Timberframe housing site control
      source: TRADA

Timber frame construction is a dry lightweight method of building, easily and quickly constructed to a weatherproof stage using factory-made wall panels and roofs similar to those used with masonry construction.

Site control should ensure the quality of the construction, the adherence to specifications and a smooth construction process to obtain the full benefits of the timber frame method.

This WI sheet serves as a checklist concentrating on the most important aspects of the assembly of the timber frame and the ancillary operations involved in completing the superstructure of the building.

Building tolerances quoted are within acceptable limits using a reasonable degree of care.

A. Concrete Base and Foundations
B. Sole Plates (or Bottom Plate of Wall Panel if No Sole Plate
C. Fixing Down Sole Plates (or Bottom Plates of Panels)
D. Delivery of Components
E. Site Work on Wall Panels

A. Concrete Base and Foundations

1. Set out accurately, using a steel tape or equivalent. The timber frame panels will be to a high level of accuracy, and an accurate base will ensure simple and quick erection.
2. Lengths of wall should be within +/- 12mm. Check that diagonals are of equal length to ensure that the base is square.
3. Ensure that walls or slab supporting base plates are levelled to +/- 6mm. And the perimeter lined within +5mm - 10mm.
4. Level concrete slabs to +/- 3mm within 3.0mm length, and avoid excessing 12mm variation generally.
5. For suspended timber floors ensure that there is a dpm over the whole area of the underfloor and that there will be 150mm between blinding or oversite concrete and the underside of joists and 75mm to the underside of plates. Underfloor vents should be located at 2.0m maximum centres and 450mm maximum from corners.

B. Sole Plates (or Bottom Plate of Wall Panel if No Sole Plate

1. Check that these are pressure treated with CCA preservative where specified and brush apply preservative to site cut ends and drilled holes.
2. Ensure that there is a dpc under sole plates lapped 100mm at joints and overlapping at dpm.
3. Sole plates must be accurately aligned and form 90ºcorners. They should not overhand their support at any point by more than 10mm.
4. Sole plates must be level within +/- 5mm. Fully bed the plates on mortar (and shims if necessary) if base is not level. Bedding should not exceed 12mm thickness, other than in small localised pockets

C. Fixing Down Sole Plates (or Bottom Plates of Panels)

1. Type, number and centres of fixings to be as Schedule and inserted in accordance with specification or manufacturers' recommendations, with adequate penetration to ensure that the plate is rigidly secured to the base.
2. Check that mild steel brackets, shoes and straps are adequately protected by hot dip galvanising.
3. Replace any plates split or damaged during fixing.

D. Delivery of Components

1. Check wall frames, floor panels and trussed rafters on delivery for:
   - quality of material
   - quality of assembly (correct nailing and jointing)
   - stress grading marks/stamping
   - correctly dimensioned within stated tolerances
   - moisture content
   - preservative treatment schedules (where applicable)
   Also check any ancillary components fitted to wall frames, e.g. breather membrane, wall ties, cavity barriers, etc.
   
2. Unload and handle all components carefully
3. Stack components off ground on level dry area to avoid soiling and distortion. Protect from rainwater and ground moisture but allow adequate ventilation.

Walls

1. Lift panels on to sole plates (if provided), fix temporary bracing and loose tack.
   - Ensure that panels are correctly positioned and the right way up.
   - Plumb and line within +/- 3mm over any 2.4m length.
   - Ensure that inside faces of panels are flush.
   - Tightly butt joint panels together
   - Adjust temporary bracing to maintain accuracy.
   - Nail panels together and to sole plate (or fix to base if no sole plate) as Schedule.
2. Nail separate head binder (if required) to top of wall panels and ensure that:
   - Binders cover panel joints.
   - Binder joints occur over studs (particularly if joists are not coincident with stud positions)
   - Binders are continuous over the whole wall length.
3. Use only light metal straps to connect separating wall leaves during construction, as rigid connections will impair the sound insulation of the completed wall.

