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Timber Frame Construction

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Outline and purpose

Timber frame installers should have a checklist for construction to ensure that their installation work is compliant with the instructions and the specifications provided by the timber frame manufacturer for each aspect of construction. The checklist should be fully assessed before starting the job and any issues that arise should be taken directly back to the timber frame manufacturer to gain their advice and guidance on a solution or variation that is required to continue with compliant work.

This section outlines the key points in the construction process where these checks should be carried out. This list is not all-inclusive and should be used in addition to any checklist provided by the timber frame manufacturer.

Member alteration and structure exposure

No member of the timber frame panels should be altered on site without the consultation and written approval of the timber frame manufacturer.

Exposure to the elements poses a high risk to the timber frame structure, what provisions are in place to protect the panels when delivered to site and stored, and for when they are installed in position.

Foundations

Provide foundations to all chimneys and load-bearing partitions. Internal wall foundations must be of the same depth and thickness as external wall foundations.

Width: Minimum width of foundation to be three times the thickness of the wall it is supporting.

Depth: Minimum depth of foundation excavation below finished ground level 600 mm.

Thickness: Minimum concrete thickness 300 mm.

Different foundations may be required depending on the building, ground condition, load requirements etc. For specialized foundations, a qualified engineer should be consulted for the design and inspection of foundation after construction. Large elements may also require increased size foundations (e.g. chimneys, columns etc.).

Substructure dimensions

The following substructure dimensions should be adhered to:

  • Proprietary vents/weepholes should be located at 1.5 m centres.

  • The level for the substructure walls should have a max deviation of ±5 mm.

  • A clean cavity drain should extend 150-225 mm below the level of the DPC.

  • Straight lengths of substructure wall should be within ± 10 mm maximum deviation.

  • Diagonal dimensions should have the following maximum deviation:

    • Lengths up to 10m ± 5 mm.
    • Lengths greater than 10m ± 10 mm.

Sole Plate and Bottom Rail

The following should be noted:

  • Sole plate and bottom rail should be level and within ±5 mm.

  • Wall vents and weepholes should be unobstructed.

  • Accurate setting out must be adhered to in order to ensure complications during later construction are avoided.

  • Material should not be damaged and if sole plate is damaged whilst in position it must be replaced.

  • Where packing is required it must be continuous with no gaps, using mortar or slate with a maximum thickness of 15mm.

  • This should be secured to the substructure walls as per the manufacturer’s instructions. Similarly, they can be secured through use of 4 mm dia. stainless steel masonry nails at a maximum of 300mm centres; the nails must be long enough to provide a minimum of 50mm embedment into the substructure. Alternately to the nails, stainless steel anchor clips may be used with a maximum spacing of 1.2m centres.

  • Where overhangs and ledges are unavoidable, they should not protrude more than 10m.

  • Sole plate must be protected by breather membrane.

  • DPCs must extend over the full area of the ground floor timber wall panels.

  • DPCs that are damaged must be repaired or replaced.

  • DPCs must be properly lapped at joints between the ground floor wall frames.

  • Wall panels must be installed vertically on the sole plate with a 10 mm tolerance between the eaves and the bottom of the panel.

  • Cavity drains must be located between 150-225 mm below the level of the DPC must be free from any obstruction.

  • A compressible seal must be installed between the sole plate and bottom rail or between the sole plate and the top of the substructure wall.

  • Any anchor straps that are used must be marked with the manufacturer's name and type of anchor for identification.

  • The timber frame manufacturer will specify the required fixings and depth of masonry cover.

  • Strap bridges must be embedded into the horizontal mortar bed joint below the DPC level.

  • Anchor straps stall be installed at openings and corners created between panels on full height studs intermittently at 1.6 m max centres for 400 mm stud centres and 1.8 m max for 600 mm max centres. The timber frame manufacturer may specific another fixing method other than anchor straps.

  • Timber frame pieces that are cut on-site need to be pre-treated with an appropriate preservative before installation.

  • A DPC should be provided under the entire surface of the DPC; the minimum lap that is allowed between sheets of DPC is 100 mm. Subsequently, the DPM subsequently should be sufficiently lapped underneath the DPC.

  • The minimum cavity width should be 50 mm ±10 mm.

  • Insulation that is damaged or wet should be removed and replaced.

  • The vapour control barrier must be correctly installed to create a seal around services passing through opes.

  • Excess mortar and debris from the masonry leaf construction must be removed from the cavity, wall ties, cavity barriers, etc.

  • Wall ties should be installed horizontally at a maximum 405 or 605 mm centres depending on the centre of studs, and vertically at a maximum of 450 mm centres (225 mm around opes).

  • Damaged breather membrane should be replaced or repaired.

  • Breather membrane should be correctly installed and wrapped into reveals.

  • Breather membrane should have a minimum lap of 100 mm vertically and 150 mm horizontally.

  • A minimum of 50 mm lapping is allowed between the breather membrane and the DPC.

  • All nail fixings should be in accordance with the site fixing schedule.

  • Panels are required to be installed with internal faces flush and tightly jointed at edges.

  • Lintels must be support by pre-designed cripple studs.

  • Shrinkage gaps are required at any opes through the studwork frame.

  • Extra studs should be installed as per the manufacturer's instructions to support heavy loads.

  • Cavity barrier size should be appropriate for the size of the cavity.

  • DPCs are to be lapped and sealed around all external opes, sills, lintels, etc.

  • Timber cavity barriers installed horizontally should be protected by the DPC.

  • Damaged vapour barrier should be repaired or replaced.

  • The vapour control barrier should have a minimum lap of 100 mm vertically and 150 mm horizontally.

