Choosing and Building a Base for a Garden Room

The Ground Is Not Being Asked to Carry Much

Add up what a garden office actually weighs and the number is smaller than the anxiety around it. A 12.6 square metre timber-frame building with an insulated floor cassette, 140 millimetre stud walls, a warm flat roof and a set of bifolds comes to somewhere around three and a half tonnes as it stands. Add the domestic imposed floor load from BS EN 1991-1-1 and its UK National Annex — Category A residential, because a garden office is a room in a house by any structural reading — and the total is roughly 50 kilonewtons, or about four kilopascals over the footprint. Put a 150 millimetre concrete slab underneath — near enough 45 kilonewtons of concrete on its own — and the whole assembly reaches roughly 100 kilonewtons, or a little under eight kilopascals.

Eight kilopascals is nothing. The figure people reach for as an allowable bearing pressure in the absence of a site investigation is 150 kilopascals — itself a placeholder rather than a measured value, and nearly twenty times what this building applies. That redirects the worry, which is the point of doing the sum: bearing capacity is almost never what limits a garden building. What limits it is seasonal ground movement, differential settlement across made ground, and the fact that a light, tall, closed box in an open garden is primarily a wind problem.

Run the arithmetic once anyway, because a number you produced yourself is harder to argue with than a rule of thumb. Enter the whole footprint as a single footing and the whole building weight as the load, and what comes back is the average contact pressure the ground sees. A real footing design checks each pad on its own and applies partial factors from BS EN 1997-1 or the equivalent, and the allowable value belongs in a geotechnical report rather than a default box — but for the question actually being asked, the average is the right first pass.

The one case where the number bites is soft ground, and gardens hide more of it than fields do: a filled-in pond, the spoil from a 1980s extension tipped over the lawn and turfed, ash and broken brick from a demolished outbuilding. Dig a 600 millimetre trial hole at two opposite corners before choosing anything. Made ground announces itself — brick fragments, coal, plastic, a sudden change of colour, sides that will not stand up — and water standing in the hole an hour later is telling you something too, though not about bearing.

Taking the Fall Out Before Anything Else

Seven hundred millimetres of fall across 4.2 metres is a one-in-six slope, and it has to go somewhere. Cut into the bank and you win a level platform at the cost of a retained face, drainage behind it and several cubic metres of spoil with nowhere to go. Build up and you are asking a filled platform to behave, which it will only do if the fill goes in as controlled layers and is compacted, with density checked against a Proctor reference to ASTM D698 or D1557 or the BS 1377 equivalent. Cut half and fill half is the usual answer and the one that keeps the spoil on site. What you must not do is set pads or a slab edge on uncompacted fill; it consolidates under its own weight for a season or two whatever sits on top of it.

For a slab or a pad base, the layer doing the real work is the sub-base: an unbound granular mixture, what UK merchants sell as MOT Type 1, specified against Series 800 of the Specification for Highway Works and graded to BS EN 13285, or a dense-graded crushed aggregate base in North America. It is compacted in layers a hand-guided plate can actually reach, which is thinner than the depth people tip in one go. Two 75 millimetre lifts, each compacted, beat a single 150 millimetre lift that has been walked on.

Order it by weight rather than volume, because that is how it is sold and the two numbers differ. Suppliers quote compacted densities for crushed stone base across a range — commonly 1,700 to 2,100 kilograms per cubic metre depending on gradation — so a 15 square metre platform at 150 millimetres compacted is between 3.8 and 4.7 tonnes, four to six bulk bags. In a garden with no vehicle access that is fifty-odd barrow loads through the gate before a single bag of cement moves.

Two details earn their keep underneath it. A separation geotextile between a soft or clayey formation and the stone stops fines pumping up into the base, which on wet clay is not optional. And the sub-base should run 150 to 300 millimetres past the footprint on every side, so the slab edge or the outer pads are not standing on the very edge of the compacted material with nothing to confine it.

The Slab, If the Garden Allows One

A raft slab under a garden room is not a driveway with walls on it. Done properly it is a layered assembly, and every layer is a separate quantity bought a separate way — stone by the tonne, fabric and membrane by the roll, insulation by the board, concrete by the cubic metre. That is four suppliers and four delivery problems, against a screw base that is one pallet.

