Cavity walls

Openings, Piers


Wall Openings

The layout of openings in walls should be designed so that they do not over-stress the masonry. Openings should be designed correctly by the designer at the design stage.

Diagram C66 -Typical opes in external walls

TGD A Provisions

The following are some provisions from the technical guidance document (TGD) A –Structure, Building Regulations 2012 in relation to openings and buttressing for houses and small houses.

  • The combined width of the openings should not exceed 2/3 the length of the wall, i.e. ope C + ope E should not be greater than 2A/3.

  • No individual opening should exceed 3 m in width, i.e. width C or width E should not be greater than 3 m.

  • A pier between an opening and a return wall should be at least 1/6 the ope width, i.e. pier B should be at least C/6 and pier F should be at least E/6.

  • A pier between 2 openings should be at least 1/3 the sum of the 2 openings, i.e. D should be greater than (C+E)/3.

Piers & Openings

High stresses occur in narrow piers between openings; the width must be controlled to avoid this. Ensure a lintel is provided above all service boxes.

Diagram C67 - Inappropriate meter box location

Care should be exercised when choosing the position of the meter box. The head of the meter box should be supported by a lintel and a stepped DPC must be provided here also. Generally, the gable wall is a good location for the meter box. Holes for ducts should be neatly cut in service boxes.


In walls

Both TGD A – Structure, Building Regulations 2012, and Irish Standard I.S. EN 1996-1-1: 2005 Eurocode 6: Design of masonry structures – Part 1-1: General rules for reinforced or unreinforced masonry structures (Eurocode 6) provide guidance in relation to chases in walls. Chases should never pass through lintels and should not affect the stability of the wall. Chases are not allowed in buildings where hollow blocks are being used.

Vertical Chases

The following guidance should be adhered to with respect to chases in walls:

  • Where a wall has a thickness between 85 mm and 115 mm the maximum dimensions of a vertical chase permitted is 30 mm deep and 100 mm wide.

  • Where a wall has a thickness between 176 mm and 225 mm the maximum dimensions of a vertical chase permitted is 30 mm deep and 150 mm wide.

  • The horizontal distance between chases should always be greater than 225 mm.

Horizontal & Inclined Chases

According to clause 8.6.3 of I.S. 1996-1, unless the wall is designed to accommodate horizontal chases, in walls of thickness 85-115 mm, horizontal chases are not allowed. In walls of thickness 176-225 mm, a 10 mm chase is allowed to unlimited length and a 20 mm chase is allowed to a length of 1250 mm.

Projecting Windows

The subsequent detail sketch shows good practice for the construction of projecting windows with regard to weathering. This method of construction should only be undertaken in small areas. In larger and/or exposed areas, other methods should be considered, such as properly fixed tile hanging.

A suitable detail should be designed to account for the fact that the external rendered surface will most likely remain wet continuously in winter conditions.

In order to reduce the risk of moisture penetration, create a cavity behind the external rendering. This is achieved using vertical and horizontal treated battens.

Ends of header joist, floor joists and bridging pieces should be treated with 2 coats of preservative.

Diagram C68 - Projecting windows - typical bay window detail

Apply the cement render or rough-cast finish to corrosion resistant metal lathing; fix the metal lathing to the horizontal treated battens using corrosion resistant nails. A key should be formed by the render behind the expanded metal. Place sarking felt behind the expanded metal. Subsequent details illustrate how the felt is placed at window junctions.

A breather membrane should be fixed to the sheathing material as a last line of defence, as shown. In order to bell-cast the render out over the window, a proprietary render stop bead should be provided as shown.

In order to stop the lead flashings from staining the render finish, it is necessary to apply 2 coats of patination oil to the lead.

Cracks in the render (due to poor construction) lead to moisture penetration, which causes extensive dampness.

Diagram C69 - Detail A - first floor cill

Diagram C70 - Detail B - ground floor cill

The designer should design and detail appropriate structural supports for the framework and the roof of a projecting window. It is not permissible for window frames to support load and frames should only be installed after the structural support has been put in place.

Diagram C71 - Typical projecting windows structural support