings.

Cavities between lightweight external cladding and timber frame walls should be vented or ventilated depending on cladding type. Cavity vents should be:

  • installed to promote through ventilation where required
  • protected to prevent the ingress of rain, insects, birds, and vermin
  • located to drain moisture from the cavity.

Drainage and ventilation openings should be provided to the base of external lightweight cladding at/near ground level, above horizontal cavity barriers and flashings, and above openings (such as windows and doors).

Horizontal battens used to support lightweight external cladding should be spaced off the frame using vertical counter battens to provide a path for drainage and ventilation. Battens or carrier rail systems supporting lightweight cladding should be located over and fixed to studs. Vertical battens and carrier rail systems should have joints at floor zones to accommodate differential movement.

DPCs and Cavity Trays#

DPCs should be:

  • fitted at openings to prevent rain penetration
  • installed below the sole plates of ground floor walls and internal partitions
  • lapped with the DPM and AVCL to enhance air tightness at sole plate level
  • installed over horizontal timber cavity barriers (except under eaves and verge) and lapped behind the breather membrane by at least 100mm.

Cavity trays should:

  • be installed over openings, at abutments, and where specified at sole plate level
  • have weepholes to deflect moisture out of the cavity over openings, or perpend ventilators where cavity trays are continuous
  • be lapped behind the breather membrane by at least 100mm to deflect moisture away from the sheathing
  • be marked to BS EN 14909 and have satisfactory assessment by an appropriate independent technical approvals authority accepted by NHBC.

Clause 6.1.17 contains further guidance on the installation of cavity trays in masonry cladding.

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
  • be suitably drained to prevent water build-up.

The lowest timber should be a minimum of 150mm above finished ground level. This may be reduced to 75mm in situations where the site is not subject to a high water table or where the cavity will not have standing water.

Last updated: 2nd January 2024

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