Specify the concrete rather than ordering 'concrete'. BS 8500-1 gives designated mixes for exactly this: a GEN-series mix for an ordinary domestic slab, and the FND designations where BRE Special Digest 1 Concrete in aggressive ground puts the site in a sulfate or acid class that demands them — brownfield gardens, made ground and some natural clays all reach that threshold. Where ACI documents govern, ACI 332 and ACI 360R cover the same decisions. Fabric reinforcement to BS 4483 controls cracking rather than carrying load, and only does that if it is on chairs at the right height rather than pulled up by boot after the pour has started.

Then there is the arithmetic that decides whether the slab happens at all. That 4.2 by 3.0 metre footprint at 150 millimetres, with a modest thickened edge, is about 2.2 cubic metres — five and a quarter tonnes of wet concrete. Through a wide gate onto a drive, that is a ready-mix delivery and a comfortable morning. Through an 860 millimetre side passage with a step, it is roughly forty barrow loads of material that sets whether you are ready or not, or a pump and its hose run, or close to two hundred bagged mixes — a 25 kilogram bag yields about 0.0125 cubic metres, so 2.2 cubic metres is a pallet and a half of them. The choice between a slab and everything else is very often made right here, and it is a logistics decision wearing a structural coat.

Whatever the route, buy for the hole you actually dug. A sub-base that dips 20 millimetres over a 12 square metre slab is a quarter of a cubic metre of extra concrete, and concrete is the one material on the job that cannot wait while somebody goes for more.

Pads: Count Them From the Bearers, Not the Perimeter

A pad base carries point loads, so the count follows the framing rather than the outline. A garden room floor cassette spans between bearers; the bearers span between pads; the pads sit on ground. Work inwards from that. A 3.0 metre wide building on 145 millimetre joists will normally want three bearer lines rather than two, and a pad under each end and at intervals along each line — which is a different number from anything the perimeter alone produces.

The perimeter figure is still worth having as a floor to check against, because it catches the base drawn with four pads at the corners and nothing in between. A standard residential deck spacing of six to eight feet along the beam line, applied to the perimeter, gives the minimum count an ordinary beam can bridge. Take it as exactly that: a lower bound from a spacing convention, which knows nothing about your interior bearer lines or the span table for the timber you are actually buying.

Pad size, though, is not set by bearing at all — the second section already showed the soil barely notices. It is set by three practical things: enough plan area to take a bearer with its packers and its fixing without any of it overhanging, enough depth to get through topsoil and made ground onto undisturbed material, and enough depth again to sit below whatever seasonal movement the site has. On ordinary non-shrinkable ground in the UK, 450 millimetres is the usual minimum starting point; where ground freezes, IRC Section R403.1 requires footings below the frost line; and near trees on clay, the tables in the previous section govern and run deeper than either.

Filling the Holes Without Buying Twice

Pad holes swallow more concrete than anyone estimates by eye, because a hole is a cylinder and cylinders grow with the square of the diameter. A 300 millimetre auger hole 600 deep holds 0.042 cubic metres, and across nine holes that is 0.38 of a cubic metre. Take off what a 100 millimetre post displaces and it comes back to about a third of a cubic metre — around thirty-three 20 kilogram bags, two-thirds of a tonne, arriving on a pallet that will not fit through the gate and has to be split on the drive. Bracket-mounted pads displace almost nothing, so on those the gross figure is the one to buy against.

That is also the number that decides between bagged mix and site mixing. Below about ten bags, buy bags. Above forty, a mixer with ballast and separate cement is cheaper, faster and gives control over consistency across the pours — which matters, because pads cast at different water contents on different afternoons set to different strengths, and it is the weakest one that finds out first.

Two site habits are worth stating. Dry-pouring bagged mix into a hole and wetting it afterwards is a method some manufacturers publish for fence posts and others explicitly do not — follow the bag, because the ones that permit it also specify how. And do not cast a bracket into a hole with 50 millimetres of slurry in the bottom; bail it, and if it refills faster than you can bail, you have learned something about the water table that changes the base type rather than the pour.

The Timber Frame That Sits on Top

Above pads or screws sits a bearer frame, and its geometry comes from the floor above rather than from a rule. Garden room cassettes are typically 45 by 145 joists at 400 millimetre centres with insulation between and an 18 millimetre deck over, and the bearer spacing is whatever that joist section can span at those centres for the species and grade being used. Look it up in the span tables for the timber you are buying; do not inherit a spacing from a shed.

