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Cladding for timber frame buildings

Cladding for timber frame buildings

Cladding for timber frame buildings

Thermowood Horizontal cladding Redwood D S/F Excl 6th PMV

Cladding for timber frame buildings

This Wood Information Sheet (WIS) describes the main types of cladding for use on timber frame buildings with insulation between studs and an external cavity wall to provide ventilation for the timber frame and drainage of any water that may penetrate the cladding. It outlines the broad requirements of building regulations to assist in the correct specification and installation of external cladding on timber frame structures.

Thermowood Vertical cladding Redwood D S/F Excl 6th
Thermowood Vertical cladding Redwood D S/F Excl 6th PMV

Building regulations differ nationally so check local requirements. This WIS is based on the England and Wales requirements for buildings of less than 18 metres, as timber frame buildings seldom reach over six storeys.

More detailed guidance on the design and detailing of different types of cladding for timber framed buildings is included in the Exova BM TRADA publication Timber frame construction [1]. Further guidance on timber cladding can be found in the Exova BM TRADA publication External timber cladding [2].

This Wood Information Sheet (WIS) is an overview of the subject, with signposts to more detailed sources that are listed at the end.

We recommend Eurocode 5 Common rules and rules for buildings
[3] for structural design of timber. However, this WIS includes a summary of variations when using BS 5268-2 Structural use of timber. Code of practice for permissible stress design, materials and workmanship [4], which the BSI has withdrawn.

Contents
• Design requirements
• Self-supporting cladding
• Non self-supporting cladding

Figure 1: Timber frame house with a combination of vertical and horizontal timber cladding and rendered cladding
Photo: Copyright Scotsframe

Whilst steel posts and beams can be used to support this cladding, cost, order of works and weatherproofing mean that this option is not normally practical. Therefore we recommend that lightweight cladding be used and supported by the timber frame. Consider also the requirements for the surface spread of flame (reaction to fire) performance and provision of appropriate fire resistance from outside to the upper building. If the external wall above the step is formed by one of the party wall leafs, consider thermal performance requirements, stud sizes and thermal insulation. It may be necessary to alter the party wall stud sizes to accommodate insulation.

Differential movement
Differential vertical movement can occur between the timber frame inner leaf and self-supporting brick/block cladding.
Contributing factors include shrinkage due to reduction in the moisture content of horizontal timber elements (such as rails, binders and floor and roof joists), slight expansion of clay bricks, compression of the timber studs into rails, and settlement of construction gaps. Therefore, any material or component attached to the timber superstructure and which overhangs the brick or blockwork (such as attached cladding, cantilevered floors, window sills, roof eaves and verges) or penetrates through the masonry (such as flues, overflow pipes, services or balconies) should have a clear gap beneath it and the top of the masonry cladding below to allow differential movement to take place.

For a typical softwood timber element, allow a 1% shrinkage across the grain for every 4% reduction in moisture content. This effectively means that 1mm should be allowed for differential movement for every 38mm of horizontal solid timber. Historically, this measurement has been shown to be adequate to allow for settlement of the timber frame after loading, drying out of the timber frame and potential expansion of the external brickwork due to thermal changes. Note that some warranty providers specify larger gaps than those specified in this WIS.

The formula using the 1% for 4% rule does not apply to engineered wood products, such as LVL, prefabricated I-joists and metal web joists. Obtain the rule for % dimension change for % moisture content change from the product manufacturer and make allowances for compression and settlement of the construction and variations depending on construction quality. Typically, 2mm–3mm of differential movement allowance for compression and settlement of floor joists that use engineered wood products will be sufficient. Shrinkage of the solid timber horizontal plates and rails must still be allowed for.

In this wall type, the vapour permeability of each construction element is of the utmost importance, including each layer in the element, such as foil facings on rigid insulation materials, for a condensation risk analysis to ensure that no interstitial condensation will develop in the timber frame elements. We recommend relevant third party approval for this wall type.

Self-supporting cladding

Brick and block
A brick outer leaf is the most popular type of cladding used in the UK as the aesthetic requirement for a ‘traditional’ appearance can be achieved at the same time as utilising the advantages
of modern timber frame construction.

Brickwork should comply with:
• BS EN 1996-1-1 Eurocode 6. Design of masonry structures General rules for reinforced and unreinforced masonry structures [7]
• BS EN 1996-2 Eurocode 6. Design of masonry structures Design considerations, selection of materials and execution of masonry [8].

Brickwork can be self-supporting up to six storeys with appropriate bricks/mortar and wall tie selection. The timber frame test building, TF2000, provided evidence of successful masonry construction to this height. Special differential movement details and bed joint reinforcement may be needed. Consult the Brick Development Association on any design above 12m.

