How to Build a Timber-Framed Garden Room Base

As with anything, there’s always more than one way to skin a cat. The same applies for a timber-framed garden room base. I’ll try and show you a few different methods whilst focusing on the method I use in most installations.

Raising the Timber Base

Each timber-framed garden room base needs to be raised off the floor to prevent the timber from sitting in wet earth, as moisture will slowly rot the wood. It also allows air to circulate beneath the building, which keeps the timber dry. This can be achieved using different methods, with the main goal to prevent the timber from touching the ground.

We also need to ensure the frame isn’t too high off the ground so that the building can remain within permitted development rules, which restrict the building’s height to 2.5 meters. (assuming you don’t want to apply for planning permission). You can read more about permitted development on garden rooms here. The aim is to build as close to the ground without compromising moisture exposure. That way, you can maximise the internal room height, which would ideally be 2m or more.

The Timber Frame

The timber frame is simple enough; it needs an outer frame with inner joists and noggins to add strength and support the floor boards. The exact size and shape will depend on your specific build. The following examples are designed with a 4×2″ C24 timber for a minimal height. I’ll discuss timber sizes in more detail below.

Timber Frame Garden Room Base Foundation Options

Consider one of the following methods to construct a suitable raised timber frame.

Pier Foundations (Bricks or Breeze Blocks)

A series of brick or breeze-block pillars is built on small footings to lift the timber frame off the ground. The garden room frame is then positioned directly on top of these piers. This creates an elevated, breathable base that prevents moisture from reaching the timber

Ideal applications:

• Sites with reasonably firm soil
• Small to medium-sized garden rooms
• DIY builds where simple, affordable materials are preferred
• Locations where you want a low-impact, easily repairable foundation

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Ground Screws

Steel helical piles are driven into the ground using specialised machinery, creating deep, stable anchors. Brackets are then attached to hold the timber frame. This system avoids large excavations and performs well in a wide range of soils.

Ideal applications:

• Sloping or uneven ground
• Medium to large garden rooms
• Areas where digging is difficult or undesirable
• When minimal ground disturbance is preferred
• Professional installation recommended

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Concrete Pile Foundation with Threaded Bar

Deep holes (around 600mm deep and 250mm wide) are filled with poured concrete. A 24mm galvanised threaded steel bar is embedded into each pile. Adjustable support plates attach to the bar, allowing fine height levelling of the timber frame.

Ideal applications:

• Heavy or oversized garden rooms
• Sites with variable levels that require precise adjustment
• Long-term or semi-permanent installations
• Builders who prefer strong, fully customisable supports

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Concrete Pile Foundation with Metal Bracket Set in the Concrete

Concrete piles are created by digging 600mm deep × 250mm wide holes and filling them with poured concrete. A metal bracket (usually a U-bracket or post support) is cast into the wet concrete, creating a fixed, extremely strong connection between the pile and the timber frame.

Ideal applications:

• Large, heavy garden rooms or workshops
• Sites requiring maximum stability
• High wind-load areas
• Permanent structures where movement must be minimised

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Pre-Formed Concrete Pads with Adjustable Brackets

Factory-made concrete pads, each with an adjustable metal bracket, provide up to 150mm of height adjustment. They come in rated capacities (1.5-tonne, 2.5-tonne, and 5-tonne). Installation requires a 500mm deep × 500mm wide hole filled with compacted hardcore to create a stable footing.

Ideal applications:

• Non-permanent or relocatable structures
• Medium to large garden rooms where easy levelling is important
• Sites where concrete pouring is not desirable
• DIY projects where predictable load ratings are beneficial

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Plastic Adjustable Pads

Durable plastic pads with screw-style height adjustment. These require a very flat and well-compacted sub-base (similar to one used for patios or concrete slabs) to ensure proper support.

Ideal applications:

• Small garden rooms or studios
• Builds where speed and simplicity are important
• Installations on existing concrete slabs or perfectly level compacted bases
• Lightweight timber structures

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What’s the Best Timber to Use on a Garden Room Base?

