Lift Thickness

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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Backfill goes in thin layers, and each one is compacted before the next. Thin lifts feel slow and save the most money.

The big idea

Backfill goes in layers, and every lift is compacted before the next goes on. Dump it all at once and the bottom stays loose, then settles for years. That is how driveways crack and patios tilt. Thin lifts feel slow, and they save the most money.

How thick is a lift?

There is no single number. The Part 9 text I read gives no lift thickness or compaction level (Section 9.12 only says to place backfill so it does not damage the wall, drain or dampproofing, and to grade it away from the foundation). The number comes from your drawings, the geotechnical report, the designer or the building official. Here is what the sources I could read say, as examples:

Two things hold across the examples: thin lifts, and a test to prove the result. I did not find a plate-compactor maker's published lift limit, so use the manual for your machine, and keep the lift thinner than the maker's maximum next to a wall.

Example (numbers made up). A 2.4 m (7 ft 10 in) deep backfill placed in 150 mm (6 in) lifts takes 2.4 ÷ 0.15 = 16 lifts. In 300 mm (12 in) lifts it would take 8. Each 150 mm lift of a 0.6 m (2 ft) wide granular zone along a 36 m (118 ft 1 in) wall is 0.6 × 36 × 0.15 = 3.24 m³ (4.2 yd³) loose, and 16 lifts total 51.84 m³ (67.8 yd³).

Moisture and weather

• Granular fill compacts best slightly damp. Too wet leaves a spongy fill, and too dry will not bind. One municipal policy asks for moisture within 3% of optimum before the next lift, which needs a test to know.
• Do not use frozen material or place fill on frozen surfaces. The same policy says so, and it is why spring settlement happens.
• Do not compact saturated fill. Cover the work at the end of the day so the next morning's lift starts on a good base.

Near the wall

• Keep the wall supported at the top until the floor system is on, or follow the designer's bracing (see Backfill Types).
• Use hand tampers or light equipment close to the wall and the drainage layer, and place fill so the dampproofing, board and drain are not damaged (Part 9, Article 9.12.3.1).
• Bring up both sides of the wall in matching lifts where there is soil on both sides, to avoid pushing it over. That is general practice, not a code sentence I read.
• Keep heavy compaction back from the wall by the distance your designer approves. Many walls are not designed for compaction loads.
• Check the dampproofing, board and pipe for damage as you go.

Equipment

• Plate compactor: thin lifts of granular fill, and pipe zones. A maker's brochure describes reversible plates as for granular and mixed soil.
• Jumping jack (rammer): confined trenches and tight spots.
• Vibratory roller: large open areas, at the lift the machine maker allows.
• Excavator bucket: not a compactor. Tamping with a bucket compacts only the surface.

Match the machine to the soil and the lift, and read the maker's manual.

Signs of poor compaction

• A settlement trench along the foundation months later.
• Cracked or sinking patios, walks and driveways.
• Water collecting beside the house.
• Soft spots when someone digs there again.

Measure lift depth with a marked shovel handle or grade stick, compact each lift fully before adding the next, and on engineered jobs record the dates, lifts and tests.

Common mistakes

• One thick lift
• Heavy compactor against the wall
• Compacting wet, frozen or spongy fill
• Using a bucket as a compactor
• Skipping the test under slabs and walks
• Copying another job's lift number