Advice
Lime or cement mortar: which does your house need?
This is the single most important decision on any repointing job, and the one most often got wrong. Use cement on a soft Victorian stock brick wall and you will damage the brick. Use lime where cement belongs and you have spent more than you needed to.
The simple rule
If your house was built before roughly 1919, it almost certainly needs lime. Solid walls of that era were built with soft, hand-made or early machine-made brick and lime mortar, and they are designed to breathe - moisture enters the wall and evaporates back out through the joints.
If your house was built after about 1930, with cavity walls and harder machine-made brick, a cement-lime mix is normally appropriate. The wall was designed for it.
The awkward period is the 1920s, which goes either way. That is what the survey is for.
What cement does to a period wall
Cement mortar is harder and far less permeable than the brick around it. Moisture that gets into the wall can no longer escape through the joints, so it escapes through the brick face instead. When it freezes there, the face spalls off.
It is also rigid. Old buildings move seasonally; lime mortar accommodates that with fine hairline cracks that self-heal through carbonation. Cement does not flex, so the movement cracks the bricks rather than the joints.
The result, visible on thousands of London terraces, is intact cement pointing standing slightly proud of eroded, crumbling brick. That damage is not reversible - the bricks have to be cut out and replaced.
Types of lime, briefly
NHL (Natural Hydraulic Lime) comes graded NHL 2, 3.5 and 5, from soft to relatively hard. Most repointing on London stock brick uses NHL 2 or 3.5, with NHL 5 reserved for very exposed positions such as chimneys and parapets.
Lime putty and hot-lime mortars are softer still and used on the oldest and most delicate work, including much listed-building repair. They take longer to gain strength and need more protection while curing.
Cost difference
Lime typically adds 20-40% to the pointing element of a job. Materials are only part of it; the real difference is application time and the protection needed during cure.
Set against the cost of replacing spalled brick faces on a whole elevation in ten years' time, it is not really a choice.
FAQ
Lime vs cement mortar - common questions
Lime mortar is softer, usually paler and slightly sandy, and can be scratched with a coin or a key. Cement is grey, hard and rings when tapped. Cement pointing on an old house also tends to sit proud of the brick face rather than flush with it.
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Honest advice from a family-run brickwork specialist serving Richmond and South West London since 1946.