Way back when I was at primary school, the formidable dinner ladies force-fed us liver for the main course. As compensation, we were sometimes “treated” to a dessert called bricks-and-cement. From memory the bricks were a sort of chocolatey rice crispy affair that would undoubtedly achieve a pretty good U-value. They were, however, ruined by spray cream that was the “cement”. Of course, had my six-year-old self had had a greater technical understanding I would have corrected the dinner ladies on their use of cement rather than mortar…
Perhaps cream would be a more appropriate choice than some of the mortar I see in my surveys. Historic properties were generally constructed with lime mortar. Lime mortar acts as the sacrificial element of a wall should any movement occur. It also has a degree of flexibility that can absorb minor settlement. Should movement exceed this capability then a crack will develop along the joints as the weakest element of the wall. This is clearly preferable to a crack developing through the bricks.
Straight to the point(ing)
All too often, however, I see repointing on old brickwork carried out with modern cementitious mortar. This stuff is much harder and more brittle than lime mortar. In addition, modern mortar does not transmit moisture as effectively as lime mortar. This results in a greater tendency for walls to remain damp once saturated by rain. Rising damp through capillary action can also become an issue in the wall behind.
Old, damp bricks are more liable to shatter on their external faces (“spalling”) when the trapped moisture expands in freezing conditions. This action exposes the weaker internal structure that then becomes even more liable to crumble. This risk is enhanced when insulation is added to a cavity (or internal wall insulation installed on a solid wall). As the external leaf is more isolated from heat loss from the house, so it becomes more likely to freeze.
The image below-right (one of my clients’ properties) illustrates the problem beautifully. A layer of hard cement-based repointing remains in situ whilst the brickwork above and softer lime mortar behind has eroded.
Damp issues
The inappropriate use of cementitious mortar can increase the risk of damp issues within the house. Over a longer time period the wall will tend to absorb water more quickly that it can dry out. Fibre insulation in a cavity construction is also more likely to become saturated over time. In the worst cases it can potentially slump in a soggy mess down the bottom of the cavity.
Mortar can also cause issues where it bridges physical damp-proof courses, again especially on older buildings before the use of engineering bricks that did not draw damp up from ground level at the same rate as older more porous bricks.
Another issue to be aware of is the deposition of mortar “snots” in a cavity when walls are being constructed. If these end up on wall ties or accumulate with other debris in the base of the cavity then this can act as a physical bridge for damp. Things can get worse when insulation is injected as this prevents air movement from drying the cavity.
Identifying and remediating poor pointing
Traditional lime mortar mixes usually lose a few grains of sand when you run your finger over a joint. Mortars with a lower proportion of lime can be more difficult to differentiate. One easy way to determine if mortar has some lime content is to dab with descaler solution. Any lime present will react with the acid and make a gentle fizzing.
The most sensible option is often to remove ill-advised cementitious mortar pointing before it causes further damage and repoint with a lime-based mortar. If damage to the bricks has gone too far then sometimes rendering is the only reasonable course of action to protect the wall from catastrophic damage. A coat of breathable masonry paint or clear breathable sealant may also remediate the problems by repelling rain, although there is a risk that rising damp may become worse.
Damp issues
These are all issues that are easily avoidable – I suspect as ever that part of the problem is a lack of understanding on the part of (often otherwise diligent) tradespeople who really should know better. My proposal to the fix the problem therefore is to retrain dinner ladies to deliver courses on sustainable construction techniques, standing over the apprentices until they have finished the wall properly. So long as there is no confusion between cement and mortar then future generations can surely look forward to dry, long-lasting well-insulated homes well into the future.
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