Lime Pointing
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Ah, the long awaited lime pointing blog post! I know that some of you have been waiting for this for a little while, so thank you for your patience. I really wanted to be able to publish this ahead of a post on Lime Plastering, and as I’m planning to begin some more lime plastering tomorrow, it seemed like time to squeeze in an extra post. I really hope the below is helpful, and give you a bit more of an insight into pointing than I’m able to give over on Instagram. If there’s anything I’ve missed, or if you have any questions, drop them in the comments! On with the pointing…
What is Pointing?
Definition: To fill and finish the joints of masonry with cement or mortar.
Pointing is a technique used to ‘finish’ the gaps between stone or brick in construction. It uses mortar, which is a workable paste made by combining a binding material such as lime, cement, or earth, with an aggregate, most commonly sand, and water. Mortars are used in the construction of walls, to ‘bed in’ courses of stone or brick, and are also used to ‘finish’ a wall and make it watertight through pointing. Incorrect pointing can cause huge damage to older buildings, so it’s incredibly important to take care to maintain pointing correctly.
Repointing, which is the focus of this post, is the process of repairing or replacing mortar that has cracked, failed, or is not made of an appropriate material. Mortar is applied wet, and hardens as the water in it evaporates. When applied correctly, mortar bonds with the host masonry of a property (the stone or brick with which the property is built), and creates a durable, long-lasting finished surface.Repointing should improve the structural integrity, water tightness, and breathability of a building.
Fresh lime pointing in the Cottage bedroom.
Why is Pointing important?
The purpose of pointing is four-fold;
To waterproof a wall by sealing gaps and cracks, in order to limit moisture penetration into a property. Mortar acts a bit like a sponge between layers of stone or brick to draw moisture away from the host masonry
To maintain the structural integrity of a wall as a property settles and moves. Mortar is a flexible material, that is softer than the host masonry, which can flex to accommodate slight movement as the masonry expands and contracts in fluctuating temperatures.
To allow a wall to ‘breathe’, by allowing water within the fabric of the building to enter and leave through the mortar without causing damage to the host masonry.
To create a decorative finish to a wall
Different types of mortars
Mortars vary from property to property, decade to decade, and also by geography. Where in the country, or world, you are based, as well as the age of your property, will impact the type of mortar used in the construction of your home. In the UK, solid walled properties built pre-1919 were built using either Lime or Earth based mortars, depending on proximity of the property to a lime kiln. Our Farm, for example, is a solid walled property built in the late 1800s using lime mortar from a kiln in the next village. UK properties build after 1920 tend to have been built using a cavity construction method, with Portland Cement mortar.
Earth Mortar
Typically, earth mortars were used in historic, pre-1919 builds as the bulk mortar used in the construction of walls. The face of the wall was usually pointed in lime for durability, as earth mortars tend to be very soft. Earth mortars are rarely used in modern construction, but are still used in some eco builds.
Lime Mortar
An example of damage caused to stone by cement strap pointing. Source: Back2Brick
Used in most property built prior to 1919, lime mortars were used historically as they are strong and durable, but softer than the host masonry used to build a property. Lime mortar allows moisture to move through solid-walled structures, creating what we define as a breathable building.
Cement Mortar
Cement, originally known as Portland Cement, was invented in 1824 by Joseph Aspdin, a British bricklayer and builder, but didn’t become commonplace in domestic construction until the early 20th century. Houses built post 1920 tend to be built with a cavity, rather than as a solid wall, and are often constructed and pointed with cement. Cement grew in popularity in the mid-20th century, as it is relatively easy to work with and quick to set, but it can cause problems when used incorrectly.
If you’re unsure of the type of mortar you have at your property, you can either have your mortar tested in a lab, or you can DIY a test yourself. Lime is an alkaline material, and will fizz violently in acid. Chip off a small section of mortar, and drop it in a dish of white vinegar. If it fizzes violently, it’s most likely lime! Be aware that cement does also fizz a bit, but slowly and not as frothily as lime. Lime is most often white or cream in colour (whereas cement tends to be grey), but can sometimes be darker coloured if a dark sand, charcoal or coal dust was used as an aggregate. Lime is also much softer than cement, and sometimes can be scratched with a fingernail.
