Garden Office Build Guide

1. Preparation & advice

Every kit ships with a customer instructions booklet that acts as your construction guide — familiarise all contractors with its technical notes as well as this page. If your site is in a high wind area, seek advice on increasing the screw frequency.

Preparing your base

Make sure the base area is flat and free from debris — lay a weed suppressor down if needed. Any foundation should be level and substantial enough for the intended use of the building.

When you receive your kit

• Store panels in a dry place — moisture can make fitting more difficult
• Identify all panels once you've stored your kit
• Some panels are joined with slips for transit and identification — remove them, apply adhesive and fixings, then refit

General SIP building advice

• The building design and materials have been calculated and tested — it's the project manager and installer's responsibility to follow correct methods
• Follow appropriate health and safety procedures when lifting and fixing panels
• Use bracing until walls are fully joined, and props while constructing the roof
• Screw and glue panels with an appropriate adhesive for strength and airtightness
• Check the fit of each panel before gluing, and check corners for squareness
• End-treat any treated timbers you cut, particularly if they'll be exposed to moisture

2. Foundation options

Neither the timber subframe nor the foundations are supplied with the kit — they should be prepared in advance. There are three common approaches:

Concrete piles

Piles are set into the ground at approximately 1200mm centres, with a timber or steel base sited on top of them.

Ground screw piles

Screw piles are driven into the ground at approximately 1200mm centres, again with a timber or steel base sited on top.

Concrete slabs

A concrete slab is built on site first to create a level foundation. A timber subframe is then raised from the slab — using concrete blocks or timbers, spaced to match the subframe's joints — to create an airflow gap under the building.

3. Timber subframe & SIP floor

Timber subframe

The timber subframe base should have timbers spaced at 600mm centres, sited on a proper foundation, with joists staggered and fixed evenly between the timbers. The floor SIPs are then screwed into the subframe (or bearers) to secure the floor into the foundations.

SIP floor construction

The floor is made of SIPs connected by timbers and splines:

• Floor beams: run the same direction — usually the full length of the building — and sit in the rebate created where two SIPs meet
• Floor splines: sit inside the SIP rebates where panels meet, running perpendicular to the floor beams
• Floor edge timber: fills the floor's perimeter SIP rebate, attached once the rest of the floor is fitted together
• Timber floor packers: used to extend SIP floors over small distances, typically as tall as the floor SIP so the finished floor stays level

To connect the floor: screw adjacent SIPs to their floor beams and splines, screw the floor edge timber into the floor's perimeter, then attach any timber floor packers, screwing them into the floor edge timber.

4. Basic SIP wall construction

SIPs should sit flush to one another wherever possible. They're connected using timbers or splines placed within the SIP rebates — typically a 2-inch gap in the foam at a SIP's edge — and screwed into the OSB.

Sole plate

The sole plate goes beneath the walls and acts as a guide for your build. Screw it into the floor before the walls go up, so you can confirm the layout matches your plan.

Starting your walls

Create an initial corner first to improve stability while you build up the walls.

Edge timbers

Edge timbers finish the ends of each SIP wall, sitting on top of the sole plates. They're cut shorter than the full SIP wall height to leave room for top timbers or window/door bars above. When connecting SIP walls at corners or internal sections, screws should go through into the edge timber every 200–300mm.

5. SIP connection construction

Top timbers

Top timbers sit in the SIP rebate and finish off the top of each SIP wall, creating a flush surface. Where the top of a wall already has a timber beam or steel bar for an opening above, the top timber fits around or on top of it, staying flush with the SIP's top edge.

Internal wall connection

Wall edge and sole plates for internal walls are screwed into place before the internal SIPs go in. The SIPs slot onto the wall edge timber, then both sides are screwed to it at 200–300mm spacing.

Window & door connections

Mark opening heights on the SIP before setting the door timbers, allowing room for fitting tolerance. Wider openings need greater top tolerance to allow for beam deflection, which can mean thicker timber or steel beams — see the opening details below.