I prefer to use a 5×2″ (120mm X 45mm) C24 treated timber, because it keeps the price minimal, ensures a low height, and also supports thick insulation slabs.

Maximum Practical Spans – C24 Treated Joists

The timber size dictates the minimum distance required between each support pillar. Here’s a quick span table based on the average size and weight of a garden room.

Timber Size (mm)	Timber Size (inches)	Max Span (m)	Notes
45 × 95	4×2	1.2–1.8 m	Budget timber option
45 × 120	5×2	1.8–2.4 m	Best for low-height bases
45 × 145	6×2	2.4–3 m	Very common; excellent stiffness
45 × 170	7×2	3–3.8 m	Good for larger spans
45 × 195	8×2	3.8–4.2 m	Large rooms; fewer supports needed

The span will be reduced if the room will be intended for heavy use. For example:

• Heavy gym equipment
• Plumbed bathrooms
• Thick internal finishes
• Heavy roofing (sedum, tiles)

You can read more about spans and span tables here.

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How to Build a Timber Framed Garden Room Base on Pre-Formed Concrete Pads

Step 1 – Materials & Site Preparation

Before you begin:

• Ensure you have all required materials:
  • Pre-formed concrete pads. I like to use EasyPads.
  • Treated structural timber for joists and perimeter frame (e.g. C24, size depending on span, see span-table),
  • Galvanised metal brackets (included with EasyPad),
  • DPM/vapour barrier,
  • Floor sheathing (OSB or tongue-&-groove),
  • fixings, preservative for cut timber ends, joist hangers or pad brackets, and
  • Basic tools including: spirit-level/laser-level, tape, saw, drill, screwdriver/screw gun and protective gear.
• Prepare the site: choose a flat, level, well-draining area. Clear off any vegetation or topsoil; remove debris, roots, and stones.
• Mark out the footprint of your garden room precisely (corners, walls, overhangs) and use pegs and string to get exact alignment.

Step 2 – Position the Pre-Formed Concrete Pads

Accurate placement is critical to ensure the timber frame sits correctly on the pads.

1. Using your marked footprint, mark the locations for each pad. Pads should support the perimeter and internal joists where load concentration will be greatest.
2. Spacing between the pads depends on your timber joist size, span and expected load. Refer to the above span table for the ideal spacing.

Step 2 – Creating a Solid Footing

1. Place the pads on firm, level ground. Use a level (spirit or laser) to ensure each pad is level in both directions. This ensures your base will be flat.
2. If using EasyPAD, the system includes adjustable metal brackets for timber beams; install or prepare them now (per pad manufacturer instructions).
3. Mark the pad location and remove the pad before digging a hole. The depth of the hole will depend on the desired height of the timber frame and the quality of the soil.

Here’s a guide from EasyPAD as to the depth options.

• There is a 100mm adjustment on the EasyPAD system. Some pre-formed concrete pads will not have adjustable supports; instead, the timber frame can be constructed directly on top of the pad.
• Dig the holes roughly 100mm wider than the pad.
• Ensure a good-quality sub-base is created to support the pad. A deeper hole and sub-base will be required for soft ground.
• A type 1 MOT (hardcore) sub-base should be used to fill the hole and compact before placing the EasyPAD.
• Place the pads and adjust as required. All pads need to be level and secure.

⚠️ Tip: Take time now to get the levels consistent across all pads. Any significant variation will cause your frame to twist or produce an uneven floor later.

Step 3 – Construct the Outer Timber Frame

1. Use treated C24 timber for the outer frame beams (perimeter). Depending on your design, 45×120 mm might be a good compromise between low base height and strength.
2. Cut the perimeter timbers to exact lengths, and pre-treat any cut ends with wood preservative.
3. Lay the perimeter timbers onto the pad brackets (or directly onto pads if brackets not used). Ensure they sit square and level. Check diagonals: the two cross-diagonals should be equal for a perfect rectangle/square.