The dimensions for a standard brick are: 215mm x 65mm x 102.5mm (plus mortar joint 10mm.) These dimensions are important in order to determine the overall wall thickness and opening sizes.

Correctly detailed brick or block cladding is capable of contributing to the timber frame structure’s resistance to wind loads. Annex D of PD 6693-1:2012 Recommendations for the design of timber structures to Eurocode 5 [11] indicates how this contribution can be calculated.

Both cladding types support their own weight but are laterally restrained by the timber frame structure using wall ties.

Although now withdrawn, Table 6 in BS 5268-6.1 gives minimum wall tie density when masonry cladding is to contribute to racking resistance. We recommend that a minimum of 4.4 wall ties per m2 of external wall should be used. To satisfy this, ties should be on
stud centres horizontally and at a vertical spacing of 375mm (5 brick courses) when the timber frame studs are at 600mm centres and at 525mm vertical centres (7 brick courses) when studs are at 400mm centres. In exposed locations and around openings, the tie spacing may need to be reduced. Wall ties must be fixed into structural timber, such as studs and header joists, and not fixed to the sheathing only. Stud locator tapes or markings fitted over the breather membrane by the timber frame manufacturer help in identifying stud locations.

The top of brickwork cladding needs restraint and the top row of ties should be located three courses below the top of the cladding. At sloping verges, ties should be located within 225mm of the top of the brickwork at every fourth course down the slope. Ties should be fixed at the sides of window and door openings spaced at not more than 300mm vertical centres and within 225mm of the jambs.

vertical movement joints may need to be included to avoid shrinkage cracks occurring in the width of the cladding. It is important that the type of movement gap can allow all anticipated differential movement to occur without resulting in damage to the render and maintaining adequate cavity ventilation. Normal render beads are not typically adequate to allow the relatively large movement which occurs at timber frame floor zones. Clear gaps or specialist beads may need to be used.

Tile or slate cladding
Vertical tile hanging using concrete or clay tiles, or natural or fibre cement slates can provide a long life, low maintenance cladding to timber frame structures. Mathematical tiles can provide a cladding similar in appearance to brickwork, but are not commonly used. Figure 6 shows a typical arrangement. Tiles or slates are normally fixed to horizontal preservative-treated timber battens. In severely exposed conditions, we recommend the use of vertical counter battens fixed between the horizontal battens and the breather membrane to permit more efficient ventilation and drainage of water that may penetrate the cladding.

Metal sheet cladding
As with other types of cladding, maintain the cavity between the timber frame and the external cladding to provide sufficient
ventilation and drainage of water that may penetrate the cladding.
Metal sheet cladding should not be placed directly over the outer face of the timber frame sheathing. Allow for differential
movement at floor zones and between the metal cladding and any other type of cladding that is to be installed.

References
1. Lancashire, R. and Taylor, L., Timber frame construction, 5th edition, ISBN 978-1900510820, TRADA Technology, 2011
2. External timber cladding, 3rd edition, ISBN 978-1909594005, TRADA Technology, 2013
3. BS EN 1995-1-1:2004+A2:2014 Eurocode 5. Design of timber structures. General. Common rules and rules for buildings, BSI
4. BS 5268-2:2002+A1:2007 Structural use of timber. Code of practice for permissible stress design, materials and workmanship, BSI [WITHDRAWN]
5. BRE Report 454: Multi-storey timber frame buildings: a design guide, ISBN 1860816053, BRE, 2003
6. England and Wales Building Regulations: Approved Document B (Fire Safety), Volume 1 – Dwellinghouses and Volume 2 – Buildings other than dwellinghouses, NBS, 2013, available at www.planningportal.gov.uk
7. BS EN 1996-1-1:2005+A1:2012 Eurocode 6. Design of masonry structures General rules for reinforced and unreinforced masonry structures, BSI
8. BS EN 1996-2:2006 Eurocode 6 Design of masonry structures Design considerations, selection of materials and execution of masonry, BSI
9. BS 5628-1:2005 Code of practice for the use of masonry. Structural use of unreinforced masonry, BSI [WITHDRAWN]
10. BS 5628-3:2005 Code of practice for the use of masonry. Materials and components, design and workmanship, BSI [WITHDRAWN]
11. PD 6693-1:2012 Recommendations for the design of timber structures to Eurocode 5: Design of timber structures. Part 1 General – Common rules and rules for buildings, BSI
12. BS 5268-6.1:1996 Structural use of timber. Code of practice for timber frame walls. Dwellings not exceeding seven storeys, BSI [WITHDRAWN]
13. BS 5268-6.2:2001 Structural use of timber. Code of practice for timber frame walls. Buildings other than dwellings not exceeding four storeys, BSI [WITHDRAWN]
14. BS EN 13914-1:2016 Design, preparation and application of external rendering and internal plastering. External rendering, BSI

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