How do I fix cracks?

Sep 1, 2021 | Building Issues

Last month we explained how cracks and movement in your property occur and when they start to become a worry, however knowing what steps need to be taken to fix the cracks is another issue entirely.

First, not all cracks need fixing.

A crack that’s only a hairline width – up to 0.5mm wide – will cause more trouble and look worse than being left alone, unless it’s an obvious damp entry path. If such a crack is in a cement rendered facing, the next opportunity to repaint with correct masonry paint is likely to be all that is needed.

If the cracks are 1mm or up to 3mm wide and in a brick joint, or where the mortar itself is loose and perished with age, then “re-pointing” is probably the best option – IF that work is correctly done. This is a simple process of raking out the old mortar joint, being careful not to damage bricks on either side, then moistening and applying fresh matching mortar neatly to a similar finish to the older parts. A skilful bricklayer/tradesman will be able to adjust the mix of sand, cement and other additives so the colour doesn’t stand out like the proverbial sore thumb. He’ll also neatly recreate the radius of the joint, or any special joint profile, with simple tools.

Repointing was once far easier done when softer lime-based mortar was used in soft, old bricks. It may surprise you to know that a correct lime-based mortar mix can “heal itself” in a fairly short time by chemical action, and fix its own minor cracks. Modern bricks are hard, being made in high-temperature gas-fired kilns and then laid in the wall using a hard cement-based mortar. Cement doesn’t repair itself, it just cracks and breaks. The nearby bricks may actually crack into pieces too, and the fracture naturally means that wall is now weaker.

The removal of cement mortar without causing damage is more challenging. Additionally, poor repointing techniques, such as utilising hard cement mortar on ancient, soft brickwork, can eventually cause frost damage. Normal mortar should be softer than the bricks it surrounds; otherwise, the bricks would crumble because the mortar won’t decay or give to pressure.

Do you recall being taught that water expands as it freezes? If a wet and soft brick freezes, and the expansion can’t be taken up by the mortar that is too hard for those bricks, their facings then break down, a process called “spalling”. Ugly damage and possibly substantial loss of the brick face is often the result. We have seen a honeycomb of cement mortar left around old, soft bricks that have eroded due to frost action, and the remaining brick is up to 40mm/1.5” recessed behind the original surface of the completely wrong, hard mortar that was used. Replacement of all the bricks is the only option.

Moving on, the crack concerned might be 4-8mm wide and the bricks becoming unstable. Presuming the brickwork isn’t showing signs of building movement, such as from subsidence, it could be possible to dismantle that section of wall and neatly refit, or “re-stitch” fresh bricks into the opening, to ensure the strength of the wall is not reduced. This is skilled work and few bricklayers have the skill to make a neat, satisfactory job.

Take a brick arch over a window or doorway, in a house built in the late 1800 to early 1900s. Say the bricks are slipping out of line, the arch is now weakened and support to the wall above it is lost. A good, well-formed arch is very strong, but a bad arch won’t hold up the wall above and so more serious movement is in prospect. How many bricklayers now know how to set out and re-fit bricks to form a good arch with suitable support provided as the mortar “cures”? Actually, relatively few have that expertise.

All this depends on the seriousness of the crack involved and you will need advice from a qualified surveyor, or possibly a structural engineer.

Now we have a significant crack in a critical location, or an expansion joint has not been included in this wall, which has cracks as a result. Expansion joints are intended to do what old soft lime-based mortar once did – take up minor movements. NHBC requirements say that expansion joints are needed, in part having regard to the wall length, and the orientation/exposure if the sun is a factor. Consider, in the obvious case of a long wall exposed to hot afternoon sun. A wall around 9 metres/29 ft long with no expansion joint can move enough to fracture quite badly. That is from bad design, but to overcome it, it is possible to insert or provide a flexible joint and tie the relative edges of the wall together with special components.

One such system is the “Helibar” – very strong stainless-steel ties are fitted into the mortar joints, crossing or bridging a crack or an expansion joint, usually by fixing with an epoxy adhesive similar to Araldite (for you do-it-yourselfers), but don’t try doing this at home! Leave it to the specialists, please – as an example, see:

Helix Crack Stitching

It takes quite a lot to weaken a wall enough to collapse, but everything has its limits.

Finally, what about a major crack, maybe above 20mm wide: see our picture. That one is, obviously, a rebuild job and trying to repair is a lost cause!

Les Long FRICS FISVA Principal, Eyesurvey

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