Key Takeaways
- Laying a sub-base runs in a fixed order: set out and fix the fall, dig to firm ground, deal with anything soft, membrane if the ground calls for it, then spread and compact the stone in thin layers.
- A sub-base dig goes down to firm subsoil, not an invented number. Topsoil runs anywhere from 75 to 250mm deep and every bit of it needs to come out – it’s called reducing to the formation level.
- Sub-base is compacted in layers no thicker than 150mm finished, so a 100mm patio base or a 150mm drive base is a single layer, and only bases over 225mm get split in two.
- MOT Type 1 loses roughly 30% of its depth under the plate, so about 130mm of loose stone gives a 100mm finished layer and about 195mm gives 150mm.
- A vibrating plate needs at least eight passes on each layer, worked in both directions with each pass overlapping the last, and the job is done when the plate starts to bounce. In the trade it’s called compaction to refusal, the point where the stone will not go any tighter.
- A finished sub-base should look tight and closed rather than open and honeycombed with gaps showing between the stones. An open surface means that part of the load was short of dust, and it will swallow bedding sand and settle later.
- A finished sub-base sits within 15mm of the string line at every point, measured about every metre.
From the bakers dozen or so times I’ve been hired to repair a patio because it had started rocking or a driveway with ruts down it, the surface was usually fine and the base under it was the problem. Either the installers had not dug to firm ground, or the stone had gone in as one thick lump and only the top third ever got compacted. The slabs came up, the base came out, and the second time round it was done in the order below.
If you want to know how to lay a sub-base that stays put, that order is the whole article. The sub-base explainer covers the other questions: what the layer actually is, which grade suits which job, the depth table, and the ordering arithmetic. This page is the doing part: bare ground at one end, a tight, level base you can build on at the other.
Before You Dig
Ground preparation for a patio or drive starts with three checks: where the damp-proof course sits, what is buried under the dig, and how firm the subsoil is.
The damp-proof course (the black line or the wider mortar joint low down on a house wall) sets your ceiling. The rule is a 150mm gap between the top of the paving and that line, so it is the first measurement I take and every other level is worked down from it. I used to find the DPC before I got the tape out for anything else.
Tools for Installing a Sub-Base
Get a cable and pipe detector over the area before a spade goes in. Services run under front gardens more often than people expect, and the gas main does not care how good your patio is going to be. The tools are not exotic: two tapes, one of them 10m or longer, a 1200mm spirit level, string line and pegs, a club hammer, spade, mattock, rake and barrow, plus a vibrating plate hired for the compacting day. A self-levelling laser is worth a weekend’s hire on a big area, because it is far more accurate than the bubble-vial sort.
Be realistic about time. A 50m² drive that a three-person crew turns round in about five days took a DIYer with one helper eight weekends or more, and the digging is all at the start.
Step 1: How to Set Out a Sub-Base and Fix the Fall
The fall on a sub-base is set with pegs and a string line before the first spade goes in, because correcting it afterwards costs a day.
Peg out the paving edge with string, square the corners using a 3-4-5 triangle (three units along one side, four along the other, and a five-unit diagonal gives a right angle), then measure both diagonals of the rectangle: equal diagonals mean it is square. Next, pour a line of dry sand, or white flour, from a bottle 200 to 300mm outside the string. That is the dig line, and the extra width gives you room for edging and its concrete haunch later. Sand lines rub out and redo; string does not.
Set the fall next. Establish a level line between two pegs first, using a line level, and only then drop the far end by the fall you want. Setting a line “at the fall” without finding level first is the mistake that puts water against the house. For a patio I work to 1 in 60 or steeper away from the building, so a 3m wide patio drops about 50mm across its width. The commercial minimums are 1 in 80 along the run and 1 in 40 across it, and a home patio wants to be tighter than that because you notice a puddle outside your own back door in a way you never notice one on a public footpath.
Two limits on the kit: a timber straight-edge flexes past about 4m and a taut string sags past about 10m, so longer runs need an intermediate peg. The fall is checked while you dig, against the line, not once at the end.
Step 2: How Deep to Dig for a Sub-Base
Dig depth for a sub-base is paving thickness plus bedding plus the compacted base, worked from the finished level down.
For 60mm block paving that is 60mm of block, a 40mm sand bed and 150mm of base, so a 250mm dig. For a patio on slabs, add the slab and its mortar bed to a 100mm base. For a gravel path it is the gravel depth, the membrane and 75 to 100mm of base. The compacted depths themselves, by job and by ground, are in the depth table on the sub-base explainer, and the sub-base calculator turns your area and depth into bags.
That figure is a minimum, not a target anyone can pre-determine. Topsoil is the dark, rooty layer on top, and it runs anywhere from 75 to 250mm deep depending on the garden. Every bit of it comes out, turf and roots included, since anything that can rot will rot, and the paving above drops by the same amount a year or two on. Under the topsoil is the subsoil, paler, stiffer and usually more clay in it, and that is the ground you build on. The trade calls it reducing to formation level. Never bury the topsoil under the base to save a skip. It is too soft to build on and usually too valuable to waste, so save it for the borders, unless you have no room.
