Repointing stone walls
Our building surveys frequently encounter problems with stonework. It can be costly to repair damaged stone, and expensive in terms of internal damage. A key feature of surveys on historic buildings is assessing the future cost of inappropriate measures - damage caused, ongoing deterioration, and the cost of remedial measures. Our advice will include an assessment of removal methods, suitable materials, and methods.
Britain enjoys a huge variety of different building stones - from hard, glittering granites of Cornwall and Devon, to soft buff coloured Bath limestones, and the forbidding hard, grey slates of Wales. Wherever we work on surveys, we see problems typical for the area. These often involve the use of cement to 'repoint' or render stone walls. If lime mortar, or mud, is replaced with cement, the wall can no longer lose moisture which diffuses through it. One of two things will happen:
...If the stone is very hard, like granite, slate, dolerite, or some of the very hard limestones - mortar joints become soaking wet, and the house will be physically wet inside. We've seen situations where every piece of timber in an outside wall was rotted. If the stone is soft and porous - bath stone, soft sandstones, soft limestones and siltstones (many of the cottages in Shropshire's Corve Valley are built from these siltstones) - the stone itself will disintegrate as moisture tries to exit the wall. The house internally may not be so damp - the wall is able to stay relatively dry, but at the expense of the stone itself.
A particularly bad example can be seen here: Damage to stone wall by cement. This is a Shropshire railway station building, cement rendered and very wet internally. Behind the cement, stone had almost entirely disintegrated.
The set of photos shown below are from a Grade 2 listed house near Manchester - built with soft sandstone. Terrible damage has been done to the stone, and the house is damp inside as a result:
In this photo you can see the original lime mortar joint in the back of the slot - and in the foreground, a white stain covers much of the stone where the builder smeared cement over the stone.
The original mortar joint was only about 4mm wide. It is now nearly 30mm wide - you can see the original as a dark line at the top of the slot. This is dreadful abuse of stone walling in a Grade 2 listed cottage.
This wall has been badly damaged by a builder using an angle grinder to hack perfectly good lime out of joints. Not only has it permanently damaged the stone, but it has made the joint two or three times the width it was originally.
The stonework of this wall is spalling badly because moisture can only exit through the stone. The actual mortar joints are around 6 mm wide - cement is smeared almost all over the stone of a Grade 2 cottage. Black stain is a leak from the roof causing penetrating damp internally.
Detail of wall damaged by cement - lime mortar joints are 6mm - moisture escaping stone has blown faces and stone repair is now needed. Original cement strap pointing at top of photo and falling off.
Welsh Quarrymans Cottage
We surveyed this cottage, which was sopping wet internally. Very high humidity readings. Externally it appeared dry. Built with slate, and pointed with cement. We removed cement pointing to find lime mortar behind which was literally mud. Simply removing the strap pointed cement, allowing the joints to dry out, and re-pointing in lime solved all the problems. We also recommended removal of gypsum plaster internally, and replacement with lime plaster.
Quarrymans cottage, built from slate blocks. It was sopping wet. Pointed with cement, and plastered internally with gypsum.
Cement strap pointing trapping moisture into walls. As the stone is slate, it cannot dry out, and moisture levels had risen to almost 100% within the walls. Simply taking the cement out of the joints dried the building out.
Original mortar, raked out from behind cement pointing - it is mud. It also dried out in the sun in a matter of minutes.
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