Carrying out repointing in the Cottage Lounge
Why we repoint
No mortar will last forever, and any form of mortar requires maintenance to keep it in the best condition. However, in most settings pointing tends to need to be replaced every century. As pointing ages, the mortar in the joints tends to weather and erode, causing it to crack and slowly crumble. As mortar fails, it can allow water to penetrate the fabric of a building, causing damp issues internally. If pointing has failed and is not repaired, it can allow water to ingress into the host masonry, which in colder weather may freeze and cause the host masonry to crack, shatter or blow.
Pointing should always be carried out in the material that was used in the construction of a property, or as close as you can get. If you properly predates 1919, this should be lime, and if your property is post 1920 and has a cavity, it can be cement. However, before you turn to repointing to remedy a damp issue, it’s worth inspecting other areas that could be causing the problem, such as blocked gutters, overflowing drains or missing flashing, as these are much quicker repairs than unnecessary repointing.
Why use Lime?
Lime mortars can readily handle the transmission of water and water vapour between the inside and outside of a masonry wall, owing to the complex interconnected pore structure of masonry. - Craig Frew
Despite their boom in popularity throughout the 20th Century, it’s now well known and recognised that the use of cement mortars can be hugely damaging and detrimental to solid walled and period properties in particular. This is because they impact the way in which the host masonry ‘copes’ with moisture. Cement mortars tend to be very hard - harder than the host masonry - and have a closed pore structure that doesn’t offer masonry the ability to ingress and egress moisture as it was designed to do. Because of this structure, and the strength of the mortar, cement mortar can force moisture into the stone, rather than drawing it away, and can cause huge damage to masonry. The trapped moisture in the stone can expand as it freezes, which can cause the face of the stone to literally pop off, a form of masonry damage is known as ‘blowing’ the stone.
Cornerstone Drymix from Cornish Lime.
Incorrect use of cement mortar is also one of the most common causes of what is branded as ‘rising damp’ (but is actually an issue of impaired breathability). Often, the solution offered to ‘rising damp’ is an injected damp proof course, but actually this can do immeasurably more damage to a property, and the best form of repair is to replace cement mortar with breathable lime mortar. Lime mortar must not contain any Portland cement, just natural hydraulic lime and sand.
My recommendation would be that wherever possible, solid walled properties that predate 1919 should be repointed with lime, not cement, mortars, and if achievable cement mortar should be replaced with lime.
Natural Hydraulic Lime (NHL)
The binder used in lime mortars is called Natural Hydraulic Lime (NHL), and comes in different grades which relate to the strength of the lime. The different grades of NHL are used for different applications, and are used depending on what the host masonry of a property is. Mortar should always be softer than the host masonry, to enable the breathability of the mortar to function effectively. A rough guide to different NHL grades is below, adapted from Cornish Lime.
As you can see from the table above, the grade that we use at the Farm, and is most commonly used in the UK, is NHL 3.5.
Lime mortar, as with cement, can be bought premixed or you can mix it yourself. The ratio of sand to lime will depend on the grade of NHL you are using, and the strength of mortar you require. For internal mortar, we use a 1:3 mix of 1 part lime to 3 parts sand. This gives a coarse finish, so if you prefer a finer finish, you could use a 2:5 ratio of 2 parts lime to 5 parts sand. Remember, any mortar is only going to be as good as it’s ingredients, so like when you’re cooking food, the quality of what you put into the mix will affect the quality of what comes out at the end. The sand you use in particular had a big impact on the consistency of your mortar. For that reason, and for quality control too, we use premixed mortars that we just add water to when we’re ready to use them. This ensures we have a good colour and consistency match everywhere we use the mortar. We also find this easier to store, as premixed mortars tend to come in 25kg bags, vs buying sand and lime separately often means finding somewhere to store large 1 tonne builders bags.