Step 3: What to Do With Soft Spots, Clay and Old Trenches
Soft spots in the formation get dug out and refilled with compacted stone, however deep that goes, because a solid bottom beats a flat one.
To check this properly, walk the formation once the topsoil is off:
- anywhere your heel sinks;
- anywhere the ground is darker or wetter than the rest;
- anywhere there is a line of looser fill running across the site;
is a spot that will settle under the base if you leave it. Old drainage trenches are the classic: the ground over them was back-filled loosely years ago and it is still going down. Dig the soft material out, fill the hole with MOT Type 1 in layers no thicker than 150mm, and compact each layer before the next. That is a capping layer, and it brings a lumpy formation up to something you can lay a proper base on.
What about clay in the formation level?
Heavy clay is a different problem, and it is where a lot of sub-bases go wrong. Clay swells when it is wet and shrinks when it dries, and given a wet winter it will pump up between the lumps until stone and clay are one grey porridge. Where I hit clay I added depth to the base and never skipped the separation sheet between clay and stone. The next step covers that.
Old brick and broken flag from the site only count as sub-base once they have been crushed down to 40 to 50mm and blended with fines; dropped in whole they leave voids that settle.
Step 4: Membrane Under the Sub-Base, or Not
A separation membrane earns its place over clay, over soft ground and over anything that was once filled in. Where the subsoil is firm and undisturbed, compacted stone can go straight on, but I lay a membrane anyway.
A woven, geo-textile heavy duty membrane’s job is to keep two materials apart under load: stone that would otherwise punch downward, and soft ground that would otherwise squeeze upward into the voids. I use a fabric that is at least 100 to 200 GSM, dependent on how heavy the load will be and what is available.
Where the membrane sits depends on what is going on top. Anywhere a car will park, it sits on the formation with the stone on top of it, since it is the wheel load that forces stone into soil. Under a loose gravel path or patio that only takes feet, the fabric goes over the compacted base instead, between the base and the gravel, because the failure there is the two mixing rather than the base sinking. A drive over clay can justify using fabric in both places. Where two sheets meet, lap them by 300 to 400mm, and carry the outer edges up the side of the excavation so they end up trapped behind the edging.
Choose the sheet by how heavy it is and how it is woven, matched to the load, whatever the roll calls itself, and forget weed control as a reason: the weeds that trouble paving germinate from seed blown in on top, well above anything buried 200mm down.
Step 5: How to Spread the First Layer of Sub-Base
Sub-base is compacted in layers no thicker than 150mm finished, and for most garden jobs that means one layer, because a whacker plate only reaches so far down.
Put the full depth down in one go and you compact the top third, half-compact the middle, and leave the bottom as loose as it was on the lorry. It feels solid underfoot for a summer and then it moves. For most garden jobs this never comes up, because a 100mm patio base or a 150mm drive base is one layer and one round of compacting. Only go to two if your finished base is thicker than about 225mm, which means heavy vehicles or ground soft enough that you have had to dig deep. In that case, finish the bottom half completely, compact it, and start again on top.
Work out the loose depth before you spread. MOT Type 1 loses around 30% of its depth under the plate, so 130mm loose gives you roughly 100mm compacted, and 195mm loose gives roughly 150mm. The exact figure moves with the rock (granite nearer 20%, sandstone nearer 40%), with how well the fines are blended through the lumps, and with how damp the load is, so treat it as a working allowance rather than a conversion. Barrow the stone in, rake it out to the loose depth, and rake it to the line so the fall is already in the layer before the plate touches it.
Step 6: How to Compact a Sub-Base
Compacting a sub-base properly is eight or more laps of the plate over every layer, run both ways, until the plate quits sinking and starts to bounce. Work up and down first, then across, with each pass overlapping the last by half the plate’s width. That bounce is the signal. The trade calls it compaction to refusal, the point where the stone will not go any tighter, and once you feel it there is nothing more to gain from another lap. On a narrow path you may only be able to run one direction, which is fine; on anything wider, change direction between passes because it evens out the result.
Moisture makes or breaks this. The stone packs hardest slightly damp, in the region of 8 to 13% water by weight, which in practice is the load as it usually arrives from the yard in British weather. Too dry and the dust never locks the lumps together, so the surface stays open no matter how many laps you give it. If it has sat uncovered through a dry spell, a light spray from a hose before each pass is enough. Wet enough to pool is too wet, and that case gets its own section below.
Tamping by hand, with a weighted post rammer or a sledgehammer head, will get two or three square metres of footpath tight if the lifts are thin, and the test is your heel: stand all your weight through it, and any give means more work. For a patio, and for every drive, I hire the plate. Compacting without a plate is a question I get asked enough that I will cover it properly in a future article.
Step 7: Checking the Level and Tightening the Surface
A compacted sub-base is dipped from the string line about every metre, and anything more than 15mm off gets raked or topped up and re-compacted. Dipping is simply measuring from the taut string down to the stone with a tape or a marked stick, at intervals across the whole area, and it works because the string already carries the fall. Anything within 15mm is fine, and the bed on top will absorb that. Anything outside it is a hollow or a hump that will telegraph through the paving as a thick spot of mortar that cracks or a thin one that never bonds. Rake off, top up, re-run the plate.
