Over time, buildings become prone to wear and tear and they do need preservation to retain their structural integrity. When a building needs repointing, the use of lime products, either hydraulic or putty based is a must to correctly preserve the building. Using cement based mortars are far too hard and can damage masonry.
The use of a lime mortar for pointing in general masonry allows you to use a material that is both porous and softer compared with mortars that feature cement. This allows the moisture to evaporate from the joints more effectively which reduces the level of moisture in the fabric of the building. It can also decrease the likelihood of soluble salts appearing on the surface of the stone. Over time these salts can build up and cause the brickwork to spall or the surface to break down.
The application of lime mortars will require meticulous and careful control within the drying process.
Before you begin any repointing project, it’s good practice to start by undertaking an assessment of the building.
Firstly, look around the building to make sure all of the original pointing is intact. This can be a source of water ingress which can be the start of internal damp problems if it is left for a long period of time. The external walls are there to keep moisture out of the building. If you are working with a sandstone wall, these should never be repointed with cement because it can cause damp to form in the structure of the building, particularly older buildings. Lime mortar is advantageous as it allows the structure to breathe, because of this moisture wont get trapped within a building causing a number of issues such as damp walls.
Where your inspection uncovers any defects in pointing, the correct quantity should be removed. Typically, this would be twice the width of the mortar joint but not less than 20mm. To make sure that bricks or stones aren’t forced apart, it’s good practice to use a tool called a plugging chisel or mortar pick.
Before the lime repointing work can begin on any type of construction, it is important that the surface is adequately prepared. Brush out any loose material and then dampen each of the joints, making sure that you leave plenty of time for the water to soak in. Mortars should be as stiff as possible which improves compaction and reduces smearing and shrinkage.
Many lime mortars can be putty based, and these will work best if they have been pre-mixed a couple of weeks or even months before it is required. It should be agitated and mixed again to soften them before application which reduces the likelihood of shrinkage. Pre-mixed lime mortar can be purchased which makes the process quicker and this can be applied immediately.
Lime Repointing: Step by Step Process
The first step in the process once you are ready to start repointing is to rake out the existing mortar joints to the right depth. The depth that you remove will depend on the width of the brickwork. It should be approximately double. So for example, if you were working on a 10mm wide brickwork joint you should remove approximately 20mm. There are specific tools that you can use for this task including the joint raker, mortar pick and plugging chisel which will prevent any damage to the surrounding brickwork. Carefully brush any loose debris away from the surface and joint of the brick.
The next stage is to damp down the mortar joints and the brickwork. It is really important that this process is completed carefully, making sure that you wet down the whole wall (bearing in mind each brick can retain up to a pint of water). If the wall is very dry, such as during the summer and it hasn’t rained for a while, you might want to start spraying it the day before. This will stop the mortar from drying out too quickly, if this does happen it can crack and it wont carbonate properly.
Preparing the Mortar
The mortar should be adequately prepared. If you have a pre-mixed mortar, there may be some excess water on the top of the mortar. Tip the mortar onto a board and any excess water will drain away. Agitate the mortar to soften it so that it becomes malleable and workable. The more you agitate lime mortar, the better it works. If it feels quite stiff and dry when first working with it, keep going working the material, only adding a small amount of water if needed. Be careful however, because adding too much water can increase the natural shrinkage of the mortar and result in the material cracking.
Place a small amount of mortar onto a hawk and then take a pointing iron or trowel to apply the material to the brickwork. It should be the right consistency and always try to work the mortar in the same direction (towards the original mortar), pushing it into place as you go. A narrow pointing iron should be used to keep the mortar from the face of the brickwork. Try not to scrape any wet mortar across the surface of the brick, as it will stain. Once the mortar has been applied to the wall, avoid over working it because it can weaken the material.
Leave the mortar for several hours (the length of time will depend on the weather conditions at the time and also the temperature). By this time, the mortar should be beginning to carbonate, and you shouldn’t be able to mark it with a nail or your finger. If the mortar is setting too quickly, lightly spraying it will slow the carbonation. Take a piece of wood which has been cut at a 45 degree angle and run it along the joint, applying even and consistent pressure. If you are applying the mortar in the winter, you may need to cover it once this process is complete with a polyethene or hessian sheet to stop frost attacking and damaging the mortar. In the summer months when the weather is windy or hot, you should cover the mortar with a damp hessian sheet to prevent the mortar from drying out too quickly.
You can now add the final touches, by using a stiff scrubbing brush to remove any excess mortar from the brickwork. You can also tamp the surface of the mortar with a hard bristled churun brush to stimulate some early weathering and to expose the aggregate as seen on our website.
The lime mortar pointing process is now complete and is a material that should last for a long time if you follow the right practices.