How to Build a Timber Retaining Wall

Before beginning this 'How to build a timber retaining wall' project, check with your local authority planning department as to their requirements. Many authorities require planning permission before any retaining structure above a certain height can be built. There may be certain height limitations or other such restrictions in your area, so always check first.

If your project calls for a high retaining wall or extensive earth work, consult an architect. This is also true whether you are simply cutting timber frames for your new home or designing a timber frame+hip roof. Always check first.

This example project shows how to erect a 1m high retaining wall using treated 150 x 50 tongue and grooved retaining timbers. So lets begin our 'How to build a timber retaining wall' project.

Materials Required:
Posts: Treated 100x100 sawn posts, in 2.4 metre lengths*.
Retaining Walling Timber: Treated 150x50 Tongue and Grooved with 'V' Chamfer.
Some people prefer to use treated 6 x 6 timbers, these are also a viable alternative.
Capping: Treated 38 x 100 Sawn Timber
Hardware: 125mm x 10mm Coach Screws

Almost all of these materials will be be available from your local timber yard. * Posts will be spaced a maximum of 1000mm intervals. Post holes must be at least a third of the length of each post deep. In our project each 2.4m post will be sunk into a hole at least 800m deep and twice the diameter of the post.

How to build a timber retaining wall: Part 1 Instructions
Set out the perimeter for your retaining wall. Drive in stakes at the corners or at each end of a straight wall, as required
Attach plumb line to stakes to mark out the outside boundary of the retaining wall.

How to build a timber retaining wall: Part 2 Mark out any intermediate post holes
Measure and divide the straight lines between the stakes into 1000mm intervals and drive a stake at each point. These will be the positions for
your intermediate post holes.
Dig all post holes 800m deep and twice the diameter of the post. This project will have 1m high wall. Adjust your post hole depths according to
the retaining wall height that you require.

How to build a timber retaining wall: Part 3 Cementing in the posts
Make a stiff mix of 5 parts aggregate, 2 parts sand to one part cement. The mix must be stiff enough to retain the posts in an upright position
without any supports.
Cement in the corner and/or end posts first and then each intermediate post. Check the alignment of each post prior to and after cementing -
adjust as required to maintain alignment. Also check the that each post is upright using a spirit level against two adjacent sides. Work the
concrete around the post and fill almost to the top of the hole. Leave for at least 48 hours to set. Protect from rain and frost during this period.

How to build a timber retaining wall: Part 4 Fixing the 150x50 T+G Retaining Boards
Dig a 75mm trench along the fence line to accommodate the bottom retaining wall timber(s). Lay the first retaining timber in the trench (tongue
uppermost) between the first two posts and check for level. Repeat this for each bottom rail along the fence line, making sure that all timber
retaining boards are joined only at a post and NOT in between. Drill and secure each rail with two 125mm coach bolts at each
post. Keep adding the retaining boards and secure as before, making sure that the groove engages fully with the tongue in each lower
timber. It is a good idea to stagger the joints across all of the post line for maximum strength. You may need to stagger the retaining wall
depending on the contour of the site.

How to build a timber retaining wall: Part 5 Trim the posts as required.
Once you have reached the desired height of your retaining wall, cut off the posts level with the top of the retaining boards and fix cappings, if required.

How to build a timber retaining wall: Part 6 Drill Drain Holes
To release any water retained in the soil behind your wall, you will need to drill a series of 18mm drain holes every 1m along the lower retaining board.