Floors

1. Ensure that walls are accurately aligned before installing floors.
2. Ensure that joist dimensions, grading, spacing and direction of span are correct in the specified location and that trimmed openings for staircases and ducts are vertically aligned, in the correct position and of the right size
3. Double joists must be correctly located in position shown, e.g. under partitions or as trimmers, and securely nailed together.
4. Ensure that metal joist hangers are of the correct type for their location and are firmly fixed in accordance with manufacturers' recommendations.
5. Avoid cutting the bottom edge of joists other than for hanger seating, or unless notching has been specified by the engineer.
6. Install blocking or strutting as specified within depth of joists and ensure that blocking is not oversize, causing distortion of the joists.
7. Ensure that solid blockings are used as fire stops between joists where they bear on the separating wall.
8. Non-loadbearing partitions parallel to, but between floor joists, must be supported by noggings at maximum 600mm centres or additional joists as specified by the engineer.

Flooring

1. Ensure that the correct thickness and type of floor decking and fixing is used in the specified location.
2. Joint all long edges of sheets over joists. All ends and edges of sheets other than tongue and groove should be fully supported on noggings. Leave minimum 10mm expansion gap round perimeter of deck.
3. Glue all tongue and groove joints if specified and ensure that all fixings are driven below the surface of the boards. Tongued and grooved softwood boards should be cramped tight before nailing down.
4. Protect installed flooring from mechanical and water damage.

Roof Framing

1. Ensure head binders are level and walls aligned before installing roof members.
2. Temporarily brace gable end wall panels and first truss.
3. Do not distort or strain trussed rafters during installation, or damage truss plates in any way.
4. Fix rafters or trussed rafters, parallel at specified centres and plumb within 10mm in 1.0m height, 15mm in 2.0, and 20mm in 3.0m height.
5. Ensure that roof bracing is installed in accordance with trussed rafter suppliers' instructions. All bracing should be fixed with a minimum of two nails at each connection.
6. Ensure that hatch openings are correctly located and formed, and that tank supports are positioned in accordance with trussed rafter suppliers' instructions.
7. With site-constructed roofs ensure that the whole roof structure is fabricated and fixed in accordance with the specification and that rafters are birsdmouthed accurately.
8. For flat roofs ensure that correct thickness and material of decking is used, and the noggings permit the ventilation specified.

Note: Trussed rafters must not be trimmed, cut or notched in any way without permission of the trussed rafter supplier or structural engineer. Do not install trussed rafters if incorrectly dimensioned, as they may distort wall panels.

E. Site Work on Wall Panels

Frame

1. Ensure that all site nailing is carried out as specified and that all bolts are fully tightened.
2. Ensure that any extra site fixed studs are correctly located to take point loads.
3. Ensure that all blockings, internal framing and noggings are correctly located and fixed flush.
4. Clear rubbish and timber offcuts from all cavities to be closed in, e.g. separating walls, framed walls and suspended floors.

Insulation

1. Tightly fit insulation between all wall framing members avoiding any gaps. Fix insulation in position if "push-fit" is inadequate to hold material in place.
2. Ensure that there is an adequate overlap between ceiling insulation and wall panels to avoid a cold bridge, but ensure that ceiling level insulation does not block roof ventilation.

Vapour Control

1. Do not fit vapour control layer if moisture content of framing is above 20%.
2. Check that any separate polythene vapour control layer is of "virgin" and not reconstituted material.
3. Fix separate polythene membrane and lap 150mm at all joints. Ensure that joints occur on studs or rails. Return membrane into reveals, head and sill of all openings.
4. Avoid puncturing or splitting separate membrane during installation, and repair all holes and tears with adhesive PVC tape.
5. Check that any vapour control layer bonded to the plasterboard lining is undamaged, or repaired, before the lining is fixed.