A runner layout gives the starting count — runners at roughly 600 millimetre centres across the width, each running the full length of the building, which for a 3.0 metre width comes out at six. That is a sensible default for a light structure with a conventional deck, and it is a default: a heavier build, a wider span, a stone-clad wall or a green roof all pull the spacing in, and the honest way to settle it is the span table rather than the convention.

Durability is where garden room bases quietly fail, and it fails at the timber-to-ground interface every time. Anything within splash range of the soil needs ground-contact treatment — Use Class 4 under BS EN 335, preserved to the retentions and penetrations in BS 8417, or UC4A under AWPA U1 where that system applies. The trap is that the treated timber most merchants stock for decking is a lower use class, suitable for above-ground exposure and nothing else, and it looks identical. Check the tag, then keep the timber off the ground anyway: a DPC pad or bracket between bearer and concrete, a ventilated gap under the floor with airflow that actually crosses it, and no soil or bark mulch banked against the frame after the landscaping goes in.

Screws, Piles, and the Difference the Brochures Blur

Two different products get sold under overlapping names. A helical pile is a steel shaft with one or more helical bearing plates, installed by a machine that measures torque; in North America the family is covered by ICC-ES AC358 Acceptance Criteria for Helical Pile Systems and Devices, with capacity confirmed by load testing to ASTM D1143 or ASTM D3689. A garden room ground screw is usually a shorter proprietary tube with a coarse thread or a single flight, and its capacities come from the supplier's own test data rather than that acceptance criteria. Both work; they are not interchangeable, and a capacity table for one says nothing about the other.

Where torque is measured, the industry method is a straight correlation: ultimate capacity is the installation torque multiplied by an empirical factor, Kt, published for the specific shaft. Kt falls as shaft diameter rises, varies between manufacturers, and is a property of the product rather than of the soil — which is why the value has to come off the data sheet for the shaft actually in the ground. It estimates ultimate capacity, so a factor of safety still applies before it becomes a working load.

Run one screw's numbers and the shape of the problem becomes obvious. A garden room corner might be asked for five to ten kilonewtons; a modest screw at a modest torque with a typical Kt produces an ultimate capacity several times that even before the safety factor. Capacity is almost never the binding constraint on this kind of building. Depth is — whether the screw can get down at all.

Refusal is the real risk in a garden. Old foundations, a demolished coal bunker, hardcore under what used to be a path, a Victorian soakaway, roots as thick as your arm: installers hit all of these routinely. A screw that stops at 600 millimetres in shrinkable clay near a tree has not reached anywhere useful — it is inside the zone that moves seasonally, and that is the base that presents two Augusts later as a door that no longer closes. When a screw refuses, the answer is the supplier's written rule for the situation, not a shrug.

Before any of it, scan for services and hand-dig the starter positions; gardens are full of unrecorded runs. And note that a screw's tension capacity is a different published number from its compression capacity, usually smaller — which matters, because on a light building it is the tension case that governs.

A Light Building Is a Wind Problem

Once the base is level, the remaining structural question is not whether the building will sink but whether it will lift, slide or rack off. The dead load of a timber garden room is small enough that the net uplift on a shallow-pitch roof, plus the overturning moment on a tall closed box with a large glazed face, can approach or exceed it in an exposed garden. Wind actions come from BS EN 1991-1-4 with the UK National Annex, or from ASCE/SEI 7 — and in both, local exposure, topography and the height of the surrounding boundary do more to the answer than the regional wind speed does.

Ground anchor kits are sold against a perimeter spacing convention — roughly six-foot intervals around the outside is the figure most commonly quoted — and that convention is a starting layout, not a wind calculation. It says nothing about the anchor's capacity in your particular soil, which is the manufacturer's published and strongly soil-dependent figure, and nothing about your particular exposure. Treat the count as the minimum to plan for and the manufacturer's instructions as the thing you follow.

The anchor is also only as good as the load path above it. A strap into the ground with nothing continuous between it and the roof is decoration; the chain has to run from anchor or screw head into the bearer, the sole plate, the wall panel and the roof structure, with a real mechanical connection at every step. Detail the metalwork for a wet environment while you are at it — hot-dip galvanized coatings to BS EN ISO 1461 or ASTM A123, hardware to ASTM A153 — and keep the fastener metallurgy compatible with what it is bolted to, because a galvanized base plate with the wrong screws through it corrodes at the one place nobody can inspect.