Introduction

This is an STA guide book offering generic advice on the assembly of the SIP, timber frame, timber floors, roof components and follow on trades, such as dry lining and cladding. A successful project requires every trade in the construction process to deliver the right quality work for the project’s design objectives. Good projects start with good designs, and a good design is one that is matched to the client’s expectations, both in cost and performance.

STA member SIP panels are factory manufactured to standards which are third party audited. There are many different SIP product types, each with their own third party certification, presenting specific performance and construction details. The designer and constructor should consult the SIP company on specific details that may vary from this generic guide. This guidance concentrates on site work – the accuracy of which is a vital link in ensuring the success of our buildings.

This pocket book presents best practice site checks to deliver good build quality for low rise domestic and commercial SIP buildings. The structural method covered is for SIP buildings constructed using the platform frame method of construction, with SIP external wall panels, timber frame internal wall panels, timber floor joists and SIP roof panels.

The principles can be applied to other types and methods of timber buildings – such as pre-insulated and closed panel frame systems. This digitial version has been done with the support of Swedish Wood.

This guide is of use to:

• Project managers
• Frame erectors
• Site managers
• Site inspectors
• Trade trainers
• Building control surveyors

This guide provides:

• Information for the coordination of successful projects
• Design-to-site details
• The build sequence linked to a checklist of what to look for
• Best practice guidance on the conceptual nature of principal details
• Reference papers and further reading

Using the Guide

This guide is not a substitute for the drawings, specifications or standard details issued with your project.

This guide complements the specific project design and specification. With SIP panels, the third party certification will provide specific details that override the generic advice in this guide. Where differences occur, the project design should take precedence. This is assuming that the frame design has been undertaken by an STA member, where the competence of the STA design team is part of the membership quality process.

Persons using the guide should be familiar with construction terms and have access to additional, more detailed literature.

How information is presented in this guide

Coordination Checklist

Use the checklists below to ensure successful completion of SIP projects. Reference papers and further reading is provided at the end of this guidance.

Build Sequence – What to Look For

The build sequence below outlines the key stages of SIP construction and what to check at each stage. Click on each stage for full details below.

Stage 1 – 2 Weeks Before Work Starts

✓ 2 Weeks Before Work Starts

• A full drawing package
• Detail booklet and erection instructions
• Nailing / screwing schedule
• Fire plan, Health & Safety plan and STA Site Safe correspondence
• Review plans, establish build methodology and check against standards
• Pre-start meeting completed
• Craneage and scaffolding agreed
• Agreed build programme

Stage 2 – 1 Week Before Frame Delivery

✓ 1 Week Before Frame Delivery

• CDM 2015 information provided — see STA Advice Note 9
• Top of substructure is level within tolerance
• Top of substructure is square and diagonals within tolerance
• Foundations are dimensionally correct
• Problems reported and rectified
• Scaffolding completed
• Access and plant off load available
• Crane hardstanding agreed
• Storage space available
• Check substructure against standards

Stage 3 – Upon Delivery

✓ Upon Delivery

• Check all components delivered
• Check for damage to frame
• Report shortages / damage
• Sign for goods received

Stage 5 – During Erection

✓ During Erection

• Take care to avoid damage
• Follow drawings, details and standards
• Ensure temporary erection bracing is provided in accordance with STA guidelines
• Where tall wall panels are required, check with engineer if any special measures are required for temporary stability
• Check floors are not overloaded by materials. Report to frame engineer if damage found
• Ensure safe systems of work are implemented
• Flooring is protected or cleared of excessive moisture
• Check panels are correctly nailed and secured
• Plan work to be progressed systematically, floor-by-floor
• Tidy up as you go – see STA 16 Steps
• Complete work per floor level – do not drop back later
• Ensure scaffolding progresses well ahead and safely – do not modify without authority

Stage 6 – Upon Completion of Erection

What to Check Generally

✓ General Checks on Completion

• Frame is anchored to slab
• All damage is repaired
• Check external cladding and cavity width requirements – cross reference to the design drawings
• Check that the structure is wind and weathertight – breather membrane is fully wrapped around the building
• Flashing and cavity trays are installed and lapped correctly with the breather membrane
• Windows and doors fitted prior to commencing internal work
• Structural shell is handed over and signed for, complete with Site Safe letters

