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Subject No: 17 We frequently receive applications (typically loft conversions and extensions of roof space), where the agent specifies external stud walls with a variety of cladding systems. One such system incorporates rendering (cement and sand based) applied to metal lathing onto breather membrane onto studding. Sometimes (usually for structural reasons) external quality ply is fixed to the face of the studs onto which the breather membrane and the metal lathing is fixed to receive the render. The issue I am raising is whether the rendering systems as described above meet the requirements of AD C4 Attached is a copy of the relevant part of C4 - Section 5 Cladding for external walls etc. Note specifically 5.4 Also attached is an extract from BS 5262:1991 Cl 38.7 which gives recommendations for timber framed constructions which relates to the cases I am referring to. The key issue probably comes down to degree of exposure/risk when determining an acceptable form of construction and the potential for damage as described in AD cl 5.1 Generally in situations of high exposure I would advocate the method described in Cl 38.7.3. of the BS ie; an open drained cavity behind the render - a system that I have seen used successfully and which is not difficult to do. This report was unanimously agreed by the Committee at its meeting on the 4th December 2003 Technical Statement : External Rendering Systems used to form a weather resisting cladding should be constructed in accordance with BS: 5262:1991 cl 38.7.3 ie : incorporating an open drained cavity. In areas where the cladding is likely to be subjected to high exposure to wind driven rain this method should be adopted.
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