Tips and tricks for Mortar replacement and brick pointing
These tip and techniques are particlarly aimed towards old brickwork however there should be something for everyone.
If you need to replace mortar that has totally degraded check this page first that describes how to remove and replace motar at the full brick depth. Essentially rebuilding a wall without demolishing it first.
Removing old mortar and raking out.
When old lime mortar is soft, a good scrape with an old wood chisel can do the job effectively an old dinner knife can come in handy as well. For tougher mortar a cold chisel used alongside a drill with a masonry bit will be more appropriate.
If there's portland cement over the top then care should be taken not to damage the bricks. Slicing through the middle of the joint with a diamond blade on an angle grinder then chipping it towards the slit will save most of the brick.
The old mortar should be removed to a depth around twice the joint width.
You’ll be left with a lot of dust in the slot so a good vacuum comes in handy, so much easier than trying to brush it clean. To get the debris out of the deep, narrow slots I use a slightly crushed piece of 15mm copper pipe, this can easily be held by hand in the nozzle of the vacuum hose.
A Good Soaking before Repacking and Pointing
Before pushing in new mortar the bricks and surrounding mortar need a thorough soaking with water. If you miss out this step the new mortar can become almost unworkable straight away as the brick sucks the water out of the mix. I have found the easiest way of soaking the bricks is using a pressurised garden sprayer. With the nozzle set for misting the surfaces wet up quickly.
Mortar Mix for Old Brickwork
The new mortar should be well mixed. I Use a mixer paddle on my 800 watt electric drill.
I prefer to use NHL 3.5 (Natural Hydraulic Lime) with grit sand at a mix ratio of 1:3 by volume. However for thinner joints, soft sand is easier to pack in, in this case I normally up the ratio to about 1:2.5. The NHL stays workable for longer than OPC (ordinary Portland cement), it's softer and breathable so it’s much kinder to old bricks.
I find a 2 pint saucepan good for measuring out workable quantities. Putting 2 sand into the bucket then the one lime topped off by the final 1 sand will keep the lime dust down while mixing. If you put the lime in first it tends to stick at the bottom and it’s hard to mix.
This eight pint mix will usually need less than a pint of water, so after the half pint just add a splash at a time until you get a good mashed potato like mix (well that’s every bit as good as saying soft buttery texture).
The mix does need to be quite stiff. If it’s sloppy it’ll be difficult to trowel in and then to compress in the joint.
Special Tools for Repointing
Pushing mortar deep in to joints is difficult without the right tools, I don’t find a small pointing trowel much help. What really makes the difference is a finger trowel of the right width. OR with just a minutes work you can make your own tool. Wait until the other half isn’t looking and pinch one the stainless steel dinner knives. Grind or file the edges of the blade parallel and the right width for your joints. Then bend the blade to about 30% from the handle.
The other tool that can speed things up is a mini mortar board or hawk. I use a cheap plaster trowel that I’ve bent at one end. The up-stand is useful to push against to load up the home made finger trowel.
Repointing and filling deep holes in old mortar
Using the purpose made hawk and finger trowel makes it much easier to actually get the mortar in to the joints. Each load from the trowel / knife should be well pressed in. If the gap goes all the way through, start on one side, for horizontal gaps or from the bottom of vertical gap. Build up the filling with more mortar towards the back so there’s angled build up. Press it well and it should fully fill the gap without too much dropping out or pushing through.
Deep holes should be fully tamped in to ensure the mortar and aggregate is fully compressed and compacted in the joint. I use the end of my knife for this.
Final Finish of the repointing
Slightly over fill the gaps and then leave it for ½ to 1 hour so the mortar has started to dry out. How it’s next worked depends on the finish required. I normally prefer a brushed finish rather than a sharp troweled effect. First the surcharge needs to be worked by drawing, under pressure, a tool across the joints. Depending on the tool used you can get the “bucket handle” finish or use a flat tool for a flat finish. I use the handle of my “knife” Finger trowel to give a flush finish. This stage will re-compress the joint and fill any small cracks that may have formed due to shrinkage as the mortar dries out.
(note: these pictures are from a full mortar replacement job rather than normal, shallow repointing)
The joints will still look messy with crumbly excess bits of mortar around the joints. This is best left a little longer to allow the mortar to set even more. After a while, the joints can be tackled with a stiff brush. The excesses should easily brush of and for an effect I like, a quick rub removes the surface of the mortar and exposes the aggregate.
Try not to re-wet the surface in an attempt to clean up the brick until the mortar has fully set, it’ll probably end up getting messier with the lime staining the brick. Once it is dry a good wash and scrub will leave a clean and durable surface.
Any comments or suggestion, please visit the "Chimney Jenga" Blog post.
For the full job that these photos came from start on "Chimney Jenga" at our Whitby Holiday cottage restoration page.
and for more about the cottage here's the Full index on the Whitby Holiday Cottage renovation