How Many Sleepers for a Retaining Wall? Posts and Screws (UK)
A sleeper retaining wall is built in courses of sleepers laid flat and held by posts at each joint. A wall 4.8 m long and 1 m high of 2.4 m sleepers 200 mm high is 5 courses of 2, so 11 sleepers with 5% extra, 3 posts of 1.6 m with 600 mm below ground, and 40 screws or bolts. The sleeper wall calculator counts them for your wall.
Sleepers for common walls
With 2.4 m sleepers laid flat (200 mm high), 5% extra, posts 600 mm below ground and two fixings at each sleeper end:
| Wall | Courses | Sleepers | Posts | Post length | Fixings |
|---|---|---|---|---|---|
| 2.4 m long, 0.4 m high | 2 | 3 | 2 | 1.00 m | 8 |
| 4.8 m long, 0.6 m high | 3 | 7 | 3 | 1.20 m | 24 |
| 4.8 m long, 1 m high | 5 | 11 | 3 | 1.60 m | 40 |
| 7.2 m long, 1 m high | 5 | 16 | 4 | 1.60 m | 60 |
| 9.6 m long, 1 m high | 5 | 21 | 5 | 1.60 m | 80 |
Post length
A post is the wall height plus the depth below ground. For a 1 m wall:
| Depth below ground | Post length |
|---|---|
| 400 mm | 1.40 m |
| 600 mm | 1.60 m |
| 800 mm | 1.80 m |
The depth is a rule of thumb, and the soil and the load set the real figure. Do not shorten posts to save timber.
Keep it low
A retaining wall holds back a lot of soil. This calculator is meant for low garden walls up to about 1 m. A higher wall, one near a boundary, or one that holds a building, a drive or a slope, needs a structural engineer and may need Building Regulations approval. Do not rely on a count for those.
Drainage
Water behind the wall is the usual cause of failure. Line the back with a membrane, put gravel behind the lowest course, and let the water leave through a drain or weep holes. The gravel calculator sizes the gravel.
Building it
- Dig the post holes first, set the posts plumb and in line, then slot the sleepers between or against them.
- Stagger the joints so that no two courses meet over the same post position where you can.
- Fix each sleeper to the posts, and to each other, with long screws or coach bolts.
- Use sleepers rated for ground contact, and treat the cut ends.
- Wear gloves and eye protection when cutting.
See the block calculator for a masonry alternative. This counts timber only: get advice for anything higher than a low garden wall.
Frequently asked questions
How many sleepers do I need for a retaining wall?
Divide the height by the sleeper height for the courses, then multiply by the sleepers in each course. A 4.8 m by 1 m wall takes 11 of 2.4 m, with 5% extra.
How many posts do I need?
One at each joint and each end: 3 for a 4.8 m wall with 2.4 m sleepers.
How high can a sleeper wall be?
A low garden wall up to about 1 m is a typical DIY job. Higher walls, and any wall holding a building, drive or slope, need a structural engineer.
How deep do the posts go?
A third to a half of the wall height is a common rule of thumb. The soil and the load decide, so ask your supplier.
Does it need drainage?
Yes: a membrane and gravel behind it, with a way for the water to leave.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.
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