How the two differ in kind
The first thing to notice is that these two are not the same component doing the same job better or worse. A skid frame is a base placed UNDER a floor the shed already has: pressure-treated runners bedded on compacted stone, spaced across the shed's width so the floor joists cross them, holding that floor flat, level and up in the air. The shed arrives with its own timber deck and the base's entire task is to keep it straight and dry. A slab is not under the floor. A slab IS the floor — the walls stand on its surface, you sweep it, you stand on it, and the timber deck the shed came with is now either left out of the build or laid down on concrete where it has nowhere to breathe. So the honest first question is not which base is better but which one your shed is designed to meet, because a kit with a framed floor and a slab are two floors trying to occupy the same space.
The cost shapes differ as much as the components do, and knowing which way each one scales is worth more than any figure. A skid frame is a linear-metre purchase: the runner count follows the width at the frame's runner spacing, each runner is as long as the shed, and beneath them goes a shallow bed of stone. Add a runner and you have added the length of the shed in timber, nothing more. A slab is a volume purchase, and volume has a multiplier hidden in it — area times thickness, where thickness is a single decision that then applies across every square metre of the footprint. Around that volume sits a perimeter cost in formwork and any thickened edge, and beneath it an area of digging, stone and carted-away spoil that arrives whether you were thinking about it or not. One of these is labour you supply in evening-sized pieces; the other is a weather-bound day with a delivery on it and no pause button once the load is on site.
Then there is the part that has nothing to do with either material. A skid base is reversible in the most literal sense — the name comes from the fact that the building can be dragged. Change your mind, sell the house, redesign the garden, discover you put it where the afternoon sun was, and the shed moves and the runners come up and get used again. Breaking out a cured slab and carting it away is harder work than placing it was, and it is the reason a slab poured under a shed that lasted three years is the most expensive mistake available in a garden. That permanence can also be read by someone other than you: many authorities treat a fixed, permanent foundation as one of the things that separates a movable structure from a building, which can pull the same shed into permit, setback or assessment rules it would otherwise have sat outside. This varies from one authority to the next and floor area and height usually matter more than the base does — but it is a free question to ask and a costly one to answer with concrete.
The factors that actually differ
| Timber skid frame on gravel | Poured concrete slab | |
|---|---|---|
| What the base actually is | A support system beneath a floor the shed already owns. The runners hold the joists; the shed keeps its own deck and the base never becomes a walking surface. | The floor itself. The walls sit on its face and the shed's own timber deck is either deleted from the build or laid on concrete with nowhere to dry out. |
| How load reaches the ground | Along a few lines. Everything heavy has to end up over a runner or over a joist spanning between them, so a machine parked in the wrong place is loading a span, not the ground. | Across the whole footprint at once. A point load can land anywhere on it without anyone working out which member is underneath — which is exactly what a workshop full of movable heavy things wants, within whatever the slab's thickness and reinforcement were designed to carry. |
| Where the cost scales | With linear metres. Runners follow the width, each runner's length follows the shed, and the stone bed is shallow. The material list grows in a straight line. | With volume — area multiplied by thickness — plus a perimeter cost in formwork and edging, plus an area of excavation and spoil to remove before any concrete is ordered. |
| The shape of the work | Owner-sized and pausable. Lay stone one evening, set runners the next, walk away mid-job with nothing spoiling. | One committed window. Base, formwork, pour and finish arrive as a single operation on a weather-dependent day, and the clock starts the moment concrete does. |
| Levelling, later | Adjustable for life. A runner that has settled is packed or shimmed and the shed is true again before lunch, as many times as the ground asks for it. | Levelled exactly once. Whatever the float left behind is the floor this shed stands on permanently, and a slab that cured out of true stays out of true. |
| Underside and moisture | An open, ventilated gap. Air moves under the floor and dries it, and the stone drains — the standing job is watching the points where treated timber meets damp ground. | A capillary path unless a membrane goes down first. Without one, ground water wicks up through the slab and into everything stored on it, quietly and continuously. |
| Anchoring against wind | Essential and separate. A light shed on runners is a sail with mass beneath it but nothing gripping the earth, so ground anchors or stakes are part of the design, not an extra. | Built in. Concrete takes a fixing anywhere you want one, so holding the walls down is a bolt rather than a scheme. |
| The void beneath | A real space. It ventilates the floor and it also houses whatever decides to live there; skirting it solves the second problem and undermines the first. | No void at all. Nothing gets under a slab, which removes both the airflow and the tenants in one stroke. |
| Cost of changing your mind | The cheapest reversal in the garden. Drag the shed, lift the runners, use them again somewhere else; the ground goes back to being ground. | The dearest. Removing cured concrete is more effort than placing it was, and the spoil has to go somewhere it cannot be persuaded to go cheaply. |
Which one, and when
Choose timber skid frame on gravel when…
- The shed came with its own timber floor and the base's only job is to hold that floor flat and out of the wet.
