7 Step Gravel Garden Mulching Method with Cedec Self-Binding Gravel Over a 15-Metre Bed

Measure the bed before ordering. A 15-metre bed that is 1.2m wide gives 18 square metres. At a finished depth of 50mm, Cedec self-binding gravel settles to roughly 1.9 to 2.0 tonnes per cubic metre once compacted, so the order wants to land around 2.2 to 2.4 tonnes. Do not buy to a bare minimum. The yellow-brown Cedec packs down hard because the fines are mixed through the aggregate, and it takes more material than a loose decorative 20mm shingle would suggest.

Most builder’s merchants deliver it as an 850kg dumpy bag on a pallet. For this length of bed, that means handling three bags. Ask for the bags to be placed as close to the work as the wagon can reach. Self-binding gravel is dense, and the fines make it cling to the barrow, so long barrow runs turn the job into slow hauling.

Strip the run before any gravel arrives

Laying Cedec straight over the existing surface is the mistake that ruins the finish early. Dig out the top 75 to 100mm across the whole run. That depth gives room for a sub-base where needed, a membrane, and 50mm of compacted gravel while keeping the finished level flush with the surrounding lawn or paving.

Grade the excavated base with a slight downhill run. A fall of about 1 in 60 along 15 metres gives roughly 250mm of drop from end to end. On a flat boxed-in bed, water sits in the fines; by the second autumn the surface can turn greasy and mossy. The fall does not need to be visually obvious.

Pull out obvious roots and old perennial weed crowns while the bed is open. Bindweed and couch grass push through weaknesses in a membrane, so the digging stage is the time to remove what you can. Tread the base firm or run a vibrating plate over it once before adding the next layer. A loose base leaves softness under the finished surface even after the top has been compacted.

Heavy clay needs more preparation. If the ground holds water, spread 40 to 50mm of MOT Type 1 sub-base over the graded soil and compact it first. On free-draining sandy soil, the Type 1 can be left out and the membrane laid straight onto the graded subsoil.

Use woven membrane where the surface is meant to stay fixed

Use woven polypropylene, not the thin spun fleece. A woven landscape fabric such as Plantex, or a 100gsm-plus woven geotextile, lets water pass through while stopping the gravel from migrating down into the soil and stopping most weed seed from pushing up. The cheap white fleece sold in 1m rolls tears as soon as a fork catches it and tends to clog within two seasons.

Lay the membrane with 100mm overlaps at the joins. Run it 50mm up any edging board, then pin it every 750mm with galvanised ground pegs, adding extra pegs along the overlaps. For planting, cut a cross with a sharp craft knife, fold the flaps back under, and keep the fabric tight around the stem so the hole does not become a weed pocket.

If you are planting through the membrane into a bed, the fabric permanently limits how easily plants can be moved and fed. An ericaceous camellia feeding routine, with sulphur chips and ericaceous mulch twice a year, is awkward under a sealed membrane. Leave generous unmembraned planting pockets, or treat the gravel around acid-lovers as decorative while feeding takes place at the crown.

Spread the Cedec damp, in short sections

Tip the gravel in heaps along the bed, spacing them every two or three metres so the barrow is used as little as possible. Spread it with a landscape rake to a loose depth of about 65mm, which should compact down to 50mm. Work in two metre sections and keep checking the level against a string line pulled tight between two pegs at finished height.

Cedec needs moisture before it binds properly. After spreading, water it with a rose on the can or with a sprinkler until the layer is wet through without standing in water. The fines need enough moisture to lock the angular stone together.

Compaction decides the finish

A wacker plate, the vibrating compactor, is what turns loose Cedec into a firm self-binding surface. Hire one for half a day if there is not one already on site. Run it over the damp gravel in overlapping passes, with two passes as the minimum, until the surface stops shifting under the plate and the fines have worked upward to bind the top. The colour deepens as the surface tightens.

Moisture has a narrow working range. Dry material sheds fines, and the stone will not knit together properly. Too much water turns the fines into slurry, which can set unevenly and leave soft patches. After compaction, press a thumb into the finished surface. Correctly compacted Cedec takes a thumbprint only with real effort.

The plate cannot reach tight around plant stems, so compact those margins by hand with the end of a fence post or a hand tamper. These hand-tamped areas are usually the first to loosen. Watch the 100mm ring around each planting hole through the first winter and top it up where needed.

Where the new gravel meets a paved path or a step, finish the Cedec flush with the hard edge or a few millimetres lower. A proud gravel edge spills stone onto the slab every time the line is walked, and loose material can be tracked across the patio within weeks. Run the wacker plate right up to the hard edge so the join is compacted.

Open sides need an edge to hold the depth. Kerb edging or a treated timber board keeps the 50mm layer in place. Without an edge, the gravel creeps sideways into the lawn and the mower throws stones. A 150mm-deep treated board pegged at 1m centres will hold for years.

Quantity check for the full bed

The 15-metre by 1.2m example gives 18 square metres. A 50mm finished depth is 0.9 cubic metres of compacted gravel. Cedec compacts at close to 2.0 tonnes per cubic metre, so the compacted gravel layer alone comes to about 1.8 tonnes.

Allow for shrinkage between loose spreading depth and compacted depth. Laying at roughly 65mm loose to finish at 50mm is about a 30 percent reduction in volume, which brings the order to roughly 2.2 to 2.4 tonnes, or three 850kg bags. If clay ground needs a 40mm Type 1 sub-base, the extra calculation is 18 square metres at 40mm, which is around 0.72 cubic metres and near 1.5 tonnes of Type 1 on top of the Cedec. Doing the arithmetic before calling the merchant can avoid a second delivery charge, often as costly as a third of a tonne of stone.

After the surface has settled

Compacted Cedec is a working surface, suitable for walking and wheelbarrows. It should not be expected to hold moisture in the soil the way a 75mm bark mulch does, so it is a poor substitute for deep organic mulch around thirsty plants.

Weed seed still lands on the surface and germinates in the fines, especially below deciduous planting where leaf litter rots into the top layer. A quick hoe in spring lifts seedlings before they root through into the membrane. Where traffic has thinned the surface, add a thin 10mm refresh layer every three or four years, water it, and tamp it by hand.

Once the surface has tightened, it behaves less like loose mulch and more like a small hardstanding threaded between plants. How much of the bed is meant to stay open for roots, feed, and future planting?