Services

1. Do not notch any studs to accommodate service runs and ensure that notching of floor joists and drilling of studs and joists is within limits
2. Do not run plumbing in external walls.
3. Ensure that all penetrations of the vapour control layer for services are tightly sealed by PVC tape or mastic.
4. Where services have to be located in separating walls, ensure that the necessary additional precautions are taken, e.g. additional backing plasterboard to outlets, to avoid any reduction in the fire and sound insulation of the wall.

Note: No services may be run in separating walls in Scotland.

Linings

1. Plasterboard must be kept clean, dry and flat during storage.
2. Test all plumbing and service connections in walls before linings are fixed.
3. Ensure that the correct specification and thickness of board is used in the right location, e.g. fire resistant, "vapour check" board, etc.
4. Following manufacturers' recommendations for all fixings and jointing, including staggering joints when more than one layer is to be used.
5. Ensure there is an adequate nailing surface behind all sheet edges and that all edges of plasterboard are supported.
6. It is recommended that ceilings are lined before walls.
7. Openings for electrical outlets should be accurately cut to the shape and size of the outlet boxes, and where plasterboard with an integral vapour control layer is used, any gap between plasterboard and outlet should be sealed with mastic.

Breather Membrane

1. If fitted on site ensure that the breather membrane is of the correct specification and is fixed and lapped in accordance with manufacturers' instructions.
2. Additional PVC strips stapled over the breather membrane at stud positions will reinforce the membrane and simplify the correct location of wall ties.
3. Damaged areas of breather membrane should be repaired locally, with the replacement material lapped correctly around the existing material and securely stapled.

Cavity Barriers and Fire Stopping

1. Locate cavity barriers in positions specified and ensure tight joints where end of cavity barriers are butted together.
2. Sleeved mineral wool cavity barriers should be fixed with stainless steel staples at spacings recommended by the manufacturers.
3. Timber cavity barriers should be of treated timber and cut ends should be brush coated with preservative. A separate dpc should be fitted on the outside to protect the timber and a strip of mineral wool between the dpc and masonry will ensure a tight seal. Horizontal timber cavity barriers should be protected by a dpc above, overlapped by the breather membrane.
4. Ensure that fire stops across the end of separating walls are correctly fitted to close the cavity between frames.
5. Ensure that fire stops are fitted over separating walls under the roof and across the eaves void and that the gaps between tiling battens are filled with mineral wool.

Damp-proof Courses and Cavity Trays

1. Fix dpc's around openings and return behind window and doorframes, lapping to ensure that any water is diverted to outside.
2. Ensure that horizontal dpcs are dressed up behind breather membrane and lapped minimum 100mm
3. Dress flashings over cavity trays where these are installed.

Brick or Block Cladding

1. Ensure that the cavity between masonry and sheathing is a nominal 50mm and that this is maintained within +/- 15mm tolerance.
2. Keep cavities clear of mortar droppings.
3. Nail wall ties securely to timber studs at specified centres, and fully bed ends of ties in mortar joints.
4. To ensure ventilation and drainage of cavity provide open perpends at minimum 1500mm centres at base of wall, at horizontal cavity barriers, and over lintels (minimum 2 open perpends over window or door)
5. Ensure that there is sufficient clearance between window sills, soffits, verges, balconies, abutments etc, to allow for long-term shrinkage of timber frame.
6. Ensure that all cavity fire barriers adequately seal cavities.
7. Ensure that all steel lintels are adequately protected from corrosion, solidly bedded, and clipped back to the timber frame (if this type is employed). Ensure that breather membrane overlaps back of lintel if no flashing is used.

Other Claddings

1. Fix preservative treated battens and counter-battens as specified. Cut ends should be brush coated with preservative.
2. Ensure that moisture content of timber cladding is in accordance with specification before fixing as specified.
3. Provide sufficient gaps for differential movement between cladding and timber-frame at intermediate floors, and at junctions between light cladding and masonry.
   

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