Garden Room Materials Guide
What to buy, what to avoid, and where to save money
Timber Frame
The structural skeleton of your garden room. The most common choice is 4×2 CLS (Canadian Lumber Standard) softwood, widely available from builders merchants.
| Primary choice | 47×100mm (4×2) CLS KD C16/C24 timber — use C24 for longer spans or if you want extra rigidity |
| Wall stud spacing | 400mm or 600mm centres. 400mm is stronger and provides more fixing points for insulation and boarding. |
| Sole plate | 47×100mm treated softwood — use pressure-treated (green tanalised) for the bottom plate in contact with or near the base. |
| Top plate | Double 47×100mm across all walls for rigidity. |
| Roof joists | 47×150mm or 47×175mm depending on span. Use online span tables or a structural engineer for larger builds. |
Tip: Buy from a builders merchant rather than a DIY store — you'll get better selection and much lower prices. Order slightly more than you calculate (5–10% waste factor).
Insulation
Good insulation is what separates a usable year-round garden room from a glorified shed. PIR (polyisocyanurate) board is the best option for a well-insulated build.
| Walls | 100mm PIR board (e.g. Kingspan K7, Celotex TB4000 or Recticel Eurowall) — achieves ~0.18 W/m²K U-value |
| Roof | 150mm PIR board in the rafter space for excellent thermal performance. Ventilate above if using a cold roof build. |
| Floor | 100mm PIR under screed or between floor joists |
| Budget option | Rockwool or mineral wool in the stud cavities — cheaper but takes up more space for equivalent performance |
| VCL | Always install a vapour control layer (polythene sheet, 1000 gauge) on the warm side of the insulation to prevent interstitial condensation |
Tip: PIR boards can be bought as offcuts or seconds from insulation merchants — often 40–50% cheaper than new. Worth checking before buying full price.
OSB Sheathing
OSB (Oriented Strand Board) is used to sheathe the frame — providing structural racking resistance and a substrate for cladding and internal boarding.
| External sheathing | 11mm or 18mm OSB3 on the outside of the frame, under the cladding. Provides racking strength and wind barrier. |
| Floor decking | 18mm or 22mm T&G OSB3 on floor joists. 22mm is better for larger spans. |
| Internal boarding | 12mm plasterboard or 12mm OSB for internal walls — then skim, paint, or clad over it. |
| Roof deck | 18mm OSB3 on roof joists for the EPDM roof deck. |
Tip: OSB3 is the structural grade for use in humid conditions. Don't use OSB2 for anything structural or external — it doesn't cope with moisture.
External Cladding
External cladding is both structural (weather protection) and aesthetic. There are many options at different price points.
| Shiplap softwood (budget) | 19×100mm or 25×125mm treated softwood shiplap. Cheapest option, needs painting/staining every 3–5 years. |
| Siberian larch (mid) | Beautiful, naturally durable, can be left to silver or stained. ~£20–30/m² supply. |
| Western red cedar (premium) | Extremely durable, lightweight, excellent thermal properties. ~£30–45/m² supply. |
| Composite cladding | Low maintenance, consistent colour, 25-year warranties. Higher upfront cost but low long-term cost. |
| Fibre cement boards | e.g. HardiePlank — very durable, fire resistant, paintable. Popular for contemporary look. |
Tip: Install cladding with a ventilated cavity of at least 25mm behind it (over breather membrane on the OSB). This dramatically improves longevity by preventing moisture build-up.
Roofing
Most DIY garden rooms use a flat or low-pitch EPDM rubber roof. It's the most DIY-friendly waterproofing system and has a 25-year lifespan when properly installed.
| EPDM membrane | 1.2mm or 1.5mm EPDM sheet in a single piece where possible. Bonded with adhesive, then trimmed and finished with trims. About £8–12/m² for the membrane. |
| GRP fibreglass | More durable than EPDM but harder to DIY. Typically installed by professionals. Used where aesthetics are important. |
| Pitched roof — felt | Standard 3-layer felt (torch-on) or a breathable roofing membrane under tiles. Used for pitched builds. |
| Minimum fall | EPDM flat roofs need a minimum 1:80 fall (1.25cm per metre). Build this into your roof joists. |
| Roof build-up | 18mm OSB deck, 150mm PIR insulation, 18mm OSB, EPDM — this gives an excellent warm roof spec. |
Tip: For EPDM, buy from a rubber roofing supplier rather than a builders merchant — typically 20–30% cheaper. Measure carefully and order in one piece where possible (seams are the main failure point).
Windows & Doors
Windows and doors are often the most expensive single line item. uPVC double-glazed units give the best value; timber or aluminium cost more.
| French doors (standard) | uPVC 1800×2100 double-glazed french doors — £450–700 supply. Easy to install, excellent insulation. |
| Bifold doors | uPVC or aluminium bifolds — £900–2500 depending on size and specification. More impressive but harder to install. |
| Fixed windows | uPVC fixed-light or tilt-and-turn double-glazed — £150–350 each depending on size. |
| Aluminium | Slimmer frames, contemporary look, excellent durability. Typically 50–80% more expensive than uPVC. |
| Timber | Traditional look, requires maintenance. Similar price to aluminium for good quality. |
Tip: Buy online from window manufacturers (trade or direct) rather than B&Q or Screwfix — you can get made-to-measure double glazing at significant savings. Allow 3–4 weeks lead time.
Electrics
Running electricity to your garden room transforms it from a shed to a proper room. This must be done by a registered electrician or notified to Building Control.
| Supply cable | Armoured cable (SWA) run underground from your consumer unit to the garden room. Minimum 6mm² for most builds, 10mm² if running high loads. Bury at 450mm–600mm depth. |
| Sub-board | Small consumer unit (4–8 way) inside the garden room for circuits. |
| Circuits | Typically: 1× lighting circuit, 1–2× ring mains for sockets, 1× dedicated circuit for any high-demand appliance (heater, A/C). |
| RCD protection | All circuits in the outbuilding must be RCD protected (already standard in modern consumer units). |
| Earth bonding | Any metal services or structures must be bonded to earth. |
Tip: Get a registered electrician (NICEIC or NAPIT) to at least certify the work even if you do the rough-in. This gives you an Electrical Installation Certificate which is essential for resale.
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