BREATHING
Hard cement traps the damp
The SPAB says cement pointing makes the bricks the most permeable part of the wall, so they suffer the greatest frost and salt damage.
Why it happensWHAT CONSERVATION GUIDANCE SAYS
Let the mortar
take the weather.
In an old wall, the joints are meant to wear first.
Check a wall
SOFTERthan the brick
Lime mortar is softer and more breathable than the bricks around it, so frost and salts do their damage in the joints. Repoint with something harder and the bricks take the hit instead.
At a glance
- Repoint only when needed
- Match the old mortar
- Never harder than the brick
- Small joints, long-lived walls
1 : 3
Lime putty to local sand, by volume
The US National Park Service's Preservation Brief 2 says traditional mortar was generally 1 part lime putty to 3 parts sand, a recipe that stayed largely unchanged for centuries.
TRADITIONAL MIX · US NPS
2–2½× joint width
Depth to remove old mortar
Old mortar should be removed to at least 2 to 2½ times the width of the joint, which for most brick joints is about ½ to 1 inch. The SPAB gives twice the joint width.
RAKING OUT · US NPS
TWO NUMBERS WORTH KNOWING
An old recipe,
a careful method.
One figure on what traditional mortar was,
one on how deep to go when repointing.
Portland cement was patented in Great Britain in 1824 and named after Portland stone from Dorset. Salford City Council advises that, as a general rule, buildings constructed before 1900 must be repaired with lime.
A QUICK WALL CHECK
What state is
the pointing in?
Pick what you see on an old brick wall to see what conservation guidance suggests. The bar shows how strongly the guidance points to action.
Based on SPAB advice on repointing. For listed buildings or serious damage, ask a conservation specialist or your council first.
THE SHORT ANSWER
The joint should
give way first.
Old walls were built to let moisture out through the mortar.
Guidance is about keeping it that way.
TIMING
Soft is not the same as failed
Repointing is premature until mortar has weathered back about as deep as the joint is wide, or is very loose. Usually only patches need doing.
Check a wall
METHOD
Rake deep, cure gently
Guidance says to rake out old mortar to about twice the joint width, avoid winter work and protect new pointing from frost, sun, wind and rain.
How it is done
FOUR STEPS FROM THE GUIDANCE
Repointing,
the gentle way.
Good repointing is slow work. These steps come from SPAB advice and the US National Park Service's brief on repointing historic masonry.
Find the original mortar
Old pointing is often best preserved in sheltered spots such as under the eaves. Match its material, colour and finish.
Keep the mix weaker
Any new mortar must be weaker than the bricks and at least as breathable, so the joint, not the brick, takes the wear.
Rake out deep enough
Remove old mortar to about twice the joint width, carefully, so brick edges are not chipped.
Protect while it cures
Avoid winter. The NPS suggests wall temperatures between 8 and 38 °C, and shade from strong sun to slow drying.
WHY CEMENT HARMS OLD BRICK
Moisture needs
a way out.
Walls get wet. In a lime-built wall, moisture moves out through the joints. The NPS explains that when water evaporates it leaves salts behind, and salts crystallising inside a brick can push its face off.
Salford City Council's guidance describes cement mortar as acting like a glue that does not move with the building and stops moisture evaporating from the joint, leaving brick and stone faces crumbly or powdery.
Questions
Questions people ask
Does soft, sandy mortar mean my wall needs repointing?
Not on its own. The SPAB says repointing is not justified simply because mortar is soft. It becomes due when mortar has weathered back to about the joint width or is very loose.
Why not just use cement mortar?
On old buildings, hard cement pointing makes the bricks the most permeable part of the wall, so they take the frost and salt damage instead of the joints. New mortar should be weaker than the bricks.
Which buildings need lime mortar?
Salford City Council advises that, as a general rule, buildings constructed before 1900 must be repaired with lime. The SPAB notes lime mortar was widely used for pointing on pre-c1919 buildings.
What is natural hydraulic lime?
Lime from limestone with silica and aluminium impurities that sets harder. It is graded NHL 2, 3.5 and 5. The SPAB notes that the more hydraulic a lime, the stronger but less permeable and flexible it is.
When is the best time of year to repoint?
The SPAB says to avoid winter where possible and protect new work from frost, rapid drying and rain. The US NPS suggests wall temperatures between 8 and 38 °C.
Sources
Where the facts come from
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