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Gravel Beds

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Clean stone is the path the water takes to reach the drain. Dirty stone plugs it.

I am a:

Tap your level and the boxes below open to the advice for you. Remembered on this device.

What it isThe clean stone around the drain pipe, and under the slab, that water flows through.
Your jobOrder clean stone, check every load, place it without voids, and keep soil out.
The hard ruleCoarse, clean stone with no more than 10 percent passing a 4 mm sieve. Sand is not a substitute.
The checkLook at each load for dust, clay and fines before it goes in, and photograph the depth before cover.

By the end you can

  • Say what clean drainage stone is and reject a dirty load.
  • Place stone around the pipe and under the slab without voids.
  • Use filter fabric to keep soil out.
  • Work out how much stone you need.
Stop and read

You are working in a trench or beside a tall wall. Treat any excavation 1.2 m (4 ft) or deeper as a trench, and follow the Trench Safety page and your provincial OHS regulator. Dusty stone is a breathing hazard, so wear a dust mask or respirator. Keep loaders and trucks clear of the trench edge.

The big idea

Clear stone is the path the water takes to reach the drain. If it is full of fines or clay, it plugs and the drain stops working. Gravel beds are cheap and easy to get right if you pay attention to what goes in and what stays out.

What the code says

From Part 9 as printed in the Vancouver Building Bylaw 2014:

  • Drainage stone is crushed stone or other coarse clean granular material, with no more than 10 percent passing a 4 mm sieve (9.14.4.1). The same limit applies to the stone around a drain pipe (9.14.3.3).
  • Around the pipe: at least 150 mm (6 in) on the top and sides (9.14.3.3).
  • A stone layer used instead of pipe: a continuous layer at least 125 mm (5 in) deep beneath the footing, extending at least 300 mm (12 in) beyond the outside edge of the footings, and graded to a sump (9.14.4). On wet sites, add enough stone to keep the top 125 mm free of soil.
  • Under floors on ground: a drainage layer is installed below the slab (9.16.2.1). The text I read gives no thickness, and the older 100 mm figure is struck out. Garage, carport and accessory-building slabs are exempt. Fill need not be compacted where it is coarse clean granular material with 10 percent or less passing a 4 mm sieve (9.16.2.2).

Check the thickness and material for your slab with your building department and drawings, because the edition matters.

What the stone does

  • Lets water flow to the pipe.
  • Keeps soil fines out of the pipe.
  • Supports the pipe, so it does not crush.
  • Reduces water pressure against the wall.

Specification

  • Clean, washed stone with few fines. A building-science forum says washed gravel should have no fines in it, and that fine dirt wicks water up and blocks the gaps that let it drain. Washed stone sold as "3/4 in" can be coarser than 3/4 in, so ask the supplier what you are getting.
  • Free of clay and organic matter.
  • Uniform size drains best.
  • Sand is not a substitute.

Example. A 5 kg (11 lb) sieve sample with 0.7 kg passing the 4 mm sieve is 14 percent, which fails the 10 percent limit, so send the load back. The sample is made up.

Placement

  • Place a bed under the pipe and cover the top and sides with at least 150 mm (6 in). Follow your drawings for the depth under the pipe.
  • Carry stone up against the wall as the design says.
  • Fill evenly, with no voids, and keep native soil out. Soil falling in is the most common problem.
  • The stone fully surrounds the perforations, and the pipe never sits directly on soil.
  • Under a slab, rake the stone level to the drawn depth and compact if the drawings call for it.

Example. A 10 m by 8 m (32 ft 10 in by 26 ft 3 in) slab with a 100 mm (4 in) stone layer needs 10 × 8 × 0.1 = 8 m³ (10.5 yd³) in place, plus an allowance for compaction and waste. The 100 mm is made up.

Filter fabric

  • Use a geotextile that suits the soil and the stone.
  • Wrap the stone, not the pipe. Lap the seams generously and fold the fabric over the top.
  • Keep it tight and in place while you backfill.

Backfill

  • Use free-draining backfill against the wall. Where crushed rock backfill is used, it extends to the footing level (9.14.2.1). Confirm any fines limit with your building department.
  • Do not push soil into the stone.
  • Compact in lifts, and keep heavy compaction off the wall. The Backfilling section covers it.

Common mistakes

  • Dirty stone
  • Not enough depth
  • No fabric where it is needed
  • Soil falling into the stone
  • Pipe sitting on dirt
  • Uneven depth
Pro note

Look at the stone pile at the supplier. If it is dusty or has clay in it, send it back.