  • The vapour control layer should not installed where a moisture content of 18% or higher is in the timber frame components.

  • The correct position of all fire stops is vital to ensure no gaps
    are present.

  • Mortar should be used to seal any gaps between the masonry leaf and the timber cavity barrier.

Fireplaces and Chimneys

The following requirements should be met with respect to the installation of fireplaces and chimneys:

  • Timber cavity barriers should be adequately protected by a DPC.

  • Timber cavity barriers should be a tight fit to ensure no gaps are present.

  • Combustible materials should be kept a minimum of 200 mm away from the chimney flue and 40 mm from the outer surface of the chimney.

  • The movement gap between the chimney and the timber frame should be packed with a non-combustible material.

  • Party wall plasterboard should run behind the proprietary chimneys in the party wall.

  • Proprietary chimneys should always be installed as per the manufacturer’s instructions.

  • A gap should be left between the proprietary chimney and the timber frame; this gap should be filled with a non-combustible material.

Party Walls

The following requirements must be met with respect to part walls:

  • Two layers of plasterboard should be installed with a minimum overall thickness of 30 mm: normal configuration is 12.5 mm plasterboard laid over 19 mm plasterboard with the joints staggered in between.

  • Each layer of plasterboard requires its own independent fixing.

  • The construction of any masonry walls should be compatible with the timber frame system being installed.

  • Firestops to be installed in the correct position (e.g. eaves level, wall junctions, etc.).

  • Sheathing is to be installed a minimum of 600 mm at the junction of the party wall and external wall.

  • Proprietary chimneys should be installed as per the manufacturer’s instructions.

  • No services should be located within the party wall cavity or penetrate party wall linings.

  • There should be no gaps or perforations in the party wall.

  • The overall width of the party wall inclusive of the 2 party wall frames should be a minimum of 220 mm. No matter what width is achieved overall, a gap of 40 mm must be maintained at all times.

  • Sound insulation must be properly secured to ensure that there is no sagging creating gaps.

  • Party walls must not be connected by any means. The only exception is the use of metal ties at 1.2 m centres installed to provide temporary stability during construction.

  • A non-combustible building board must be secured to spandrel frames at roof level and cover the party wall.

Floors

The following requirements must be met with respect to floors:

  • Floor panels should be correctly aligned with wall panels.

  • Joists must line directly up with wall studs unless a head plate is provided to support the joists.

  • Sheathing required over header joists to close any gaps between the floor panel and the wall panel.

  • There should be no gaps between floor panels and any wall panels.

  • Floor joists should only be notched or drilled in accordance with the timber frame manufacturer’s instructions.

  • One stud must only support one joist unless otherwise designed.

  • Bridging should be provided between joists to support partition walls.

  • Header joists are to align correctly with the top of wall panels and not protrude into the cavity.

  • Floor joists around chimneys and fireplaces are required to be trimmed.

  • Floor joists should have adequate bearing on the timber frame walls.

  • Joist hangers to be secured to the timber frame wall panels.

Floor Decking and Roofs

Floor Decking

  • Short edges must be staggered.

  • Full support must be provided to butt joints.

  • Decking must be of sufficient thickness.

  • Decking should be fixed in accordance with the manufacturer’s instructions.

Roofs

  • Damaged roof underlay must be replaced or repaired.

  • Lateral restraints must be installed where necessary.

  • Wall bracing must be fixed to spandrel panels.

  • Trusses must be supported by individual studs unless a head plate is installed.

  • Trusses must have a maximum spacing of 600 mm centres, be appropriately trimmed, plumb, and be effectively braced.

  • Non-combustible building board must be used to fill the gap at the top of the party wall.

  • A shrinkage gap must be left between the masonry leaf and roof structure at the verge and eaves level.

  • The water cistern, if located in the roof space, must be adequately supported, with extra timbers installed if necessary.

Internal work before dry-lining

The following should be complete prior to dry lining:

  • Vapour control layer should be a minimum of 75 mm dressed into reveals.

  • Vapour control layer should be of material as specified by the timber frame manufacturer.

  • Correct installation should be adhered to for all party wall firestops and insulation.

  • Insulations should be installed as per the manufacturer’s instructions ensuring correct fixing and coverage to allow no gaps.

  • The timber frame structure should be dry and fully weather-tight before any of the internal works begin.

Cavity Barriers and Fire Safety

Cavity Barriers

  • All cavity barriers must have a minimum fire rating of 30 min.

  • Cavity barriers are required at these locations:

    • All opes in the timber frame structure.
    • At the top of each timber frame panel at eaves level.
    • At ceiling level in the party wall.
    • At the junction of the external walls and party walls.
  • Cavity barriers should be marked with the manufacturer’s name and the basic specification of the cavity barrier.

  • Cavity barriers should be overlapped a minimum of 150 mm at joints both horizontally and vertically.

  • Cavity barriers should not be taken around corners or services in such a way that may cause damage or gaps to occur internally.

  • Austenitic stainless steel staples or nails should be used to secure cavity barriers to the timber frame and should be protected suitably by a DPC to protect from moisture.

  • The cavity barrier should be relative to the external cavity width.

  • DPCs protecting cavity barriers should have a minimum 1000 polythene gauge.

  • Solid timber cavity barriers should have a minimum dimension of 38 mm.

Firestops/Cavity barriers are required:

  • Horizontally within the party wall at the level of each floor.

  • At the top of the party walls.

  • At junctions between the external walls and the party walls.

Notes:

1. Timber cavity barriers that are cut on-site are to be tight-fitting with no gaps and cut ends should be treated with preservative.

2. Mortar should be used to fill any gaps between the masonry leaf and the cavity barrier.