Why Patio Bases Need Gravel Before Concrete

Compacted gravel spreads load and helps water move away from the slab. Pouring concrete on soft or organic soil invites settlement and cracking. The stone layer is structural support and a drainage path, not a cosmetic underlay.

Depth depends on soil and climate. Follow a local specification rather than copying a neighbour’s patio blindly. Clay that holds water, made ground, and frost-prone regions often need more care than a free-draining sandy plot.

In UK practice you will hear hardcore or MOT Type 1; in US practice, compacted crushed stone or gravel base. Names differ; the job is the same: remove unsuitable material, place compactable aggregate in lifts, and compact it before forms go final.

Run the Gravel Calculator with the sub-base footprint and depth. Run the Concrete Calculator with slab thickness. Add them in the Cost Estimator as separate lines. If the patio includes a thickened edge or footings, those concrete volumes are extra — note them explicitly.

Worked example: a 10 by 12 ft patio (120 sq ft). Sub-base at 4 inches: 120 × (4/12) = 40 cubic feet ≈ 1.5 cubic yards of gravel. Slab at 4 inches: another ≈ 1.5 cubic yards of concrete. Those are not interchangeable materials on one ticket.

Metric: 3.0 by 3.6 m = 10.8 m². Sub-base at 100 mm → 1.08 m³ gravel. Slab at 100 mm → 1.08 m³ concrete. If you thicken the slab to 150 mm for a heavier use, concrete becomes 1.62 m³ while gravel stays on its own depth decision.

Place stone in lifts and compact each lift. Dumping the full depth and waving a plate over the top once is how soft spots survive. Ask whether the supplier’s coverage assumes loose or compacted depth, and order a little extra for compaction if you are working to a finished compacted level.

Geotextile over soft soil can separate subgrade from stone and reduce pumping of mud into the aggregate. It is not magic — you still need the right depth of compactable material. Price fabric as its own line.

Recycled hardcore can work when it is clean and compactable. Contaminated fill with plastics, timber, or clay lumps creates soft spots. Ask what is in the load. The Sand Calculator belongs in the picture only if your method uses a thin sand blinding over compacted stone — sand alone is not a substitute for a proper sub-base on soft ground.

Patios usually need a slight fall away from the house so water does not sit against the wall. That fall can change average concrete thickness slightly if the top is levelled while the dig follows the slope — discuss finished levels before you lock a concrete order.

Against a house wall, flashing, damp-proof courses, and weeps matter as much as cubic metres of mix. Holding a slab hard against porous masonry without detailing is a damp invitation, not a materials win.

If the patio ties into existing paving or a doorway threshold, set levels from those fixed points first. A perfect volume calculation on the wrong datum still produces a step you trip over.

Have forms, reinforcement, and a screed board ready before the mixer truck or pump arrives. Cold joints from delayed deliveries show forever on a patio finish — timing matters as much as volume.

Confirm access. Narrow side returns may need a pump or barrowled ready-mix; that labour belongs in the Cost Estimator beside the concrete line. Bag mixing a full patio is possible but rarely pleasant — use the Concrete Calculator volume to decide honestly.

Plan curing: keep the slab damp and protected from extreme heat or frost according to the mix guidance. Walking on the slab too early can mark a finish that looked perfect at pour. Light foot traffic often waits a day or two; heavier loads wait longer.

A quote that prices “concrete patio” without naming dig-out, sub-base depth, and disposal is incomplete. Ask for those lines. Recalculate gravel and concrete yourself so a low headline number cannot silently assume you already excavated and compacted.

Compare thickness, mix, reinforcement, and base depth across bidders. A cheaper pour on thinner stone is not the same patio. Your Gravel Calculator and Concrete Calculator results are sanity checks, not accusations — good trades explain gaps calmly.

Waste and spoil: excavated soil has to go somewhere. Skip hire or haul-away can rival the stone cost on a tight urban plot. Put disposal next to materials so the budget reflects the whole dig-to-pour sequence.

Retaining edges, steep slopes, poor fill, or a patio that will carry a hot tub or heavy masonry barbecue are beyond casual DIY thickness picks. Volume tools still help you understand quotes; they do not replace engineering judgment where loads or ground are unusual.

If an old patio failed, dig a small inspection pit and look at what was underneath before you pour the same mistake thicker. Often the failure was the base, not a shortage of decorative finish on top.