Getting straight to the (re)point

(Warning: this is an insanely long post. It’s largely an information dump so that I don’t forget what I’ve learned, so please don’t feel obliged to read it all!)

How do you solve a problem like lime mortar? With some difficulty, as it turns out...

While the weather stayed unexpectedly mild in November, we decided to focus on the outside of the house. As well as painting the stonework and weeding, one of the priorities was fixing the pointing on the bricks at the front.  

What's the problem?

The main issue was that some absolute chimp had smeared mortar across the brickwork as though icing a cake. It looked awful and couldn't be left as it was. 

The cement mortar coating the brick faces

There were also sections of mortar missing, with big gaps between bricks. And the other problem was that it appeared to have been repointed at least twice before - but with cement mortar.

Cement mortar sitting proud of the brick faces

I didn’t realise there was such a thing as the ‘wrong type’ of mortar. But after falling down a massive rabbit hole of blogs and YouTube videos, I learned that up until the 1920s or 1930s, most buildings were constructed using lime mortar. Lime lets water evaporate, which is important with old bricks, which are softer than their modern equivalents. Cement is a lot harder than lime - so if you use cement on old buildings, the water ends up going into the bricks instead and popping the faces off, eventually destroying the wall.

The prep work begins

The first job was to chip the mortar off the faces of the bricks (a great excuse to buy a club hammer and bolster). The cement was a lot harder than the old Victorian red bricks, but by chipping the edges bit by bit it was possible to remove it without taking any of the brick with it.

I chipped out the cement repointing too. The blogs I read said to do it to a depth of twice the height of the gap between the bricks; but mainly I stopped once I reached the original lime mortar, as I couldn’t see the point of removing this if it was still sound. 

The original, waxy 1890s lime mortar - so much prettier than the uniform dull grey of modern cement

The original lime mortar was actually quite lovely, if you’re into that kind of thing. You can still see the white lumps, which I presume are pieces of unmixed lime:

White pieces of unmixed lime

and black bits, which I think might be wood ash they might have used as a pozzolan (another term I learned - see the bottom of the post!).

Black ash in the mortar

A big piece of ash

I then mixed up one part of hydraulic lime to three parts of sharp sand. Unlike builders sand, which is soft and uniform, sharp sand has differently sized pieces of gravel in it, which helps the mortar to bond. 

The YouTube videos said to mix all the lime with half the sand, to ensure it was thoroughly blended, before adding the rest of the sand.

Sand and lime before adding water

I then added very small amounts of water at a time (adding too much water makes the mortar more likely to shrink and crack) and mixed it for about 20 minutes. Forget about going to the gym - I’ll have forearms like Popeye after this.

If it stayed on a board when I turned it upside down, it was ready. Apparently if you leave it for an hour then work it up again, it gets ‘fattier’ and easier to use, but I used it straight away.

I cleaned the cracks with a stiff brush to get rid of any dust, then brushed water over the wall using a large paint brush. This helps to prevent the bricks sucking the water out of the mortar and drying it out too fast.

Starting the repointing

Once the water had soaked in, I began repointing! Instead of a pointing trowel, I used a tuck pointer. By holding the mortar on a board just below the gaps, I could push the mortar in and do a whole row quite quickly. I found I got a better finish if I smoothed the mortar just shy of the brick face. I tried not to overwork it, too, as this can pull the water to the surface.

My lovely little tuck pointer, and an old piece of fibreboard I found in the house which I used as a pointing hawk

The next bit was my favourite part (yes, I’m quite sad…). Once the mortar had gone ‘green hard’, meaning it had firmed up but wasn’t set solid (about an hour in the cool November weather, although the shallower the mortar the quicker it dried), I took the stiff brush and gave it a hard scrub. Like magic, it turned from this:

The mortar before being brushed

to this!

 A lot smoother, and you can see the gravel in it, which is quite pretty!

Over the next few days I kept brushing it down with water to ensure it dried slowly, although it didn’t matter much with the weather being cool and damp. In hot weather or frosts, it’s also probably best to cover it with sheeting, as it can take several weeks before lime mortar reaches its maximum strength.

Brick replacement

It also turned out the smeared mortar was hiding some spalled bricks, so I decided to replace some of them. This involved chipping the mortar out from around the brick. I used a long-handled screwdriver for the hard to reach bits. Another trick I read was to drill a line of holes into the mortar, and it's then much easier to chip out.

