How Deep to Rake Out Mortar Before Repointing?
Quick Answer
About twice the joint width, minimum 20mm for a standard 10mm brick joint (twice the joint width)
Based on SPAB (Society for the Protection of Ancient Buildings) best-practice guidance to rake to twice the joint width, and the standard UK brick joint of 10mm, which is also the default raking depth in our repointing calculator. Wider stone joints need deeper raking; fine ashlar needs less. Always stop only when you reach sound, well-bonded mortar, even if that means going deeper than the minimum.
Raking Depth by Joint Type
| Joint type | Rake to | Note |
|---|---|---|
| Standard brick joint (10mm) | 20mm | twice the joint width; never less than 20mm per SPAB best practice |
| Rubble-stone joint (wider, irregular) | 25-40mm | or deeper still until you reach sound mortar (Heritage House minimum) |
| Fine ashlar joint (narrow) | 10-12mm | shallower on purpose, to avoid disturbing the stone arris |
Figures from SPAB and Heritage House historic-building guidance (see sources below). The 10mm brick joint row matches the raking depth used by default in our repointing calculator.
Worked Example: A Standard 10mm Brick Joint
UK bricks are laid with a standard mortar joint of 10mm, both the horizontal bed joints and the vertical perp joints (the same 10mm joint this site’s brick and repointing calculators use as their default).
- Joint width: 10mm
- Twice the joint width: 10mm × 2 = 20mm
- That matches SPAB’s “twice the joint width” rule and is the minimum you should rake to before repointing a standard brick wall
Worked Example: A Wider Rubble-Stone Joint
Rubble stonework is usually laid with a wider, less regular joint than brick. Heritage House’s sourced minimum raking depth for rubble stone is 25-40mm, which under the same twice-the-joint-width rule implies joint widths of roughly 12-20mm. Take a 20mm joint, at the top of that range:
- Joint width: 20mm
- Twice the joint width: 20mm × 2 = 40mm
- That sits at the top of Heritage House’s quoted 25-40mm range for rubble stone, raked out “until sound mortar” is reached, which can mean going deeper still
Why Twice the Joint Width
The new mortar needs enough depth to key into the joint and grip the brick or stone on both sides. Rake out too shallow and you are left with a thin skim sitting on the face of old, often weathered mortar. Our own step-by-step repointing guide is explicit that shallow repointing, under 10mm on a standard brick joint, will not last, and SPAB’s twice-the-joint-width rule puts the practical minimum for that same 10mm joint at 20mm. That skim has almost nothing to bond to, so it debonds, cracks and falls out again within a few years, sometimes months on an exposed wall.
Rake to Sound Mortar, Not Just to a Number
The depth figures above are a minimum, not a target to stop at regardless of what you find. If the old mortar is still soft, crumbly or hollow-sounding at 20mm, keep raking until you reach mortar that is hard and firmly bonded to the brick or stone, then stop. Heritage House’s guidance for rubble stone makes this explicit: rake “to a minimum depth of 25-40mm or until sound mortar”, whichever is deeper.
Tools: What to Use and What to Avoid
A plugging chisel and lump hammer, or a battery-powered hand mortar rake, give you control over the depth and keep the cut square-backed. SPAB and other historic-building guidance both warn against using angle grinders or disc cutters for raking out: they are hard to control at the joint edge, tend to overcut into the brick or stone face, and can chip or damage the arris (the sharp edge of the brick). If you do use a grinder attachment on a large modern wall, work carefully and keep well clear of the brick faces.
Lime vs Cement Mortar
The raking depth is the same principle regardless of mortar type, but what you rake out and put back should not be. For buildings from before about 1919, use a lime mortar: it stays more permeable than the brick or stone, so moisture can move through the joints rather than concentrating frost and salt damage there. Repointing an older wall with hard cement mortar is a common cause of spalled (blown) brick faces. See our mortar calculator for mix ratios, and the full step-by-step repointing guide for the whole process from raking out to curing.
Figures cross-checked against SPAB (spab.org.uk) and Heritage House historic-building guidance, July 2026. If the wall is listed or in a conservation area, check with your local conservation officer before starting; a specialist survey is recommended for any building showing structural cracking, not just worn pointing. Last updated: July 2026.