Our lime mortar mixes

On our ‘working with lime mortar’ course, Hans Norling convincingly addressed a simmering dispute amongst those who use lime mortar – ratios of sand to lime. There is a common rule of thumb that it should be around 3 sand to 1 lime, by volume. It turns out this is most likely from the days when quicklime was used. When quicklime is slaked, it expands by 1.8 times. Making an equivalent mixture directly from slaked lime (using NHL or lime putty) would need nearer 2 units of sand to 1 unit of lime to get an equivalent mix.

We have made the choice to do most of our building with lime, including all the concrete blockwork more than 1m above ground. Below 1m, it will be tanked on the inside face and have the liquid DPC (synthaprufe) on the other i.e. there would be nowhere for lime mortar to breath. For those who work with cement mortar mixes of 6:1 or 4:1 and who pay £5 per bag rather than our £12 for lime, this might seem extravagant. We are paying a premium, but lime is a lot less dense than cement, so goes quite a bit further. We really do want to make the structure as breathable as possible. It is also great to work with, it stays workable for a lot longer than cement mortar. If we have left over mortar at the end of the day, we put it back in the mixer next day, add new sand and lime and carry on as normal. Cleaning up is quicker and easier. We do have hessian to keep the sun & wind off and a canvas to work under when it is wet, but most of the time, it is all very well behaved.

The downside to lime is that, over time, it starts to bind to metal objects such as trowels and the cement mixer, forming a hard crust of what I expect is calcium carbonate. In the cement mixer the mortar sticks to it, preventing it mixing properly and making it a bit of a swine to empty. At intervals I have spent a happy hour with a chisel tip in our light SDS drill, chipping it all out.

Our normal mix for non-visible mortar has indeed been 3:1 by volume, using building sand:NHL 3.5 and whilst it does go quite plastic after 20 minutes mixing, it is more workable with plasticiser added. It also tends not to stick very well as it starts to dry out. For laying blocks, we are making a slightly wetter mix which works well enough. When building rubble wall, we need the stiffer mix and stickiness is not such an issue. I quickly found the value in splatting even non-adhesive mortar into narrow gaps to fill them. If you miss, it is messy and a waste of mortar. If you get it right, it does fill all nooks and crannies. Strength-wise, 3:1 is quite adequate for us.

I experimented with increasing the lime content and decreasing the plasticiser. As I got towards 2:1 and no plasticiser, the result was indeed smoother, sticker and easier to work with for longer. It is also significantly more costly! So we will go with 3:1 for the bulk of the building, but I will use 2:1 for final pointing, where adhesion is very important. We will also be using our sharp sand from Bridgend quarry in place of building sand. We have two big bags, this should do for most of our final pointing. We will experiment with the pointing mortar at a future date, we can adjust the sharp sand by adding building sand or a coarser grit if we need to.

Training to use lime mortar

We were tipped-off by another steading-converter (the far side of Ellon from us) that a 2-day ‘working with lime mortar’ course was scheduled for the week we were arriving, to be held in Banff. This was organised by the Scottish Traditional Skills Training Centre, who do a variety of courses such as ‘managing woodland’ or ‘traditional window design’. We enquired and were instructed by Ric to attend. So we spent two days at Banff castle, about a 50 minute drive over to the Moray coast. This was run by master-mason Hans Norling, a Swede who has lived in Scotland for 16 years, giving him a most unusual accent. He co-founded his current company Masonry & Lime ltd and employs 16 staff. He was very laid back and really knew his stuff – we covered the theory & practice of picking out mortar, mixing lime mortars, pointing stonework, applying harling and using limewash. This may sound pretty dull, but when faced with loads of walling to build and several hundred square metres of picking & pointing, it has been a lifeline.

We got to meet 9 other like-minded souls including a couple over Fyvie way and, a bit spookily, the owner of a farmhouse just south of us, at Auchnabo. He is between us and our Architect, I guess his building is one of the points of light we see at night.

Most of us had some previous experience working with old stone buildings and there were sighs of relief at various points as we realised we had not irretrievably wrecked whatever we had been working on. Which is not to say that we will do a better job now we know how! My moment was when we got his blessing for using Singleton & Birch NHL 3.5 lime – good stuff, he said. They were using St Astier on the course, which gives a rather whiter finish. He was also increasingly adding quicklime to his mixes, to make them sticker and more workable and to give a softer finish. His view is that old lime mortars would have included locally-produced quicklimes.