Then look at the texture. A good surface looks tight, closed and slightly polished where the plate has been. A surface that reads open and honeycombed, with visible voids between the lumps, has too few fines in that part of the load, and it will swallow bedding sand into those voids and settle under the paving. Brush grano dust or grit sand across it, run the plate over again, and it closes up. It’s known as “boney”, and the answer to boney is fines and another lap, not more water.
Step 8: Building Up the Depth and Finishing
A sub-base over 225mm is built as two lifts, the second spread and compacted like the first, then checked against the fall one last time. A 250mm base is two lifts of 125mm compacted, or one of 150mm and one of 100mm; it is not one of 250mm. The second lift is easier than the first because you are spreading onto a firm, level floor rather than onto the formation. Go back over the finished surface with your tape, measuring down from the string line every metre or so. The string already carries the fall, so that measurement should come out the same everywhere. Check it along the house wall and along the low edge where the water is meant to run off, because those are the two places a mistake costs you most. Then give it one more lap. I have never once regretted the extra ten minutes.
Wet Ground, Rain and When to Stop
Compacting a sub-base on waterlogged ground makes it worse, because every pass draws more water up through the layer until it moves like a waterbed.
Damp is good and wet is a no-go. If you run the plate, and:
- the surface ripples; or,
- water rises around the edge of the plate; or,
- the stone stays soft however many passes you give it;
the ground under the layer is saturated and the layer is not going to compact that day. More passes will not fix it.
Stop, and if it is a small patch dig out the soft material and replace it with fresh dry Type 1 in thin lifts; if it is the whole area, cover it, wait for it to drain, and if water is standing on the formation after a day or two you have a drainage problem that the base cannot solve. Put a soakaway or a perforated land drain in while the excavation is open, because cutting one in after the base is down means digging the base back out. The pit version is in build a soakaway, and the trench version, which the trade also calls a land drain, is in how to build a French drain.
Rain on an open sub-base is less of a worry than people think. Stone rained on and compacted once it has drained a little is fine; stone compacted while water is still running off it is not. Leaving a finished base open for a week or two before paving does it no harm, and I used to prefer it, because anything that was going to settle got the chance to.
What Goes on Top of the Sub-Base
Every surface sits on the same tight base, and only the layer between them changes.
| Surface | What goes between it and the sub-base | Depth of that layer |
|---|---|---|
| Block paving | Grit sand, screeded flat. Never soft building sand, which holds water and ruts. | 40mm |
| Paving slabs | A full mortar bed under the whole slab, not five dabs. Method and mix on laying paving slabs on a dry mix. | See the dry mix guide |
| Loose gravel | A second membrane over the compacted stone, then the gravel on top of it. | Gravel at roughly twice its own stone size |
| Artificial grass | A blinding of grano dust, or sharp sand at a push, screeded flat so the turf lies smooth. | Enough to fill the surface texture |
| Shed or garden building | Nothing. The bearers sit straight on the finished base. | n/a |
On a gravel build, the second sheet is laid over the compacted stone, and that is the moment to run a hose over it and watch where the water goes, before any gravel is tipped. On bare stone the water just vanishes downward, and under a layer of gravel you would never see it, but across the membrane it runs as a sheet, so the direction of fall and any hollow show up straight away, and if something is wrong the fabric rolls back in a minute and the base gets corrected underneath. The full builds are on the gravel patio guide and the gravel driveway guide.
Next Steps
We supply MOT Type 1 by the 850kg bulk bag or the loose load, delivered nationwide, and one bag covers about 4.4m² at 100mm compacted or 2.9m² at 150mm, with the sub-base calculator doing the loose-depth allowance for you. If you have any questions, drop us a line and we will work through it with you.
If you would rather talk your sub-base through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.
Have you ever built a sub-base from scratch? How many layers did you go with, and did you hire a plate or do it by hand? We would love to hear how it held up in the comments.
FAQs
Should I wet MOT Type 1 before compacting it?
Yes, MOT Type 1 packs hardest slightly damp, so a light hose spray before each pass helps if the load has dried out. Wet enough to pool is too wet, and it will not compact until it drains.
Do I need to compact the soil before laying a sub-base?
No, the formation layer under a sub-base needs to be firm rather than machine-compacted. Walk it, dig out anywhere your heel sinks, and refill those spots with compacted stone before the first layer goes on.
How flat does a sub-base need to be before paving?
A sub-base needs to sit within 15mm of the string line at every point. The bedding layer above it absorbs that much and not much more before slabs start rocking.
How many passes with a wacker plate does a sub-base need?
A sub-base layer takes eight laps of the vibrating plate at the very least, half-lapped and run in two directions. The layer is done when the plate bounces rather than sinks.
Can you lay a sub-base on top of grass?
No, a sub-base laid on grass will settle as the turf and topsoil rot down beneath it. Strip both first and put the stone on the firm subsoil below.