Penetrating damp caused by using cement on lime mortar beds | Preservation Expert
Posted by David Bailey
Hard cement pointing can destroy stone buildings within a few decades
It’s a sad irony that within 100 years of the introduction of cement and its complete dominance in new buildings, that the downside should be the destruction of buildings which have stood for hundreds of years; well before cement was invented.
Cement is a truly wonderful product, easy to use, forgiving and offering incredible strength it’s not surprising that it’s applied almost everywhere.
I mixed my first batch at work in my father’s damp proofing company in 1976, instructed by my favourite uncle; Derek.. “Three t’ one, lad, that’s it…. Just a squeeze of fairy liquid… lovely”
And so it has been for many lads joining the building trade in the 70’s. One shovel of the grey powder and three of the sand; usually bright yellow builder’s sand, adulterated with finer red sand of exactly the wrong sort for the job – but it was such a pleasure to use.
I know better now.
For those that don’t I’ve written this short post to describe in simple terms why cement should be avoided when applying mortar to external surfaces, as either pointing or render and what to use instead. I am not a conservation specialist so here I am referring to ‘old’ buildings – not historic fabric which may need much more careful consideration and consultation with the local conservation officer.
By ‘old’ I mean any stone building, even if brand new (remember – stone is always old) and all brick buildings which are pre-war. There are arguments against cement even on new-build, but let’s leave that on one side for now and ask why it matters, particularly on older existing ones.
The above image shows destruction of stone on a building in Yorkshire. Look close and you can see that the pointing is standing proud of the stone edges and in effect the stone appears to have almost shrunk away from the pointing; leaving an undercut I could get my fingers in. Think about it; the chap who applied this pointing couldn’t have trowelled that mortar like that; this has happened since the pointing set; against the stone.
Is the cement corrosive or something? What’s happening?
Well, the first thing to remember is, we live in a wet country; it rains lots. These stones soak some of the rain up and have been since they were hewn from the quarry and put in this building. This getting wet and drying out, is all part of a natural cycle and is not that damaging to the stone. The lime mortar bed joints too, soak up this water, maybe even a bit quicker and they too let the moisture back out as the wind blows and the sun shines.
There are buildings like this which have stood for hundreds of years, with little sign of the sort of erosion seen above and then in the middle of the 20th century things changed.
The thing is, lime bed joints don’t last forever and the weather takes its toll. Eventually after decades the beds may be badly recessed and water can really get in and cause damage. The answer is re-pointing and many old buildings have been repointed several times with no ill effect; until a few decades ago when the well-meaning builders used the new stuff – cement. Unlike lime it sets fast, can be built up quickly – is less susceptible to frost damage and it retains water better in dry weather – rock hard too, so it lasts so much longer. No wonder the builders chose it – anyone would.
So, we have now repointed the old chapel with fancy cement mortar and even used it’s workability to produce a fancy ‘strap’ appearance as below (shudder…).
The next week it starts raining and once more the wall soaks up water. Through the stone mainly, because now the beds are sealed up with this dense cement. The wall gets pretty saturated and moisture gets behind the cement via the stone and the tiny cracks which form between hard and soft materials. Some of these cracks are too hard to see with the naked eye but they are there; cement is sometimes so hard that even the act of curing against a soft stone will cause the face of the stone to be pulled away by tension, induced as the cement imperceptibly shrinks.
Once the sun comes out drying begins, but now the cycle is altered.
Evaporation still happens in the stone, but the water in the bed joints, behind the cement is not wicked away and the beds stay moist for much longer. There are now some subtle additional things to consider; a wet wall is a cold wall because as water evaporates it uses energy. The wall is cold so overall there is less energy to drive evaporation and so the time needed to dry the wall in depth is much longer. Two things lead on from this; it is likely that the wall will be rained on again before complete drying has happened, leading to a wall which is wet for a much longer period. For much of the year it is now never truly dry.
Fast forward into winter and that’s when trouble starts. Night falls and ice forms in the surface of the stone, especially around the edges of it next to the bed-joint. Moisture is concentrated here because its pathway out of the wall is blocked by the bead of impenetrable cement, so it is slowly ‘wicked’ around the cement via the adjacent stone. To make matters worse the cement pointing has the strap detail, so it also offers handy little ledges for standing water to dally, allowing even more saturation of the stone.
When water freezes it expands; ice cubes are always bigger than when they were poured into trays as water. When this happens inside the stone the very structure of the material is burst apart, leaving the characteristic friable surface seen below – often accompanied by a pile of sand on a windowsill just below that.
The tiny cracks at the interface between stone and bed ices up and the expansion puts stress equally into the cement bed and the stone…. which is harder? It should of course be the stone, which was the case, before the lime bed was chased out and replaced with rock hard cement. Something has got to give.
There are other things going on too, but the big killer is the freeze thaw cycle, which may have been there before the cement was applied, but the new conditions have amplified the problem and accelerated it many times over.
The evidence is all over the place and is growing. I’ve seen many buildings where the general condition of the stone is excellent, apart from one area which was cement pointed say 40 to 60 years ago. That is a very short time indeed, especially when you think that actually all stone is millions of years old – why is it suddenly changing back to sand?
You know why now.
For all relevant external repairs I specify natural lime mortars. Occasionally we’re using moderately or weakly hydraulic types when it cannot be adequately protected and fat lime is too risky. This type of pointing won’t last forever – nothing can. But as it succumbs to the weather it will sacrifice itself for the good of the building, rather than selfishly hanging on like a parasite, whilst its host withers away.
For more information try SPAB (Society for the Preservation of Ancient buildings), who have a huge library of useful stuff and contacts galore on their web site. The use of lime is no longer just the preserve of specialist conservation companies, because frankly there are not enough of them to deal with the epidemic of damage which is in progress now. All builders and maintenance companies should, like I, have themselves and some key staff attend at least a basic ‘lime day’ of the type SPAB organise, so that those healthy stone and old brick buildings awaiting pointing, get the right material and don’t end up looking like those in the above images.
If your building is suffering already then the best solution is to remove the cement pointing and replace it with lime. However, this is not always appropriate if the material is so hard that its very removal will cause damage. In these cases and only with specialist advice and careful consideration, the use of a water repellent cream such as Storm Dry may help. This will at least reduce the absorption of rainwater so that less time is needed for evaporation and drying – and the freeze thaw cycle is shorter or negated altogether.