6.2 External timber framed walls

MATERIALS STANDARDS

6.2 - M1	Materials standards
6.2 - M2	Timber
6.2 - M3	Timber preservation
6.2 - M4	Sheathing
6.2 - M5	Breather membranes
6.2 - M6	Cavity barriers and fire-stops
6.2 - M7	Holding down devices
6.2 - M8	Nails and staples
6.2 - M9	Vapour control layers
6.2 - M10	Wall ties and fixings
6.2 - M11	Insulation

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 for external timber framed walls including wall panels and dwellings which are substantially timber framed 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.

TIMBER PRESERVATION

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

SHEATHING

Plywood sheathing should be:

• in accordance with BS 5268 Part 6
• bonded to WBP of BS EN 636 and BS 1203
• at least 5.5mm thick
• appropriate to the exposure of the building.

Oriented strand board should be OSB3 to BS EN 300 and be at least 8mm thick.

Medium board should be in accordance with BS 1142 Type HME or HMN.

Impregnated soft board should be in accordance with BS 1142 Type SBI or SBS.

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

In areas of Very Severe exposure (as defined in Appendix 6.1- A) the wall construction should include a 50mm cavity between the sheathing and the cladding and:

• a high performance breather membrane, or
• a masonry cladding which is rendered or clad with an impervious material, or
• plywood sheathing treated by 10 minute immersion in organic solvent preservative complying with type F/N of BS 5707.

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:

• phosphor bronze
• silicon bronze
• stainless steel to BS EN 10095
• mild steel with zinc coating to BS 729 or BS 1706.

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

NAILS AND STAPLES

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

Nails for fixing sheathing, breather membranes or timber treated with CCA should be galvanised, sheradized or austenitic stainless steel, phosphor bronze or silicon bronze.

VAPOUR CONTROL LAYERS

Minimum 500 gauge polyethylene sheet or vapour control plasterboard should be used.

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

Foil backed plasterboard or the integral backing of insulation is not acceptable as a vapour control layer.

WALL TIES AND FIXINGS

Wall ties should be capable of accommodating any anticipated differential movement. The dimensions are given in Design clause D6. Only certain ties are suitable for buildings of four or more storeys.

Wall ties should comply with BS EN 845 and be of austenitic stainless steel, phosphor bronze, silicon bronze.

Materials for wall ties and their fixings should be compatible. Stainless steel, phosphor bronze and silicon bronze are compatible with each other.

INSULATION

Wall insulation should be of a type that breathes, eg mineral wool (rock or glass). Proprietary insulation systems should be assessed in accordance with Technical Requirement R3.