Step 4 – Install Internal Joists

1. Based on your span calculations and pad spacing, mark the positions of internal joists on the perimeter beams. Typical joist centre spacing for garden-room floors is 400–600 mm (400 mm gives a stiffer, more load-resistant floor).
2. Cut the joists to length. Pre-treat cut ends with preservative.
3. Set each joist into the pad-bracket (or joist-hanger) support position, or let it bear on the perimeter beams (depending on design), and secure with appropriate fixings (galvanised screws or nails).
4. Use a spirit or laser level to check that all joists are level and flush. Also re-check frame squareness after joist installation.

• It’s important to add noggins to provide additional strength and prevent timbers from twisting. Noggins can be staggered for ease of attachment.

Step 5 – Install 100 mm Insulation Between Joists

• Fit 100 mm insulation (typically rigid PIR boards like Kingspan or Celotex) snugly between the floor joists. Measure each cavity individually; joist spacing often varies slightly, and cut insulation to size so it friction-fits without gaps.
• Use a standard panel saw to cut the PIR insulation boards
• Ensure the PIR boards sit flush with the top of the joists or slightly recessed if adding a service void. Use foil tape to seal joints between boards where needed to reduce air leakage and improve thermal performance.
• Support the insulation from below with stapled battens or insulation hangers to prevent sagging and to maintain a clear ventilation gap beneath the structure.
• Check that the insulation is continuous across the floor area. Avoid gaps and cold bridges around edges, joist ends, and structural pads. Once insulation is secure and level, proceed to lay the subfloor decking.

Step 6 – Add Damp-Proofing & Separation from Ground

• Install a DPM (damp-proof membrane) or vapour barrier under the floor if the building will be used as a habitable/insulated garden room. This helps protect from ground moisture.
• Ensure there is an air gap/ventilation under the floor (if pads raise the floor above ground level), which helps reduce damp and rot risk.
• Ensure that soil, turf, or vegetation does not come into contact with the timber, especially horizontally. Where needed, add a layer of hardcore or pea-gravel around the pads to keep soil away from timber.

Step 7 – Lay Subfloor & Finish the Floor

1. On top of the joists, lay your floor decking/subfloor, e.g. 18 mm structural plywood, 22 mm tongue-and-groove chipboard, or OSB3. I prefer to use Egger Protect boards, which have a higher moisture protective barrier. Use galvanised screws (and adhesive if using T&G chipboard) to fix floor boards firmly to joists.
2. Check the floor for level and rigidity. Walk across and listen for creaks or movement; if you find any, reinforce joists or add additional blocking where needed.

Step 8 – Final Checks & Maintenance Preparations

• Re-check that the base is square, level and stable at all pad support points.
• Ensure no timber touches soil and there is good drainage/ventilation around and under the base.
• If the timber is untreated at any cut ends, treat them now with preservative.
• Make a note of pad spacing, timber sizes, joist layout, and ventilation gaps. This is useful maintenance documentation for the future.
• Plan for periodic inspections of the underside after heavy rain or in damp seasons, check for moisture, rot, or settlement, especially if the soil is soft or prone to shifting.

Conclusion

After more than a decade designing and installing garden buildings, I’ve learned that the success and longevity of any project always come down to the quality of the base. There are many ways to raise a timber-framed floor, and each method, whether piers, ground screws, concrete piles, or pre-formed pads, has its place. But the goal is always the same: keep the timber dry, stable, and as close to the ground as possible without compromising durability or permitted development rules.

Over the years, I’ve found that selecting the right treated timber, working within safe span limits, and taking the time to level and align the structure properly is what truly makes the difference. A well-built base not only supports the weight of the garden room but also prevents future issues like moisture damage, floor movement, or uneven settling.

While every site and every build is unique, pre-formed concrete pads have become one of my go-to solutions, they offer reliable support, easy adjustment, and minimal disruption to the ground. But whichever foundation you choose, getting the basics right is essential.

