Repointing Heritage Brickwork
Keeping Thin Joints Thin
We rarely see repointing done well in standard 10mm mortar joints so imagine what we find when encountering thin joint construction or gauged brickwork with joints of around 1mm thick? We commonly find thin joint construction in old buildings and generally speaking, the thinner the mortar joint, the stronger the building. Buildings are either left with deep open joints in the belief that these buildings can’t be repointed or mortar specification is incorrect and looks like its been applied with a catapult. Poor application, incorrect tools and lack of specialist training are all problems but mortar specification is a primary consideration that is often overlooked. Imagine trying to repoint a a 1mm mortar joint when aggregate size is 1.5mm in diameter, the effect would be similar to trying to fit a square peg into a round hole.
There are a number of techniques for repointing thin joint lime construction in old buildings but the primary consideration has to be the mortar mix. Aggregate size is critical and if you are to stand any chance of successfully repointing then you’d be wise to have the mortar analysed. There are a number of UK based lime companies who offer this service now. If you are repointing a 1mm thick joint then the sand diameter needs to be 0.5mm. This would produce an extremely fine lime mortar. Generally speaking, this would still be an NHL 3.5 lime mortar for most external masonry applications but consideration is given to BS8104, the British Standard for assessing building exposure; the mortar analysis service we use also supplies our mortar after blending it to match existing and provides a certificate of conformity. It costs nothing to send the mortar sample in, and as was said to us recently, “It’s as easy to get it right as get it wrong.”
We recently encountered a fully repointed chimneystack, all correctly done in hydraulic lime and to a reasonable standard. The only problem was that the mortar was not colour matched and was stark white when compared to the pale yellow colour seen in the original mortar, simple colour matching would have avoided this problem. Chimneystacks are one area where repointing needs doing particularly well due to high exposure and poor access for ongoing maintenance but repointing here is often no better than seen in other areas of external masonry. Here we have a beautiful ornate chimneystack constructed in 1877 but aesthetically, and no doubt structurally, damaged by poor quality, ill informed repointing work. Note the thicker joints where mortar has been smeared across rather than pressed into the thin joints.
Finding a true craftsman who can deal with high quality maintenance work to gauged brickwork or thin joint construction is incredibly difficult and when you do find one they are generally booked up for weeks or even months ahead. These skills are all but lost to the mainstream construction industry and I would prompt any builder with an interest in heritage brickwork to sign up for one of Dr Gerard Lynch’s Courses, which can be found here… Heritage courses . Dr Lynch is one of the few remaining master craftsmen left in the UK and is thankfully passing on his skills.
Gauged bricks or ‘rubbers’ are very soft bricks, hand cut and rubbed to size. Buildings constructed with gauged brickwork and built to incredibly high tolerances and joints are unlikely to exceed 3mm in width. Where mortar has eroded in gauged brickwork then it is often best not to repoint unless absolutely necessary since raking out mortar joints can damage the soft brick arris, thereby doing more harm than good. If repointing is absolutely necessary, for instance lets say that a brick has slipped in a brick arch, then this will need a specialised repair. You would use a fine hacksaw blade to to help ease the brick back into position prior to pegging it into position with slivers of lead. The joints are then temporarily sealed with 2 coats of liquid latex, which on drying is then injected with a fine lime mortar from a 50cc syringe. The recommended mortar will be lime putty normally mixed with refractory brick (Fire brick) powder but in practice this can be extremely difficult to get hold of. Refractory brick powder or HTI powder, is a natural pozzolan that gives the lime putty its chemical set and is mainly comprised of silica and alumina. With regard to particle size, studies have shown that particles of 75 microns or less are pozzolanic, whilst particles of 300 microns or more act as porous particulates. You could have your own refractory brick powder made by having fire bricks crushed in a roller mill but we would generally use Argical M-1000, which is made from burnt clay and contains primarily silica and alumina; particle size is around 80 microns or less and you can find a data sheet for this product Here. The latex is peeled away from the joint once the mortar has set. Carpet tape can also be used as an aid to repointing thin joints; it is laid across the joint then split down the centre of the joint with a sharp craft knife. The folds are then pressed into the joint and this will aid you pressing fine lime mortar into the joint with damaging or spoiling the brick arrises. You would use extremely thin pointing irons for this work and sometimes they need to be purpose made. Assessing whether your contractor is capable of carrying out repairs to heritage brickwork may be a simple case of asking what’s in his tool bag because even many qualified brick layers will not carry these tools or understand the specialised requirements.
As with most things good preparation is critical and only the most patient and exacting tradesmen are prepared to expend the time and commitment to ensure that joints are carefully hand raked to the required depth and cleaned, so be prepared to pay a premium. Joints should be washed out with clean water prior to repointing and should still be damp when repointing commences. This will reduce suction on the mortar and promote better adhesion in the joint. Generally speaking, joints should be raked out to a depth of at least two times the joint width. However, you will often find that this principle is fairly meaningless when dealing with thin joint construction since erosion often far exceeds this depth by the time repointing is required. Maintenance and repair of heritage brickwork is a large and extremely complex subject area that I can’t possibly cover within the limitations of this blog but if it is a subject area you’re interested in then I’d highly recommend reading Gauged Brickwork by Dr Gerard Lynch but volume 2 of Practical Building Conservation is a good starting point.