Wall Construction to Check

✓ Wall Construction Checks

• DPCs are under all ground floor walls in contact with slab
• Cavities are clear and properly ventilated
• Panels are the right way around, so that stud groups or posts are in the correct location
• All joints are aligned and tight to tolerance
• All fixings as per schedule / specification
• Breather membrane laps are present and repaired if necessary
• Under beam loads – check for SIP spreader rail or post
• Partitions are plumb and square
• Vertical DPCs are fitted to all external openings
• Locating plate and headbinders fitted
• Compartment wall straps fitted to internal timber frame wall leafs per storey height, as close to floor and ceiling diaphragm as possible at minimum 1200mm centres or as fixing schedule
• External ground levels are at least 75–150mm below the lowest timber level. Less than 150mm requires drainage details

Floor Construction to Check

✓ Floor Construction Checks

• Flooring is protected or cleared of excessive moisture
• Joists are in accordance with design drawings, with decking correctly screwed or nailed to them – do not rely on adhesive alone
• Joists have adequate bearing – cross reference to the design drawings. Typically minimum 45mm, but some joist hangers and support conditions may require more
• Open web joists are braced using strongbacks and noggins are installed above and below partitions – cross reference to the design drawings
• Joist connections are nailed and tight as per design
• Joists are level and even
• Stair is trimmed correctly with fixings to the design
• Engineered timber joists are not to be modified or notched – if there is a clash then ask
• Joist hangers are fully nailed and close fitting – check joist hanger bearing and size compliance, do not use oversized hangers
• No excessive loads are applied to the floor, for example dry lining board stacks
• Timber packing plates are installed over walls between the top chords of top chord supported open web joists – refer to drawing

SIP Roof Construction to Check

✓ SIP Roof Construction Checks

• Profiled timber wallplates or headbinders, if required, are fixed into the SIP wall panels and onto purlins in accordance with the fixing schedule to resist uplift
• Roof panels are prevented from sliding down the roof in the temporary condition prior to final fixing
• Final fixings are installed in accordance with fixing schedule to resist sliding and uplift
• Where a tied roof is required, ensure that thrust blocks are fixed in accordance with fixing schedule
• Where valley purlins are designed to resist the thrust from a tied roof, ensure that valley purlins are not subjected to out of balance loading prior to removal of temporary works
• Where parapet panels are required check that they are fixed in accordance with the fixing schedule
• Roof is watertight before starting internal work
• Spreader rails or posts are fitted to ensure the continuity of vertical load paths to foundations under concentrated loads
• Reinforcing timbers around rooflights, dormers and openings are installed correctly
• For pitched roofs, a breather membrane is fitted to the top of the SIP before the counter battens, to create a ventilation void of at least 50mm
• Where truss rafters or cut roofs are used, check that adequate cross ventilation is provided at eaves and ridge using vent ducts or breather membranes with third party accreditation
• Where omission has not been proven and justified, fit VCL to the internal face of the SIP – air barriers are not VCL

Stage 7 – Before Dry Lining

✓ Before Dry Lining – General Checks

• Frame moisture content is less than 20% and watertight before fixing any additional insulation and dry lining
• Cavity barriers are fitted to separating floors and walls
• Any additional insulation to internal and external walls is correctly fitted with no gaps
• If vapour control drylining is used, or the SIP supplier has an appropriate assessment, then the vapour control layer (VCL) can be omitted

✓ If Fitting Polythene Based VCL – Check That:

• It is the correct density
• 100mm laps at all joints
• It is fitted to warm side of insulation
• Holes for services are neat, tidy and taped to specification
• Splits etc. are repaired
• Correct sealing around socket boxes
• Vapour control layer or vapour control dry lining board is fitted to all external walls and laps to floor and roof junctions

Services to Check Before Dry Lining

✓ Services Checks

• For services in internal timber frame walls, notching or drilling as per details
• Noggins / dwangs fitted as required
• Vapour control layer (VCL) fitted – or SIP supplier has appropriate assessment to omit the VCL
• Insulation is still in place
• Check all service holes in vapour control layer are neat and tidy
• No timber contact with flues or chimneys – a suitable clearance air gap should be provided
• Services in separating walls are protected so they do not affect the sound / fire performance of these walls
• Avoid services in separating walls if possible
• No cables except service tails are located in external cavities
• Surface-mounted voids are created for services against SIP walls
• All first fix services installed as drawings