Construction notes

• Check squareness as you go — fix a couple of screws first, check, then continue
• Start at one corner for stability, then connect each subsequent SIP
• SIPs stored outside for a long period may be affected by moisture, making joints tight
• Foam adhesive makes joints airtight — fit the panel first to check alignment, then remove it to apply the foam, since it expands fast

Useful tools: a power screwdriver or impact driver (for timberlock/torx bolts), a claw hammer (for clearing foam from SIPs), and a multisaw (for cutting SIPs or OSB).

6. SIP panel sizes

Our SIP panels come in five thicknesses: 72mm, 98mm, 122mm, 147mm, and 172mm. Thicker panels give greater thermal performance and are used where the design calls for it. Every panel in your kit is labelled so you can match it against your customer instructions during the build.

7. Openings: from floor level

For all three types below, build your SIP walls and fix them in place first, then cut your timber opening sides to the distance between your intended opening height and the top of the sole plates, and glue and screw them into the SIP rebate with 50mm screws.

Type A — window/door bar above

Glue and screw the door bar or window bar into place above the opening with 50mm screws, then finish the elevation by running the top timbers through the SIP rebate up to the bar on both sides. For smaller bars, the top timber may sit on top of their core instead.

Type B — SIP above

Cut an opening top timber to sit across the two opening sides, each end inside the SIP rebates, and glue and screw it into place. Once secured, fix the SIPs and splines above it as normal, then run the top timbers through the rebate.

Type C — SIP & window/door bar above

Fit the door bar or window bar above the opening as in Type A, then fix the SIPs and splines above the bar as normal, before running the top timbers through the rebate.

8. Openings: raised

For raised openings, fix the SIPs and splines below the opening in place first, then cut and fit a timber opening bottom across the opening (each end in the SIP rebate) before adding the opening sides.

Type D — window/door bar above

Fit the window bar above the opening sides, then run the top timbers through the SIP rebate up to the bar on both sides. For lower, shorter bars the top timber may sit directly on top of it.

Type E — SIP above

Cut and fit both an opening bottom and an opening top timber, then fix the SIPs and splines above as normal before running the top timbers through the rebate.

Type F — SIP & window/door bar above

Fit the opening bottom and sides, then the window bar, then fix the SIPs and splines above the bar as normal before running the top timbers through the rebate.

9. Openings: large

Type G — window/door bar with door posts

Fit the timber opening sides, then cut window/door posts the full height of the opening and as wide as the SIP wall, gluing and screwing them onto the opening sides with 80mm screws so they sit flush. Fit the door or window bar, then run the top timbers through the rebate up to it on both sides.

Type H — side SIPs as posts for the beam

Here the SIPs either side of the opening act as the posts, so they need rebates deep enough to seat the beam — see the FAQ if yours need modifying. Fit wall edge timbers and opening sides, then door/window posts, then place the door or window bar into the gap made in the SIPs and glue and screw it into place. Wider bars may need a firring to finish the elevation, or in some cases the OSB cut out or the beam fixed on top of the SIPs instead.

Type I — steel beam above

Fit the opening sides, then window/door posts as tall as the opening and as wide as the SIP wall, fixed with 80mm screws. The steel beam is then glued and screwed into place with 50mm screws and steel tappers, before running the top timbers through the rebate up to the beam. Beams wider than the SIP wall may need the Type H modifications.

10. Door posts & ridge beams

Door posts

Door posts should be as wide as the wall they attach to, for a flush finish, and cut to the height of the opening. Fit them with 80mm screws once the SIP walls are up, attaching to either timber on an opening side or directly to a SIP face. They can also be used to pack out or widen an opening if it needs to change size.

Ridge beams

Finish all the SIP walls first, then glue and screw the ridge beam posts into place at an angle, and attach the ridge beam firring to the ridge beam. The SIPs at the gable end typically need modifying to fit the beam — scribe the end section of the beam and firring on paper as a cutting guide, then cut the SIPs, deepen the rebate if needed, and offset the timber accordingly. Secure the ridge beam with screws fixed at an angle into the edge timbers on the adjoining SIPs and the ridge beam posts, following proper health and safety procedure while lifting it into place.