One of the spalled bricks - presumably someone had hidden it with a coat of cement mortar rather than fix it

I’ll admit it was a bit scary at first taking a piece out of the wall. I started with the motto of "what's the worst that can happen if I have a go at this?" - until I actually started contemplating the worst that could happen while removing bricks from a 130-year-old wall, and decided I should probably stop thinking about it.

Err, anyone know where this bit goes?

I then followed the same process, brushing and damping the hole and the new brick. I then put a layer of mortar on the bottom and one side of the hole, and the top and one side of the brick, and pushed it in. It was difficult lining it up in three dimensions, as you have to ensure the brick sits level with the rest of the row, that the sides are equal widths, and that the brick is flush with the rest of the wall!

In all, the brickwork took longer than I was expecting; partly because I've never done it before, and partly because I had to take my time over removing the mortar without damaging the bricks. But I think it makes the house look cared for, and replacing the cement mortar with lime will hopefully ensure it’s damp-free for years to come.

That’s pretty much it for the outside of the house: join us next time as we discover what awaits us inside…

The lime addendum (sounds like a 1970s political thriller)

One website I read said that cement repointing had caused more damage to London's beautiful old brickwork than the Blitz. Here’s what I learned about lime…

What's the problem?

These days, houses are built to be basically waterproof: really hard bricks, pointed with really hard cement mortar. But this is a fairly modern method; prior to about the 1930s, most houses were built with lime mortar in the brickwork and lime plaster on the interior walls. This is the stuff the Romans used, and it’s pretty amazing. It's breathable. It's flexible, absorbing the little movements every wall has. It can even self heal: cracks will eventually disappear by the mortar absorbing carbon dioxide from the air. It’s designed to be the 'expendable' bit of the wall. Over decades, it will crumble and need replacing, which protects the bricks themselves. If you repoint with cement mortar, none of these things can happen, and it’s the bricks that suffer.

But just as not all mortars are the same, not all limes are the same (of course not - it couldn’t be that simple, could it?).

Lime putty

This is quicklime that has had an excess of water added to it. It comes in tubs and is the consistency of thick yoghurt. It is matured for three months, which apparently makes the crystals smaller (or something), and helps it to cling to the wall better. It’s incredibly breathable.

It comes covered with a layer of water. This is because it hardens by reabsorbing carbon dioxide from the air - unlike cement or hydraulic lime, there’s no chemical reaction to harden it, so it can take up to a year to properly harden on the wall. If left underwater, it will never harden.

It’s too soft for exterior work, but is used for decorative plasterwork and for interior plaster. It’s also used for Venetian plaster, which I’d love to try one day.

NHL (Natural Hydraulic Lime)

This is the stuff we needed. It’s the same as the lime powder used for making lime putty, but has naturally occurring clay impurities in it, which have a cementitious effect. This means there’s a chemical reaction, which speeds up how quickly the mortar hardens. It comes in three grades depending on the amount of impurity, increasing the hardness but at the expense of some breathability. NHL 2 is the softest, followed by NHL 3.5 which is for general exterior work (the one we needed), and NHL 5, which is hard enough to build sea defences.

The NHL 3.5 we used

Given how many old houses there are in the country, I thought this would be readily available. But crikey, did it prove hard to find. After a lot of phone calls we eventually tracked some down on a windswept industrial estate on the outskirts of Darlington. There was a sign outside saying 'public welcome' - although on reflection I think it was ironic.

Pozzolans

These are things such as fly ash and brick dust that can be used to artificially give lime putty a cementitious effect and speed up how quickly it hardens. In theory, if you couldn’t find any NHL, then you could make your own hydraulic lime by adding a pozzolan to lime putty, but it might be difficult to control how hydraulic it was.

Hydrated lime

This is widely available from builders merchants, and is often confused with hydraulic lime. It’s quicklime that’s had just enough water added to take the reactive edge off it, turning it into a powder. It's nothing to do with lime mortar: it’s used as a plasticiser in cement mortars, making it more workable.

However, although this isn't traditionally what's needed for making lime mortar or plaster, I think you could use it to make lime putty by adding more water, then topping up the container with water and keeping it for anything from 48 hours to three months to improve it. But it would have to be a fresh batch, otherwise it might have already carbonated. Traditional lime putty is usually made straight from the kiln.

I suppose you could even make hydraulic lime from hydrated lime by first turning it into lime putty then adding a pozzolan, but there are a lot of variables!