Stage 8 – After Dry Lining

✓ After Dry Lining Checks

• Lining is fixed to manufacturer’s instructions with nails or screws not overdriven
• Installation matched to standards
• All joints sealed with filler, tape or skim and jointing compound

Best Practice Advice Notes

The following sections provide detailed best practice guidance for each stage of SIP construction. Defect warnings are highlighted in red, checklists in green and best practice notes in purple.

Substructure

Check Dimensions

Measure diagonals. If they are equal, the base is square.

Sole Plates

The sole plate is the first level of timber on a project. The tolerance of the setting out of the sole plate and its fixing can influence the complete building performance. It is therefore essential to get this right.

Correctly set out the sole plate. For block up stands use a minimum 7.3N/mm² block.

Soleplates should be positioned a minimum of 150mm above finished ground level. This may be reduced to a minimum of 75mm where additional drainage provisions are provided to the perimeter of the building. Refer to the Building Designer for details.

At accessible thresholds the landing should be designed with a gradient of between 1:60 and 1:40 to ensure water run-off and an adjacent drainage slot or channel provided to protect the entrance.

Packing Under Sole Plates

Where packing under a sole plate is required, the packing option to be adopted should be detailed on the construction drawings. There are three common options.

Option 1 — Temporary Sole Plate Packing Followed by Permanent Grouting

The sole plate is levelled on temporary spacers (no greater than 0.9m centres). After the ground floor SIP wall panels, with floor over or roof structure have been erected, permanent packing is placed under the sole plates. This packing can be free flowing non-shrinkable grout along the full length and width of the sole plate, or individual durable robust packers placed under the full area of each load point where relevant.

Option 2 — Sole Plate Bedded on Mortar Levelling Bed

The sole plate is laid on a continuous level bed of mortar, prior to wall panel erection. The mortar should extend the full length and width of the sole plate. The sole plate is checked for line and level and spacers may be used to do this.

Option 3 — Double Sole Plate Sandwich

The lower sole plate is fixed along the contours of the supporting structure. The upper sole plate is fixed on top and levelled with temporary spacers inserted between the sole plates. Once the first lift construction has been erected, permanent timber or robust packing is inserted for the full area.

Depth of Packing

The packing / bed must cover the full area of the sole plate, be durable, not deform under load, and maintain DPC between packing and plate.

Fixing

The sole plate is to be anchored to the substructure to resist lateral, in-plane and vertical forces. The design drawings will provide the fixing method. Direct fixing of sole plates using shot fired nails or screw and plug to design specification and centres.

Sole plate brackets within the internal envelope should be galvanised mild steel, minimum coating z275 with matching corrosion protection fixings. If the fixing is in the cavity the brackets should be stainless steel.

Frame Erection

Ensure panels have been stored correctly: check damaged frames or out of square panels as these can cause poor fitting frames and lead to out of tolerance buildings.

Fix locater plate to sole plate. Fixings should be at least 200mm centres. Locate SIP on locator plates and fix together – and to locater plate – as per the fixing schedule. There are two methods of jointing SIP wall panels together; using edge timbers or SIP splines. Panel fixings to edge timbers and splines should be at minimum 100mm centres on both faces of SIP.

Nail or screw separate headplate into the rebate in the top of the SIP wall panels. Fixings should be at least 150mm centres on both faces of OSB panel into the locater plates and headplates. The fixing specification should be followed in all cases and ensure that binders cover the panel joints and overlap by 600mm.

SIP Wall Frame Connection

Connection using edge timbers

Connection using mini SIP splines

Typical Floor Zone Detail

Top Hung Joists

Vertical Tolerances

Alignment

Internal horizontal building face of the panel on plan shall be a maximum step of 5mm with an average of no more than 3mm to any one panel. Steps to the vertical level shall be reviewed to consider the impact on floor or roof structure above the wall.

