Backfill Types

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

Some links on this site are affiliate links. If you buy through them we may earn a commission at no extra cost to you. Affiliate disclosure

The soil you put back against a wall decides how the drain works and how much pressure the wall carries.

I am a:

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

What it isChoosing what soil goes back against a foundation wall and the drain.

Your jobUse clean, free-draining fill against the wall, keep clay, organics, debris and frozen lumps out, and wait until the wall is supported.

The hard ruleNever backfill a wall that is not supported at the top, and never push loose fill into fresh dampproofing.

The checkSupported wall, undamaged dampproofing, board and drain, clean fill, and the inspector's approval where one is required.

By the end you can

Name the fill that belongs against a foundation wall, and the fill that does not.
Sort material when you dig, not when you backfill.
Check that a wall is ready before the machine pushes soil in.
Protect the dampproofing, board and drain while you place fill.

Stop and read

Do not backfill a wall that is not supported. A basement wall is designed to be held at the top by the floor system or anchors before soil is pushed against it. Backfilling an unsupported wall, especially with a loader pushing material in, can crack or collapse it. Where the floor is not framed, the wall needs bracing designed for the job. Get the bracing and timing from the drawings, the engineer or the building department.

Keep people out of the zone beside the wall while a machine places fill, and never enter an excavation 1.2 m (4 ft) or deeper unless it is sloped, shored or boxed.

The big idea

The soil you put back decides how well the drain works, how the ground freezes against the wall, and how much pressure pushes on it. Clean, free-draining fill lets water reach the drain. Clay holds water against the wall. The right fill makes the drainage system work, and the wrong fill can undo it.

A cross-section from the inside of a basement wall outward. Inside, a slab sits on a gravel base. On the outside of the wall are dampproofing, a dimpled drain board, then clean granular backfill placed in lifts, capped by native soil sloping away from the wall. At the base, drain rock and filter fabric surround a perforated pipe. — **What goes against the wall.** Dampproofing, a drain board, clean granular backfill in lifts, and a native soil cap sloped away from the house. Drain rock and fabric surround the pipe at the footing. Not to scale. The dampproofing coat and the fabric face of the board are not distinct from each other in the drawing, and the dimple direction is hard to read at this size. Materials and article numbers: see the text and your local building department.

Is the wall ready?

Support at the top. The Part 9 text I read treats a foundation wall as laterally supported at the top when the floor joists are embedded in it or the floor system is anchored to it with anchor bolts (Article 9.15.4.3). It then limits how high the finished ground may sit above the basement floor (Table 9.15.4.2.A). For the wall types in that table, the limit is from 0.6 m (about 2 ft) up to 1.8 m (5 ft 11 in) when the top is not supported, and up to 2.85 m (9 ft 4 in) when it is.
No floor yet? One US trade article says to brace the walls with heavy timbers, about two-thirds of the way up and roughly 3.7 m (12 ft) apart in its 8 ft wall example. That is one magazine's example, not a code rule, so use your designer's bracing.
Concrete age. The Part 9 text I read says nothing on timing. The same article says to wait at least one week after the pour and calls four weeks best practice, and it gives no strength figure. Use the age or strength your designer or inspector sets.
Inspection. Many municipalities want to see the dampproofing and drain before backfill. Ask your building department which stages apply, because I did not verify them by province.

Backfill types

Material	Use it	Watch for
Clean granular fill (sand, gravel, crushed rock with few fines)	The usual choice against the wall and over the drainage layer. It drains freely and compacts predictably.	Keep it clean and protected from rain on the pile.
Drain rock (clean, single-size, no fines)	Around the perforated pipe at the footing and at sump outlets, wrapped in filter fabric where soil would wash in.	Dirty rock with fines clogs the drain. Normally not compacted.
Sand	Bedding under pipe and levelling under slabs.	Can wash out in running water. Not usually used for the whole depth unless specified.
Native soil	Often acceptable higher up and away from the wall if it is clean and compacts. Sandy or gravelly soil works best.	Check it first for clay, silt, organics and debris. Screen out boulders, roots and scrap.
Clay and silt	Only as a sloped cap near the surface, and only if designed.	Holds water, swells and shrinks, and is slow to compact. Builder sources advise against it against the wall.
Topsoil and organics	Keep for the final landscaping layer.	Rots and shrinks, causing settlement. Do not use as fill.
Engineered fill	Under structural slabs, footings and larger buildings.	Follow the engineer's grain size, moisture and compaction, with testing.