Weather note

Rain washes fines into the stone, wind blows soil into the trench, and cold makes fabric stiff. Keep the trench clean, and cover it if rain is coming.

What the inspector looks at

Stone type, depth and cleanliness, and the fabric, before backfill.

✅ Check yourselfTap a question
What does Part 9 limit about drainage gravel?
It is crushed stone or other coarse clean granular material, with no more than 10 percent passing a 4 mm sieve (9.14.3.3 and 9.14.4.1).
A 5 kg (11 lb) sample from a delivery is sieved, and 0.7 kg passes the 4 mm sieve. Does it pass?
0.7 ÷ 5 = 14 percent, which is over the 10 percent limit. Send the load back. The sample is made up, and a real test uses a proper sample and sieve.
A 10 m by 8 m (32 ft 10 in by 26 ft 3 in) slab gets a 100 mm (4 in) stone layer. How much stone?
10 × 8 × 0.1 = 8 m³ (10.5 yd³) in place, plus an allowance for compaction and waste. The 100 mm is made up, so use your drawings.
Where a gravel layer is used instead of drain pipe, how deep and how wide must it be?
At least 125 mm (5 in) beneath the footing and extending at least 300 mm (12 in) beyond the outside edge of the footings, graded to a sump (9.14.4).
Is sand a substitute for drainage stone?
No. Fines hold water and block the flow. Drainage stone needs few fines.
Soil keeps sliding into the stone in the trench. What do you do?
Stop placing, and keep soil out. Under 9.14.4.4 a wet site needs enough extra stone to keep the top 125 mm free of soil.
🧰 Gear for this stepMust-haveRent itNice to have
  • Must-haveLayout and grade:
  • Must-haveStone handling (hand tools):
  • Rent itStone handling (machine): a loader or conveyor for large volumes
  • Must-haveFabric:
  • Must-haveCompaction (hand tool): a hand tamper for narrow trenches
  • Rent itCompaction (machine): a plate compactor for slab stone
  • Must-haveChecking:
  • Must-havePPE: hard hat (CSA Z94.1), eye protection (CSA Z94.3), green-triangle boots (CSA Z195), gloves, and a dust mask or respirator (CSA Z94.4) when stone is dusty
👷 Who can do thisTap your level
Home DIY
A capable owner can place and rake clean stone around a drain pipe in a shallow, stable trench. Treat 1.2 m (4 ft) as a no-entry line, do not judge sloping or shoring yourself, and do not dig beside an existing footing or foundation. Ask the supplier for clean, washed stone, and have an inspector see it before cover.
Laborer or helper
Shovel and rake clean stone where the crew points, and keep soil from sliding into the trench. Do not enter an unprotected trench. Tell the lead if the stone looks dusty or muddy, and stop if soil is falling in.
1st-year apprentice
Under supervision, place stone to depth, wrap fabric, and keep soil out of the trench. Learn first what makes stone clean and how to spot fines or clay.
Apprentice, 2nd to 4th year
Place stone to the marked depth and wrap fabric under direction. Look at each load for dust, fines and clay, and tell the journeyperson before it goes in. Never stand in a trench that is not protected, and report slumping soil right away.
Journeyperson
You own the stone specification, depth and envelope, and the fabric detail. Stop-work triggers: reject the load and stop placing if the stone is dusty, has clay or fines, or soil is falling into the trench.
Lead hand or foreman
Check every delivery against the stone spec before it is placed, and send back dirty loads. Stop placement when soil falls into the trench. Keep the crew out of any trench that is not protected, and call the engineer if the ground or drawings do not match.
Contractor or business owner
Price stone by volume and allow for rejected loads. Keep supplier tickets and photos of depth. Specify washed, clean stone in the contract. Dirty stone plugs the drain later, so reject it at delivery.
Starting a new business
Write the stone spec and the right to reject loads into your supplier order and your contract. Keep delivery tickets. Confirm with your insurer what covers a drain that plugs later, and subcontract trench work if your crew is not trained for it.
By Province:

The technique is the same across Canada. What changes is the code edition, the stone depth and size your province or municipality asks for, and who inspects. I read Sections 9.14 and 9.16 in the Vancouver Building Bylaw 2014, which may not match your province's current edition. Confirm the clauses with your local building department.

See Canada by Province for the full comparison.

Sources:

Last reviewed October 2026. General information, not engineering or legal advice.

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