Gaps above 2mm in height and above 300mm long shall have a rigid full bearing packer inserted. Maximum steps shall be +/- 5mm before investigations are undertaken and adjustment works considered.

Non-Loadbearing Walls

Non-loadbearing walls on floor decking must be adequately supported on joists or noggins below. All non-loadbearing walls below the floor deck/joists require lateral restraint at the top rail/head binder at maximum 600mm centres. Check the design details for secure fixing.

Breather Membrane

Breather membrane is typically pre-fixed in the factory. Laps on the membrane to manufacturer’s instructions: as a guide laps are usually 100mm horizontal; 150mm vertical; lap sole plate by 25mm.

Patching tears in breather membrane

Roof Structure

Eaves to Roof Junction where SIP roof is adopted

SIP Roof at Ridge Purlin

SIP Ridge Without Ridge Purlin — Tied Roof Structure

Diagrammatic Valley Gutter

Parapet Wall to Sloping Roof

Roof to Gable Wall

Roof to Gable Wall with Opening

Roof Light

Pitched Roof Construction — Roof Finishes

Covering for pitched SIP roofs can be any type of covering that may be used on any other type of roof (e.g. tiles, slates, profiled metal etc). A ventilation void must be provided between the SIP and the roof covering, so the use of counter battens before tiling battens, profiled metal roofing or any other covering should be considered. Roofing battens should be fixed to the SIP roof with the use of screws.

Pitched SIP roofs will generally consist of the following layers:

• Tiling or slates
• Tiling battens
• Counter battens (providing a path for ventilation and water runoff)
• Breathable roofing membrane
• SIP of the required thickness
• Vapour control layer (if required)
• Battens forming a service void
• Internal lining

SIP Flat Roofs — Roof Finishes

Zero degree pitched flat roofs are not recommended for any construction and a fall is required to be agreed with the designer. Any reference to flat roofs assumes a nominal pitch.

SIP flat roofs are classified as cold flat roofs, and spans may ultimately be limited by requirements for ventilation. The SIP roof should be overlaid with battens or tapered firrings to maintain the required ventilation void, and then overlaid with a deck and roof covering.

Flat SIP roofs will generally consist of the following layers:

• Waterproof roofing membrane
• Plywood or similar roof decking
• Firrings to form a runoff and create a ventilation void
• Breathable roofing membrane
• SIP of the required thickness
• Vapour control layer (if required)
• Battens forming a service void
• Internal lining

Insulation

Site applied insulation is not necessary for SIP walls or roofs, since the insulation is an integral part of the panel. However there are some situations in SIP buildings where site applied insulation may be required.

Trussed Rafter Roofs

For trussed rafter roofs, it is common practice to have insulation at the ceiling tie level. This is called a cold roof space. Other roof types are available and the design specification should be consulted for guidance.

Fully Filled Party Walls

Fully filled insulation to party walls is a common requirement for current building regulation compliant buildings. Check if project requires fully filled insulation. See STA Advice Note on party wall insulation.

Insulation between timber frame: mineral wool or to design specification. For open timber frame panels, insulation between timber frame panels installed once the building is dry and watertight.

Vapour Control Layer (VCL)

Durability of the structure is ensured by keeping the timber dry. In most SIP constructions, this is achieved through a combination of:

• A vented cavity between the SIP wall panels and the external cladding
• A protective breather membrane on the cold side of the insulation — applied to the outer face of the sheathing
• A vapour control layer (VCL) from inside to outside, for which BS 5250:2011 can be used to determine if an additional VCL is required, or if the SIP panel can inherently provide an effective VCL

Airtightness

It should be noted that the VCL layer can be an important component in providing airtightness. The design specification should be consulted for airtight details. In addition to the designer being consulted, checks should be made to ensure that air circulation and ventilation is included in the airtightness levels of the build.

Dry Lining and VCL Options for Walls

Thermal Efficiency and Airtightness

New SIP buildings have high levels of insulation, double glazed windows and highly efficient heating systems. To ensure that the design matches the as-built performance, the site trades are to be aware of the accuracy of the construction and to apply the design details.

Continuity of the airtightness barrier is important, especially around windows, doors and other penetrations through the external envelope. At floor level, make sure the VCL (where needed) laps with the DPC.