We asked for advice on removing our 50+ square metres of concrete render along the entire north wall, he could not give us much comfort – he did unbend enough to say to use an SDS drill with chisel tip. Using mechanical tools on old stone is a bit heretical in the building preservation industry. I followed his tips and tried an area where I allowed the point to stay in any one place for only a second or two at a time. This mostly worked, the mortar was much more likely to end up crumbling and falling away, than where I left the tip in one place until it broke through the mortar. It potentially reduces the damage done to the stonework. It did not work so well where there was a thin layer of mortar strongly bonded to individual boulders. [A further tip on removing hard cement pointing from stonework, from an English Heritage publication: Run a diamond blade down the middle line of the pointing, to help break it out without damaging the stonework. I will look at getting a blade for our small angle grinder.]

2018 shopping list

As far as we know, we need to buy the following:

• Granite lintels, cills and thresholds – from Lantoom Quarry
• Around half our concrete structural lintels – Having bought a whole load of 1.5m and 1.8m lengths a year or so ago, we need to get 18 x 1.2m, 8 x 1.5m and 8 x 1.8m ones. We can do this by buying 4 x 3m & 10 x 3.6m lintels and cutting them.
• An initial tonne of NHL 3.5 lime.
• A truckload of sand, probably from the Bridgend quarry near Tipperty, for lime mortar
• A heavy canvas that we can arrange over scaffolding to protect us and the walling from the elements.
• A tarpaulin to cover the sand.
• A truck load of 440 x 215 x 100 concrete blocks
• DPC and other sundries
• Locking castors for our scaffolding so we can run a bay around the concrete slab for working inside.
• A silt trap so we can install our linear drain in front of our garage & front door.
• Blockmix, sand & cement to build a concrete apron in front of the garage up to the linear drain.
• A bilge pump to clear sludge out from our concrete tank

Depending on progress with walls, we may need:

• Structural timber to support roof trusses at each end of the north wing (where we demolished the internal rubble walls)
• Timber for wallplates, truss clips, tie-down straps etc.
• Roof trusses
• Sarking boarding
• Breather membrane and tape (may be a cheap temporary fix to protect the roof, to be replaced when we are ready to slate)

Methinks this will stretch our finances until we can sell our current house and I retire.

Concrete & Lime

Concrete

I have discovered the complicated world of ready-mix concrete. We need around 30 ‘cubes’ and want delivery trucks with elevators at the back end, so we can get the stuff within the steading walls before barrowing it i.e. no more than 10m to push each barrow load.

I looked around, there are loads of companies in and around Peterhead and Fraserburgh. I checked out a reasonable sample, none do trucks with elevators. By chance I looked nearer Aberdeen and spotted one that did on-site mixing, which I knew nothing about. I glanced at their website and Lo! They had a piccie of a truck with an elevator and a chute, that would reach up to 6m from the back end of the truck. I phoned them, they can deliver when we want it, it will take 3 loads of one of their trucks, we can have one or two trucks relaying the materials to our site. Perfect. The price, £115 inc vat per cu m, is also not bad.

It turns out the company uses ‘volumetric mixing’ trucks. They have hoppers for sand, gravel & cement plus a tank for water. They mix on site by metering materials onto a conveyor belt and into an auger screw, add the water, then the screw mixes the concrete and pushes it out into the chute. Fine by me. I texted my two builder brothers and got suspicious replies. One said he tried it years ago and did not like it because it did not mix as well as a normal barrel truck. The other asked dubiously whether they would certify their mix.

I did some due diligence on Google and discovered that the industry is ‘somewhat’ divided about volumetric mixing. In the pro camp, they all say that you only get and pay for exactly what you need, there is no transport time for it to go off, it is cheaper because the trucks can do several drops in one round trip, they can alter the mix at any time. Nothing about the quality of mixing, though. The anti camp say that it is lower quality, but also that it is unfair because the volumetric trucks are classed as ‘engineering’ vehicles not HGVs, so they pay less road tax, do not need operators licences and do not need to meet upper weight limits for their vehicles. A fully loaded volumetric truck can mix 10 cu m in one load and weighs in at around 40 tonnes. Some-one suggested that if the trucks had to meet HGV weight limits, they could carry enough materials for about 4 cu m, so they would not be economic. And so on. The debate is heated.

I checked with our architect, he said we did not need a certificate for the concrete.

A question for another day, “how hands-off can you be and still get decent quality housing stock?”

I found some US technical documents about the relative qualities of volumetric v weight batched concrete. If the equipment is set up correctly and the operative knows what they are doing, they found that volumetric concrete was marginally stronger than barrel or drum mixed. So, what to do? Actually, it was an easy decision, the plusses greatly outweigh the minuses. The mix they quoted for, C35, is so massively over the top for a floor slab that it would have to be wildly variable to cause us future problems. So I will phone them and give them the opportunity to show their expertise and customer service. At a not-bad price.