The material rules I could confirm in code text, from Part 9 Section 9.12 as printed in a BC municipal reprint:

Backfill shall be placed to avoid damaging the foundation wall, the drainage tile, and any insulation, waterproofing or dampproofing (9.12.3.1).
Backfill shall be graded to prevent drainage toward the foundation after settling (9.12.3.2).
Backfill within 600 mm (2 ft) of the foundation shall be free of deleterious debris and boulders larger than 250 mm (10 in) across (9.12.3.3).
Backfill shall not contain pyritic material or material susceptible to ice lensing, with qualifications in the full sentence that I did not fully read (9.12.3.3).

The code text I read does not say "granular only" or ban clay. Those are common practice and designer choices, so follow your drawings and any geotechnical report first.

Placing it

Sort the pile when you dig. Keep the clean granular separate and covered.
Put better material nearest the wall and drain, and reuse native soil higher up and farther out.
Do not dump or push a loader load straight against the dampproofing. Place fill so the board and membrane are not torn (9.12.3.1).
Place it in thin, even lifts (see Lift Thickness).
Finish with a cap that slopes away from the wall (see Drainage Corrections).

Example. A 36 m (118 ft 1 in) perimeter wall 2.4 m (7 ft 10 in) tall with a 0.6 m (2 ft) wide granular zone needs 0.6 × 2.4 × 36 = 51.84 m³ (67.8 yd³) of granular fill, plus the drain rock around the pipe. The sizes are made up, and a few truckloads of clean gravel cost little against a wet basement.

Common mistakes

Backfilling before the wall is supported or the concrete is ready
Boulders or construction debris against the wall
Wet, muddy or frozen fill
Pushing a loader load against fresh dampproofing
Clay against the wall because the pile was closest
Dirty drain rock with fines

Pro note

Sort the material when you dig it, not when you backfill. Keep the clean granular pile separate and covered.

Weather note

Rain saturates fill and makes it impossible to compact. Frozen fill leaves voids that settle in spring. Do not backfill with frozen lumps or on frozen ground without a plan, and cover stockpiles.

What the inspector looks at

Dampproofing, board and drain undamaged before backfill, and the fill and timing the designer set. Confirm which stages your building department inspects.

✅ Check yourselfTap a question

The excavation pile closest to the wall is clay. Do you use it against the foundation?

No. Clay holds water and is slow to compact. Several builder sources advise against it against the wall. Bring in clean granular fill, or use clay only as a sloped cap if the design calls for it.

What does the Part 9 text I read say about stones near the wall?

Backfill within 600 mm (2 ft) of the foundation shall be free of deleterious debris and boulders larger than 250 mm (10 in) across (Article 9.12.3.3(1), as printed in a BC municipal reprint). Check your provincial edition.

Why does a wall need to be supported at the top before backfilling?

The wall is designed for soil pressure with the floor system or anchors holding the top. Part 9 limits how high the ground can sit above the basement floor, and a wall not supported at the top is allowed much less than one that is. A pushed-in load on an unsupported wall can crack it.

A 36 m (118 ft 1 in) perimeter, 2.4 m (7 ft 10 in) tall, with a 0.6 m (2 ft) granular zone. How much granular fill?

0.6 × 2.4 × 36 = 51.84 m³ (67.8 yd³). The sizes are made up, and the drain rock around the pipe is extra.

Frozen lumps are in the backfill pile. What happens in spring?

They thaw and leave voids, so the fill settles and the ground can slope toward the house. Keep frozen material out and cover stockpiles.

What should the fill beside the wall let water do?

Drain freely, so water reaches the drain at the footing instead of building up pressure on the wall.