Make sure that the VCL around the floor frame, if needed, will be able to lap onto the VCL for the walls. The airtight layer around the floor should be of a suitable material to ensure that no water vapour collects at this point, do not use polythene, but a breathable airtight membrane.

Ensure the VCL around the wall to the roof laps, or where a SIP with an inherent VCL has been agreed, then junctions are suitably sealed.

Dry Lining

Dry lining should not occur until the frame is watertight. Test services in wall before dry lining. Dry lining is used for final wall finish providing fire protection and acoustic performance.

Fixing of dry lining should not occur on timber framing with greater than 20% moisture content or damp/wet OSB SIP boards. Correct fixing of the dry lining is essential to achieve the design fire performance. If the fixings are inadequate the boards will fail early.

Where the fire resistance is achieved with two layers of board, both layers should be fully nailed or screwed to specification.

Fire Stops

Fire stops are used to ensure that fire resistance requirements are met for the compartment of the building. Typically they are non-combustible board or mineral wool — proprietary systems are also available.

Fire stops are installed at:

• Wall junctions between dwellings such as roof to wall
• Where services and penetrations occur at external walls and floors
• Other fire compartment separations

Cavity Barriers

Cavity barriers are used within cavities to prevent the spread of smoke and fire. They can be rigid — preservative treated timber battens, non-combustible board — or flexible and based on mineral wool or approved intumescent-based strips.

For Dwellings, Cavity Barriers Are Required:

• Around all openings and penetrations in external walls
• At the top of the external wall cavity
• At the junction between compartment walls or floors and external walls
• At the junction between a compartment wall that separates buildings
• For non-domestic properties in England and Wales — vertical cavity barriers are required at every 10m

Cavity barriers at penetration through external wall

Scotland and Northern Ireland

In Scotland and Northern Ireland cavity barriers are also required:

• At the junction between any floor and an external wall
• At vertical or horizontal centres not exceeding 10m in Scotland
• At vertical or horizontal centres not exceeding 8m in Northern Ireland

Cavity Barrier Positions

The diagrams below indicate cavity barrier positions for a typical semi-detached or end terrace dwelling. Full details of positions of all cavity barriers and fire stops will be found on the design drawings.

Cavity Barriers and Fire Stops at Party Walls

Party Wall to Floor Junction

Masonry Cladding

Masonry cladding should be constructed using a coursing rod to ensure the right levels are achieved at the openings and eaves levels. Pre-plan the coursing to allow for a clearance between window sills, soffits, balcony structures, etc. for differential movement.

Cavity width is maintained between the frame assembly and masonry cladding — typically 50mm by design with an as-built minimum of 40mm and maximum of 60mm or specific design detail. Designer to check with warranty provider.

Typical Section – Clear Cavity

Wall Ties

Unlike timber frame stud frame construction, masonry wall ties can be fixed anywhere to the SIP panel, providing the fixings used are appropriate for the SIP board thickness. Wall tie spacing should be specified in the design.

Fixing Wall Ties

Ensure wall ties are suitable for the cavity width. Wall tie centres and wall tie sizes to the engineer’s specification. Make sure they are timber frame wall ties that can accommodate movement — single piece SIP wall tie for up to three storeys and above three storeys use priority sliding tie.

Do not straighten out pre-bent ties — bend points upwards. Bedded end must be flat against brick. Drive stainless steel screw home firmly to outer skin of SIP wall panel or to edge timbers around the openings. Typically stainless steel flange head screws should be used to provide adequate withdrawal capacity into the SIP.

Supported Cladding

Supported claddings such as timber boarding, metal cladding or proprietary render systems should be installed onto vertical treated timber battens or proprietary metal cladding rails fixed on top of the breather membrane to the SIP panels to provide a drained and ventilated cavity.

This cavity should be at least 25mm for render board and 20mm for timber cladding and must incorporate drainage and ventilation gaps at the base equivalent to 500mm² per metre at each floor level.

If deeper batten zones are required the battens should be built up in cross layers ensuring that the layer of battens closest to the SIP panel is vertical to prevent moisture traps against the SIP panel. Battens should be discontinuous across floor zones with a minimum of 10mm gap to allow for differential movement.