Preparing to repoint
Repointing should only be carried out if the existing mortar is decayed, cracked or damaged. Wherever possible, sound historic mortar should be left untouched. When preparing to repoint, it’s incredibly important to proceed with caution. It’s best, as much as possible, to chip out existing mortar joints manually, using a mortar rake or a good old fashioned lump hammer and cold chisel to a depth of 10-12mm. Be careful not to damage the brick, and it’s always best practise to wear eye protection. Cement pointing is often set so hard it can be difficult to remove this manually, so at the Farm we tend to use an SDS Hammer Drill with chisel fittings to help to remove the mortar. It’s extremely important to take care whilst doing this, as using heavy machinery can cause irreparable damage to host masonry. Do not use an angle grinder, as these can cut the stone rather than just removing the mortar, which can damage the natural glaze of masonry allowing water to enter the fabric of the building.
Some of the tools we use for repointing.
If you are replacing cement pointing, it’s really important to be as thorough as you can and remove all of the cement pointing from the joints. Any cement pointing left in situ will act as a seal around the host masonry, and the replacement lime pointing will not be able to function effectively.
Materials
This part is nice and simple! All we use for repointing is water, and Cornerstone Drymix from Cornish Lime. We use their NHL 3.5 mix in coarse grade, which is a 1:3 ratio of 1 part lime to to 3 parts sand. Cornish Lime use St. Astier Natural Hydraulic Lime in their Drymix, which is an extremely high quality matured lime.
Tools
How to repoint
Once you have prepared your wall by raking out the mortar, wet down your wall using a pressure sprayer (internal walls) or hose (external walls) and allow the moisture to soak into the wall. This helps to prevent mortar from drying out too quickly and not adhering to the masonry correctly.
Mix your mortar according to the manufacturers instructions. We do this by eye now, using a SDS drill with paddle mixer to mix the mortar in a flexibucket. If you don’t have a SDS drill, you can mix small quantities by hand using a bucket trowel. Your mortar should be thick, and should stick to a trowel.
Spray down your first joint. I recommend starting at the top of the wall, on a horizontal joint, and working your way down and across a wall, keeping a wet edge as you go.
Use your bucket trowel to transfer some mortar from the flexibucket to a plasterers hawk.
Starting at a horizontal joint, hold the plasterers hawk against the wall just underneath the joint, and use a pointing trowel to force the mortar mix from your hawk into the joint.
Repeat step 5, but on the vertical joint. Make sure you continue to wet down the joints as you go to stop the mortar drying out too much.
Use a brick jointer or the flat side of your trowel, to smooth any joints, remove any excess mortar, and create a flush finish. Your pointing should sit below the surface of the host masonry.
Once the mortar has dried out slightly (usually after a couple of house), hit the mortar with a churn brush to compact the mortar into the joints, and to add a textured finish to the mortar.
Brush off any excess mortar from the face of the masonry. If needed, you can use a damp cloth to wipe the mortar away.
It’s really important to cover your mortar, particularly if you are repointing outside, in warm or cold weather, or in an exposed location. Lime mortar sets through a process called carbonation, where water and carbon dioxide react to set the mortar. This process must happen slowly to prevent cracking, so adequate protection for mortar whilst it cures is a must. Outside, we cover fresh mortar with damp hessian for the first few days of drying. In hot weather, you may need to spray the wall down at least once a day for the first few days, to prevent the mortar drying too quickly.
It’s also important that the temperature doesn’t fall below 5 degrees celsius whilst mortar is curing. Below this temperature, the water in the mortar mix may freeze, causing frost damage and causing the mortar to fail. It’s best to consider when in the year you’re applying your mortar before you start, so you can make sure you’re adequately prepared for the work and for protecting the new mortar as it cures. From experience, it’s no fun repointing outside in the pouring rain!
I hope that was a helpful insight into repointing, and why it’s important to use lime in period properties. If you have any questions, please pop them in the comments section, and I’ll also pop some links below for further reading! Otherwise, head over to our Instagram to keep in touch with what we’re working on at the Farm.
Further Reading
Cornish Lime: About Cornerstone Mortars
Cornish Lime: Lime in Building
Cornish Lime: Lime Mortar for use in today’s restoration and newbuild projects
