How do I fit coping stones on a garden wall?

Why a wall needs a coping, the overhang and drip that make it work, and bedding and jointing copings so they do not lift or crack.

Difficulty: intermediate
Time: 3 hr
Read: 4 min
Safety: caution

Short answer

Bed copings on a full mortar bed with a damp-proof course underneath on an exposed wall, oversail both faces by at least 25mm, and make sure there is a drip groove or a weathered edge so water falls clear rather than running back down the wall.

The coping is the most important part of a garden wall and the part most often left off. An uncapped wall takes water in at the top, holds it, and destroys itself in frost. A good coping sheds that water clear of both faces.

Safety

Coping stones are heavy and awkward and are usually lifted at chest height — a dropped coping breaks feet. Lift with two people. Cement burns skin, so wear waterproof gloves. Work off a firm platform rather than a ladder where you need both hands.

Step by step

Level and clean the top course.Copings sit on a level bed. Any high spots in the top course have to be dealt with, because a coping bedded on an uneven top is either wedged or sitting on a thick weak mortar bed.
Fit a damp-proof course under the coping on an exposed wall.A bitumen or polymer DPC across the top of the wall under the coping bed stops water that gets past the coping joints from soaking down into the wall.
Choose a coping with a proper profile.It must oversail both faces by at least 25mm and have either a drip groove on the underside of each overhang or a strongly weathered top. A flat slab flush with the wall face is not a coping.
Set the fall.Saddle-back and twice-weathered copings shed both ways. A once-weathered coping falls one way and should fall towards the least important face, usually into the garden rather than onto a path.
Bed on a full mortar bed.A 3:1 sharp sand to cement mix, full bed under the whole coping. Voids under a coping hold water and crack it in frost.
Set to a line and check as you go.String line along the front edge of the copings so the overhang is consistent. A varying overhang is very visible along a wall.
Point the joints between copings carefully.These joints are the weak point. Fill them completely, and finish them flush or slightly weathered so water runs off rather than sitting. Some specifications use a flexible sealant instead of mortar.
Keep the ends and returns right.Corner and stop-end copings are made for the purpose. A mitred cut on site is acceptable but must be sealed properly — the mitre is another joint.
Clean off mortar smears immediately.Cement staining on a coping face is permanent on most stone. Wipe as you go rather than trying to clean at the end.

Tips

Dry lay the whole run first, spacing the joints evenly, before mixing mortar. It is much easier to adjust the spacing now.
Concrete copings are cast with a weathered profile and a drip. Reclaimed stone often is not — check before buying.
On a wall that gets a lot of driving rain, a wider overhang throws water further clear and makes a visible difference to how the wall weathers.

Common mistakes

Using flat slabs flush with the wall face — Water runs off the slab straight down the wall face, so the wall stays wet, stains and spalls. The overhang and drip are the whole point of a coping.
Spot bedding copings — The voids fill with water and freeze, and the copings crack or rock. Full bed, every time.

If it doesn't work

Copings lifting or rocking

Cause: Spot bedding, or frost damage in a void — Fix: Lift, clean off, and rebed on a full mortar bed. Add a DPC under the bed while it is off.

Staining running down the wall under the coping

Cause: No drip, or the drip is blocked with mortar — Fix: Clear the drip groove, or fit copings with a proper overhang. This is a design fault, not a cleaning problem.

Joints between copings cracking every year

Cause: Thermal movement in a long rigid run — Fix: Rake out and replace some joints with a flexible sealant, particularly near the ends of a long run.

Wall face spalling below the coping

Cause: Water shedding onto the face for years — Fix: Fix the coping first, then repair the spalled brickwork. Repairing the wall without fixing the coping repeats the damage.

Questions people ask

Does a garden wall really need a coping?

Yes. The top of a wall is exposed end grain in masonry terms — open joints and absorbent material facing straight up. An uncapped wall saturates and frost-damages from the top down within a few winters.

How far should a coping overhang?

At least 25mm each side, with a drip groove under the overhang. More on an exposed elevation. The point is that water falls clear of the wall face rather than running down it.