Cement Render Carrier Board Cladding

The use of alternative weather protection other than masonry is becoming common place. There is an advance in new thin coloured render coating systems, which can be applied to SIP frames using a render carrier backing board. The render carrier board shall be third party approved and secured to the SIP panel using vertical treated timber battens.

Corrosion resistant fastners form the cavity batten to the SIP panel are to be specified by the building designer so that they are suitable for both the loads and the SIP outer board thickness capacity. Movement within frame building at floor levels is a design consideration and advice should be sought from the SIP manufacturer on the movement anticipated. A movement joint should be installed through the render, board and batten. A render detail from the render manufacturer will be required.

Installing Services

Floor Joists

• Only notch or drill solid timber floor joists within specified limits
• Do not notch I-joists or open web joists
• Notch solid timber joists to designer’s limits
• Use knockouts for I-joists
• Refer to manufacturer’s recommendations for holes and services in engineered wood joists

Walls

• Ensure that studs and rails in load bearing timber frame walls are drilled and notched only when stated on the drawings — no services should be routed through SIP without approval
• Ensure service points / sockets are fully backed to achieve fire resistance requirements — consult the details
• For external service penetrations consult the specification for fire stopping and cavity barrier requirements

Other Aspects

• Pipework should be located in a dedicated service void or behind skirtings in external SIP walls and where practical, provide access to pipe bends, stop valves etc.
• Seal service penetrations of the VCL with PVC tape
• Use de-rated cabling within the framework, in accordance with IEE guidelines
• Ensure that surface mounted electrical services in SIP walls are routed in protective steel conduits

Chimneys and Flues

Check that SIP and timber frame walls are correctly built around chimneys and flues. Seek the specification for clearance.

Flues Through SIP Roof

Flue to Combustible Material Distances

Differential Movement

Why It Happens

• Timber is typically installed at maximum 20% moisture content — this reduces to around 10–12% for internal walls in the heated building
• As timber dries out, its cross-section shrinks and the structure settles
• Cladding materials also change — clay bricks expand; blocks and calcium silicate bricks shrink — but not in tandem with the timber
• It is good practice to pre-load the timber frame structure with roof tiles and internal sheeting materials prior to the installation of the masonry cladding, within structural limits

The Implications

• Any material or component attached to the timber frame structure which overhangs or projects through masonry cladding must have an adequate gap beneath it to allow differential movement to take place without damage to the structure or the cladding
• Gaps should be filled with a compressible filler, such as an impregnated foam tape

Where Allowances Need to be Made

• Window sills
• Roof verges and eaves
• Where attached to cladding, e.g. timber or boarding overhangs brickwork
• Flues and chimneys
• Any penetrations through the external cladding such as overflow pipes, flues or balcony brackets
• Traditionally built stair cores

Reducing Vertical Shrinkage

• Use I-joists, open web joists or super dried timbers
• Use of top hung floor to wall detail
• Ensure detailing is correct to allow for settlement
• Ensure adequate gaps in supported claddings and linings, plus around penetrations through masonry cladding, are left to take up the downward movement of the frame
• Keep timbers as dry as possible
• Where possible minimise the amount of cross-grain material in wall and floor buildups

Typical Movement Gap Allowances

Sillss

Allow for the differential movement of masonry cladding and the window frame and sill, which is fixed to the SIP.

Verges and Eaves

Summary

For site supervisory staff, the golden rule is attention to detail. Measure, check and check again. Remember that mistakes are more difficult to rectify later. If in doubt, ask.

Take pride in constructing homes and buildings, ensuring generations can enjoy places that are:

• High quality
• Environmentally friendly
• A joy to live in
• Economic to run

Reference Documents and Further Reading

For the Project

• The SIP manufacturer details, drawings and specification
• The architect’s details for all construction details

Good Practice

• NHBC Standards
• LABC Warranty Technical Manual
• STA – Advice notes on construction and tolerances
• STA – Cavity barrier best practice
• STA – Erector training books
• STA – SIP guidance papers on the website

Background

There is lots of useful information on airtightness and thermal efficiency within the free resources for housing professionals pages of the Energy Saving Trust website.

With Thanks

Thank you to Swedish Wood for their contribution in making this guide digital.