11. Timber firrings & angled SIPs

Firrings

Firrings are timber that sit beneath the roof and on top of the wall SIPs, creating the roof angle and helping bring water to the bottom of the building. They're screwed with 50mm screws on the short end and 80mm at the top, and usually arrive in two or more parts that make up the full length — your customer instructions will show where each one goes. Rectangular (non-angled) firrings may need packing out with spare timber once the roof is on.

Angled SIPs

On steeper roofs, SIPs cut at an angle are typically used instead of firrings. They're fitted like any other SIP, but take care that any edge timbers or wall splines won't intrude on where the top timber needs to run flush with the SIP's edge. Gable ends on an apex roof usually use angled SIPs, which may need modifying to leave space for the ridge beam.

12. Flat roof construction

Before starting, make sure the top timbers and firrings are correctly installed and secure, and use props to support the roof throughout construction.

Put the first rows of SIPs and roof splines in place, holding them temporarily with screws where needed, then fit the first roof beam and attach the SIPs to it. Attach the next row onto the roof beam and repeat until the roof is finished, filling the perimeter SIP rebate with roof edge timber as you go — some builds will also need roof facing timbers to extend the roof's edge. Finally, secure the whole roof down to the walls below, screwing top-to-bottom through each panel into the firrings or top timbers.

Beam types

• Standard roof beam: timber, running through the SIP rebate and attached to adjoining SIPs and splines
• Composite roof beam: used over larger spans, with additional timber strengthening each edge
• Steel roof beam: used in SIP rebates over the largest spans

Once the roof is secured, it's ready for the membrane wrap — see below — followed by battens for the external walls, EPDM roof, and any overhangs.

13. Apex roof construction

Before starting, make sure the top timbers, firrings, and ridge beams are correctly installed and secure, and use props throughout.

Build one side of the apex first: put the first rows of SIPs and splines in place, fit the roof beam and attach the SIPs to it, then repeat row by row until that side is finished. Fix the apex ridge cap in place at the peak — a composite timber beam that joins both sides of the roof together, screwed into the SIP rebate — then start the second side by attaching its first SIP to the ridge cap and building outward the same way, connecting each row's starting SIP to the ridge cap as you go.

Fit the roof edge timber and any roof facing timbers as you would on a flat roof, then secure both sides of the roof down to the SIP walls and ridge beam below, screwing top-to-bottom through each panel into the firrings or top timbers.

14. Membrane wrap installation

Step 1 — cover the openings

Cut enough breathable membrane to cover the edges of each opening, so the internal timbers between the OSB are covered, and fix it in place by stapling to the OSB.

Step 2 — cover the building

Once every opening is covered, wrap the rest of the building, cutting holes for the openings while leaving enough membrane to fold around their corners. Staple it to the SIPs as you go, working up the building so each row overlaps the one below by at least 150mm.

Step 3 — finish the openings

Use the leftover membrane from cutting the openings to wrap around their corners, checking carefully that every corner is properly wrapped.

15. FAQ & troubleshooting

• Expanding foam fills any voids created during the build — use it on every joint if you want a fully airtight structure.
• Timber splines can be used as an alternative to SIP splines when joining panels together.
• Splines and timber can be used as joined sections, though structural timbers should be a single length where possible.
• Small or light damage to corners and edges won't affect the structural integrity of the build.
• Panel edges can be cut square during the build if needed.
• Polyurethane glue stains hands and clothing — wear gloves.
• Sole plates can alternatively be laid as full lengths for each elevation, with doorways cut out after the SIP build.
• Modifications can be made by moving full panels around. SIPs can be cut after installation to create windows and doors — edges will need to be rebated, or cut larger and plated.
• Rebating foam for clearance: cut the SIP or OSB with a handsaw or multisaw, then use a claw hammer to chop out the foam. Polyurethane foam won't cut with a hot knife.