🧰 Gear for this stepMust-haveRent itNice to have

Must-haveLayout and checking: a tape measure, long level or laser level, grade stick and marking paint for fill depth and finished grade
Rent itMoving fill (machine): an excavator or loader with a smooth-edge bucket
Must-haveMoving fill (hand tools):
Must-haveSorting material: a screen or a clean tarp area to keep clean granular fill separate, plus tarps to cover stockpiles from rain
Must-haveDrainage layer protection: filter fabric, drain board and a hand tamper so nothing sharp or heavy hits the dampproofing
Must-haveCompaction (hand): a hand tamper
Rent itCompaction (machine): a plate compactor for thin lifts beside the wall
Rent itTesting: on engineered or structural fill, a testing company with a density gauge reports on compaction
Must-havePPE:
Must-haveDust and noise: a respirator (CSA Z94.4) where dust is heavy, and hearing protection (CSA Z94.2) near machines

👷 Who can do thisTap your level

Home DIY

A capable owner can sort fill, bring in clean granular material and place it in thin lifts once the wall is supported and any required inspection is passed. Do not backfill a wall that is not supported at the top, and do not enter an excavation 1.2 m (4 ft) or deeper unless it is sloped, shored or boxed. Hire a pro for tall walls, engineered fill or anything near a neighbour.

Laborer or helper

Move fill by hand, keep clay, roots and debris out of the clean pile, and stay well back from the machine and the wall. Never enter an excavation 1.2 m (4 ft) or deeper unprotected. Tell the lead at once if the wall moves or the fill looks wrong.

1st-year apprentice

Spread and compact fill by hand under supervision. Learn to tell clean granular from clay, silt and organics, and why the floor system or bracing must be in place before a machine pushes soil against the wall.

Apprentice, 2nd to 4th year

Sort and spread fill in thin lifts under direction, and keep out of the zone beside the wall while the machine works. Report any bulge, crack or movement in the wall right away.

Journeyperson

You own the readiness check for backfill: the wall supported at the top or braced, the strength and age the designer sets, the inspection passed, and the dampproofing, board and drain undamaged. Stop-work triggers: the wall is not supported, the concrete has not reached the designer's strength, or the fill is clay, organic or frozen. Call the engineer when the fill depth or wall is outside the drawings.

Lead hand or foreman

Do not let the machine push soil in until the floor system or bracing is in place, the inspection is passed and the designer's strength is met. Check the dampproofing and drain, reject unsuitable fill, and call the engineer when the wall or fill depth is outside the drawings.

Contractor or business owner

Price imported granular, hauling, and disposal of unsuitable soil. Keep photos of the drainage layer and the inspector's approval before backfill, and the engineer's release on file. Confirm with your lawyer or insurer who carries the cost of wall damage from early backfill.

Starting a new business

Keep the engineer's release before any backfill starts, and write an unsuitable-soil allowance into your price. Confirm inspection timing with the building department, and ask your insurer who carries the cost of wall damage from early backfill.

By Province:

The national baseline is Part 9 (Sections 9.12, 9.13, 9.14 and 9.15). The code text above comes from a municipal reprint and may be older than your province's edition. Inspection stages, concrete age before backfill, and any limits on stone size or fill type are set by your province, municipality or designer. I dropped the old per-province inspection list because I could not verify it, so ask your building department which stages apply.

See Canada by Province for the full comparison.

Sources:

Vancouver Building Bylaw 2014, Part 9, Section 9.12 (municipal reprint of NBC Part 9, possibly older than your province's edition: Articles 9.12.3.1 to 9.12.3.3)
Vancouver Building Bylaw 2014, Part 9, Section 9.15 (Articles 9.15.4.2 and 9.15.4.3, Table 9.15.4.2.A)
Builder, How to Backfill a Foundation (Ted Cushman, 2012, US trade magazine: bracing example, concrete age, material advice)
GreenBuildingAdvisor, Backfill drainage (forum Q&A, November 2025: practitioner opinion, no figures)
Fill-type advice and the idea of putting better fill lower and native soil higher are general trade practice, not code. Provincial inspection stages and concrete strength before backfill were not verified. Volume arithmetic is calculated and re-checked.

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