In the end, a garden room is only as good as the base it sits on. When the foundation is solid, level, and thoughtfully constructed, everything else becomes easier, and you set the building up to perform beautifully for many years to come.

If you’re considering a different approach, you can also explore how to build a strong concrete base for a garden room here.

Garden Room Base FAQs

1. What is the best type of base for a garden room?

There’s no single “best” base for every garden room – it depends on your soil, access, budget, and the size/weight of the building. For most timber-framed garden rooms, I commonly recommend:

• Pre-formed concrete pads with adjustable brackets for a strong, level and relatively quick-to-install base.
• Ground screws on sloping or difficult ground, especially where you want minimal excavation.
• Concrete piles for very heavy or permanent garden rooms that need maximum stability.

The key is that the base keeps the timber frame off the ground, is properly levelled, and is designed to handle the expected load of the building.

2. How high should a timber garden room base be off the ground?

The base should be raised high enough to prevent the timber from sitting in surface water or damp soil, but low enough to keep the overall building height within permitted development limits (usually 2.5m to the eaves when close to a boundary in the UK).

In most cases, a clearance of at least 100–150mm between the bottom of the timber and the ground is a good target. This:

• Allows airflow under the building to keep the timber dry.
• Reduces the risk of rot and moisture problems.
• Still keeps overall height manageable so you maximise internal headroom.

3. Do I need planning permission for a raised garden room base?

In many cases, you can build a garden room under permitted development rights, which means you won’t need full planning permission, provided you follow the rules.

The base is part of the overall height, so you must:

• Keep the total height (base + walls + roof) within the permitted development limits (often 2.5m near boundaries).
• Avoid excessive raising of the structure unless planning permission is obtained.

Always check your local authority’s guidance or Planning Portal rules for up-to-date requirements and, if in doubt, get confirmation before building.

4. How do I choose the right timber size and support spacing for the base?

The timber size (joist depth) determines how far it can safely span between supports. For most garden rooms, I use treated C24 timber and follow span tables to size the joists and set the spacing between pads or piles.

In general:

• Smaller timbers (e.g. 45 × 95mm) need more frequent supports.
• Larger timbers (e.g. 45 × 145mm or 45 × 170mm) can span further between supports.
• Heavier uses (gyms, bathrooms, heavy roofs) require shorter spans and/or deeper timbers.

Always refer to a reliable joist span table and, if you’re unsure, err on the side of larger timber and more support points for a stiff, solid floor.

5. How do I prevent damp and rot in a timber garden room base?

Moisture management is critical for the long-term health of your garden room. To reduce the risk of damp and rot:

• Keep all timber raised off the ground on pads, piers, screws or piles.
• Use treated structural timber (e.g. C24) and treat all cut ends with preservative.
• Maintain a clear air gap under the floor to allow ventilation.
• Install an appropriate damp-proof membrane (DPM) or vapour barrier as part of the floor build-up.
• Keep soil, turf and plants away from the base and consider gravel or hardcore around the perimeter.

Regular inspections under the base, especially after heavy rain, help you spot issues early before they become serious problems.

6. Can I build a timber garden room base on an existing concrete slab?

Yes, in many cases you can build a timber-framed garden room base on top of an existing concrete slab, provided the slab is:

• Structurally sound (no major cracks or movement).
• Reasonably level and free-draining.

You can either:

• Fix plastic or adjustable pads on the slab and sit your timber frame on those, or
• Lay a DPM and construct the timber frame directly over the slab with suitable fixings and ventilation where possible.

The priority is still to protect the timber from trapped moisture and ensure the structure is stable, level and properly anchored.

Full Reference List for the Article

• Span tables for floor joists – timberbeamcalculator.co.uk/span-tables/floor-joists
• Garden room insulation guide – wood-create.com/how-to-insulate-a-garden-room-summer-house-or-shed/
• Suspended timber floors best practice (UK Gov) – gov.uk timber insulation PDF guide
• Building regs flooring & damp-proofing (Planning Portal) – planningportal.co.uk/building-regulations-flooring