6.2 External timber framed walls

MATERIALS STANDARDS

Materials that comply with the design and the guidance below will be acceptable for external timber framed walls including wall panels and dwellings which are substantially timber framed.

Materials should comply with all relevant standards, including those listed below. Where no standard exists, Technical Requirement R3 applies (see Chapter 1.1 'Introduction to the Standards and Technical Requirements').

References to British Standards and Codes of Practice include those made under the Construction Products Directive (89/106/ EEC) and, in particular, appropriate European Technical Specifications approved by a European Committee for Standardisation (CEN).

TIMBER

All structural timber should be:

• of a suitable grade in accordance with BS 5268 and
• dry graded and marked in accordance with BS 4978.

I-studs should be assessed in accordance with Technical Requirement R3.

TIMBER PRESERVATION

Timber preservation should be in accordance with Chapter 2.3 'Timber preservation (natural solid timber)'.

I-studs manufactured from timber of durability class 'moderately durable' or lower should be preservative treated in accordance with Chapter 2.3 'Timber preservation (natural solid timber).

SHEATHING

The following materials are acceptable:

• plywood in accordance with BS 5268-2 or BS EN 13986 table 7
• oriented strand board in accordance with BS EN 300 type OSB/3 or 4
• moisture-resistant chipboard in accordance with BS EN 312 type P5 or P7.
• medium board in accordance with BS EN 622-3 type MBH.HLS1 or MBH.HLS2.
• impregnated soft board in accordance with BS EN 622-4 type SB.HLS.

Proprietary sheathing materials should be assessed in accordance with Technical Requirement R3 and used in accordance with the assessment.

BREATHER MEMBRANES

Breather membranes should be:

• vapour resistant to less than 0.6MNs/g when calculated from the results of tests carried out in accordance with BS 3177 at 25°C and relative humidity of 75%
• capable of resisting water penetration
• self extinguishing
• durable
• adequately strong when wet to resist site damage
• Type 1 to BS 4016 in areas of Very Severe exposure (unless the alternatives given in Design Clause D4(c) are adopted).

CAVITY BARRIERS AND FIRE-STOPS

Materials specified in statutory requirements are acceptable.

Other materials may be used if satisfactorily assessed in accordance with Technical Requirement R3.

HOLDING DOWN DEVICES

Holding down devices should be manufactured from:

• austenitic stainless steel to BS EN 10088-1, minimum grade 1.4301
• galvanised mild steel with zinc coating to BS EN ISO 1461, minimum coating 940 g/m² on each side.

Sole plate anchors within the internal envelope should be galvanised mild steel, minimum coating Z275.

All holding down devices should be as detailed in the design.

NAILS AND STAPLES

Staples for fixing breather membranes should be austenitic stainless steel or other material of similar strength and corrosion resistance.

Nails for fixing sheathing or timber should be galvanised, sheradized or austenitic stainless steel.

VAPOUR CONTROL LAYERS

Vapour control layers should be 500 gauge (120 micron) polyethylene sheet or vapour control plasterboard.

Vapour control products manufactured from recycled materials should be assessed in accordance with Technical Requirement R3.

WALL TIES AND FIXINGS

Wall ties and their fixings should be of austenitic stainless steel, comply with BS EN 845 and be capable of accommodating the anticipated differential movement given in Appendix 6.2-C.

INSULATION

Wall insulation should be of a type that 'breathes', eg mineral wool (rock or glass). Other insulation used in walls should be assessed in accordance with Technical Requirement R3 for use in timber frame wall panels.