Lime

We have found a source of lime that we will go with. In principle we want to match our mortar to what was used when the steading was built, but that may well have been produced from local materials that we do not have access to. Possibly sea shells. Or that rare outcrop of limestone up the coast towards Boddam. Who knows? We also observe that the walls in east and west wings are substantially tougher than those in the north wing, but is that because the builders used more mortar or a different composition?

So, to be practical, we looked for a commercial medium-strength naturally-hydraulic lime (NHL 3.5) that we knew we would be able to get hold of in large enough quantities, over several years. There are several candidates, so we looked at a) availability in Aberdeenshire and b) cost. Our local builders merchant sells a brand of lime, but it is over £17 per 25kg bag ex vat. Looking at on-line suppliers, their per-bag costs are lower, but by the time you add in transport, it is up to that sort of level. We kept looking until we found Singleton-Birch. They are based in the Lincolnshire wolds near Louth – close to where I was brought up – but have a stockist in Linlithgow, a stone-throw from the Forth bridges. I will drop in on our way up to Aberdeen and get 8 bags or so, at £11 + Vat a bag. If it is what we want, they will deliver tonne pallets to the steading for around £13.75 inc VAT per bag.

Lime goes off quite quickly, in the way cement does. I will take a whole load of heavy-gauge plastic bags (that we get out dog food delivered in) to wrap them up. We will see if that extends their life enough to work through a tonne batch.

Our next challenge is finding a consistent sharp sand to go with the lime, that resembles the original mix. We will start with the local stuff, which comes from a local quarry the far side of Ellon – recommended by the architect.

Getting ready for the floor slab & walls

We are getting going on organising our August/September session at the steading, just two weeks, unfortunately. Ric will go up the week before and stay on for a week afterwards.

New/rebuilt walling

Floor slabs

We will have a fairly demanding timeline to get our big job done, getting the floor slab laid:

1. Dig out the remaining steading floors- all the long northern stretch and much of the west wing. We need to screen the soil and find somewhere to deposit it all. I expect this will fill the week before we arrive.
2. Clean up the floors round the edges where the digger had trouble, level it all out to 53cm or so below finished floor. We will be doing this for a day or two, as soon as we get up there.
3. Spread 200mm hardcore, probably in two layers, using a wacker plate to get it compacted. At the edges where the slab needs to bear structural timber, we do not place hardcore, so we can thicken the slab downwards. I expect we need a bit less than a week.
4. Spread 5cm soft sand to blind the hardcore, a couple of days work.
5. Lay Damp Proof Membrane (polythene), double-taped where the sheets join. A days work.
6. Use supports to get sheets of A252 rebar mesh in place. I expect we will do this just ahead of the concrete so we keep reasonable access.
7. Get the concrete mixer trucks in with 30 cubic metres of concrete, have a fun few days getting it placed, levelled and presentable. This will be much better if we can get mixers with concrete elevators on the back – not sure yet. As far as we know, my youngest brother Geoff is still coming up to do the skilful bits. Three days work I reckon.

Obviously we do not want a lot of rain over the period, I expect showers will not stop us but heavy rain will. I foresee some long old days.

We have various sections of drain, for the bathroom, cloakroom and en-suites, that will be under the slab. We will do leakage tests on them before we cover them permanently. We stick a bung at the end of the section of pipe e.g. where it goes into an inspection chamber. We create a 1500mm head of water and check that no more than 50ml per 1-metre run of pipe leaks over two hours. We also want to run an electrical cable through the sand blinding towards where we want our polytunnel – 6mm2, 3-core.

Whilst this is going on, we will get everything needed to start building walls. We have to build a leaf of blockwork 220mm wide on the inside by laying standard blocks on their sides. We will tie them in to the existing walls. We paint liquid DPM on the outside face of blockwork and use our steel expanded mesh to tie the outer leaf of granite in to the blockwork. We will get 8 or so bags of NHL 3.5 (medium strength) lime and experiment using local sharp sand to make lime mortar. All being well, we can use any time not doing floor slab on getting some experience of actually building upwards – essential if we hope to get our roof on by the end of next year.

The shopping list is looking like this:

• 1.7 tonne digger, hire 3 weeks
• Mini dumper, hire 3 weeks
• Wacker plate, hire 1 week
• Soft sand, 18 tonnes
• 1200 gauge DPM, 25m x 4m x 3 packs
• Single-sided tape, 100m
• Double-sided tape, 100m
• A252 Rebar mesh, 240 sq m
• Mesh supports, as many as we need
• 6 sq mm armoured cable, 50m
• NHL 3.5 Lime, 25kg x 8
• Sharp sand, 2 tonnes
• Stainless steel wall ties, 200
• Expamet 20m x 65mm, 3
• Synthaprufe liquid DPM, 25 l
• Hessian 40m x 1.4m (to protect the lime mortar while it goes off), 2