- Permanence is not settled — a rented plot, a garden still being drawn, or a shed that might follow you to the next house.
- Ground that shifts with the seasons, where being re-levelable in an afternoon is worth more than being level once.
- The local rule you are staying on the right side of turns on whether the building is fixed down — ask first, then build to the answer you got.
Choose poured concrete slab when…
- Something heavy and mobile is going in: a vehicle, a machine on feet, a bench that must not move, anything on a jack.
- The floor is the point — you want to sweep it, roll things across it, and never feel it give underfoot.
- This shed is a permanent part of the property, and you have already checked what that means where you live.
- The plot is flat and drains, so pouring a surface that gets levelled exactly once is a strength rather than a gamble.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which one is cheaper?
- They are cheap and expensive in different places, which is why this site will not put a figure on either. The skid frame is a short list of timber bought by the metre, a shallow bed of stone, and labour you supply yourself over as many evenings as it takes. The slab carries a volume of concrete, an area of digging and spoil removal, formwork around the perimeter, and a single day that either goes well or goes expensively wrong. Compare them over the whole life instead of the build: a slab bought once and never touched again can beat a base that gets re-levelled every few seasons, and a slab poured under a shed that moves in three years loses to it comprehensively. Put your own dimensions through both calculators and price the quantities locally — the arithmetic is the part that transfers, the rates are not.
- Can I put a shed with a wooden floor onto a slab?
- You can, and it is the combination that disappoints people most often. A timber deck laid flat on concrete has no airflow beneath it and sits on a surface that stays damp longer than the air above it, so it takes up moisture and holds it exactly where nobody can see. If the shed must keep its floor, raise it on treated battens with a membrane under the slab and a gap for air to cross, which is a small version of the skid arrangement built on top of concrete. The cleaner answer is to leave the timber floor out and let the slab be the floor it already is — and to sit the walls slightly proud of the slab edge or step the edge down, so rain running off the roof does not have a flat path back in under the wall.
- Do skids need gravel, or can they sit straight on the soil?
- Bed them on compacted stone. Timber laid on bare earth is in permanent contact with something that holds water against it, and even treated timber gives up in that position sooner than it should — the stone's first job is drainage and only its second is bearing. It also gives you a surface you can screed flat and rake back, which is how the frame gets level in the first place and how it gets level again later. Setting the runners on blocks or paving slabs bedded in that stone is a common refinement: it lifts the timber clear of the splash line and gives you a hard, stable pad to shim against when one end settles.
- Does either one need to go below the frost line?
- For an ordinary garden shed, usually neither does — both are shallow, both float on the ground, and both will move when the ground beneath them moves. The difference is what happens next. A skid frame records the movement as a shed that has gone slightly out of true and hands you a fix that takes an afternoon. A slab records it permanently, as a crack or a tilt, and offers nothing to adjust. If the ground genuinely heaves and the building matters enough that neither outcome is acceptable, the honest answer is that this comparison is the wrong one: what you want is a foundation that reaches below the frost depth on piers or piles, and that is a different page and a different calculator.
- Will pouring a slab make my shed count as a permanent building?
- Frequently it is one of the tests, and occasionally it is the deciding one — but it is rarely the only one, and the answer is genuinely local. Many authorities look first at floor area, height and how close the building sits to a boundary, and treat the foundation as evidence of intent rather than the trigger itself. Others draw the line precisely there, at whether the structure is fixed to the ground. What does not vary is which order to do this in: find out what your own authority asks before you order concrete, because the question is free to ask and the base is the one part of the decision you cannot quietly reverse afterwards.