Lime Floors

Our approach

All things considered (cost, availability of

materials etc) we decided to go for:

15cm hérisson of 20-40 stone wrapped in geotextile
10cm lime slab using “melange” (mix of sand and stone) from our local brico and NHL5 lime mixed at a ratio of 5:12 i.e. 5 buckets of NHL5 to 12 buckets of melange. If using separate sand and stone it would have been 5:4:8. We added fibre for added strength at 1kg per m3 of limecrete.
Underfloor heating (UFH) pipes attached to the slab.
7cm screed at a ratio of 1:3 NHL5 and sand with fibre laid over the UFH.

The process

1. Measuring and marking reference points

Before removing the old floors we needed to create a reference for the new finished floor height. We made marks on the walls 1m up from the existing floor at various points around each room and then used a laser level across these points to check how level the floor was. It wasn’t. Then we worked out the ideal finished floor height and made a new mark 1m up from that around each room

2. Floor removal and excavation

We started by removing the existing tiles and screed and digging out the floors to a depth of around 35cm (our total planned floor depth). Even once we were through the tiles and screed/slab, the soil was extremely hard and required the use of a chisel on a large hammer drill. A proper jackhammer probably would have made the job easier, but we were a little concerned about excessive vibration. Given the lack of foundations in these old houses, particularly on the interior walls, which appear to be built directly onto the soil at floor-level, we decided to dig down just 10cm against the interior walls and then have a 45 degree angle to the new base level. To measure the depth as we progressed and ensure the base was flat and level, we used a laser level aligned to our 1m reference marks on the wall and a stick with a mark at 1.35m. All in all, we removed roughly 30-40 tonnes of soil, which was a lot of fun.

3. Hedgehog/Hérisson

We put down a layer of geotextile using a 2m wide role allowing for a 20 cm overlap and enough length to wrap it up over the top. We then put down our 15cm of 20/40 washed stone and used a wacker plate to compact it. I’m not sure this was strictly necessary as it didn’t compact much. Then another layer of geotextile over the top.

4. Slab

A lime slab is a lot more work than concrete! The mix needs to be quite dry, you need to be able to ball it up and break it in half, a bit like a homemade burger patty. And you need to mix each batch for at least 15 minutes, so having two mixers is a big help.

Also, don’t forget a mask!

Limecrete doesn’t need expansion joints and you definitely shouldn’t use steel rebar or mesh, but we did include a 30mm cork border around the edge of the room.

We experimented with two different methods for laying the slab. The first involved setting up the laser level to a height where the beam would just skim the top of the slab and then doing it largely by eye and using a long level as a straight edge. This worked ok, but you always have the laser level behind you so that your body is blocking the beam making it difficult. A little way into the job, we discovered a better method on a forum, which involves creating a series of flat-topped mounds about 30cm across at specific points around the room using the laser level to ensure they are all at the same hight, flat and level. We then built “bridges” between the mounds to create a series of level squares roughly 1-1.5m wide around the room. Once these had set enough, we simply had to fill in the squares using the bridges as a reference point to get everything level. The mix has a pretty quick initial set. Within an hour of putting it down it is very difficult to compress or move, which can make blending sections together a bit tricky, but because we were adding a screed on top we weren’t too concerned about minor variations or getting everything smooth.

Despite the quick initial set time, you should allow about 2 to 4 weeks for the slab to fully cure before putting anything on top of it. Misting the surface occasionally during the first 48 hours helps minimise cracking

5. Underfloor heating

We ran Multicouche pipe at 20cm centres around the rooms, taking care to keep each circuit under 100m and ideally as uniform in length as possible. We used rail clips attached directly to the slab with plastic dowels. Using Multicouche pipe rather than PER is a little bit more expensive, but means that we can in theory cool the floor in summer.

6. Screed

The process for the screed is similar to the slab. We used NHL5 and 0/4 sand at a 3:1 ratio to create a 7cm thick screed. Mixing is the same as for the slab, with quite a dry mix for 15 minutes. We started by using metal rails and the laser level to create a frame 20cm from the walls around the perimeter of the room and then filling the gap between the rail and the wall to create a narrow screed border at the correct height. We also set rails at the same height at 1.5m centres across each room to give ourselves something to work off. We then removed the outer rails and filled in the gaps, removing the metal rails as soon as the screed had hardened sufficiently to allow us to move onto the next section. Once the entire area was done, it’s simply a case of waiting 28 days before starting to lay tiles.

Calculating material quantities

Once you know the area and the depth of your floor, the calculation of quantities is quite simple although made a bit trickier depending on if you’re buying by weight or volume. The example below shows our calculations for one 25m2 room. You can download the calculators in Excel here.

Hopefully that's of some use. If you have any questions, please get in touch.

Please note, all of the information above is based on purely on our experience of doing our own floors and many hours of reading. If in any doubt about the right route for you please contact a qualified tradesperson.

We’re currently in the process of redoing some areas of the floor in our house with a limecrete slab. And having spent many, many hours researching and wading through frequently contradictory articles and posts, I thought it might be helpful to share our findings and experience so far. I’ve also included a simple calculator to work out volume of materials required.*

Firstly a bit of background on our situation:

Our house is a typical old Gers farm house. Built around 200 years ago with thick stone and mudbrick walls and repaired and renovated over the years with varying degrees of care and competence. The land around the house slopes slightly so that one end is marginally below ground level and the other is around half a meter above ground level.

Our floors were a mixed bag. Some old terracotta tiles (tomettes) laid direct onto soil, some newer terracotta tile laid onto a thin screed and some cement tiles laid onto a thin concrete slab.

Our reasons for redoing the floors were:

a) an aesthetic choice

b) to deal with some apparent damp issues, and

c) to install underfloor heating

The total area we were tackling was around 55m2 across three rooms with an additional 35m2 in an extension where we were only doing a screed on top of a newly laid solid concrete slab.

Are lime floors necessary?

The first decision we faced was concrete or “limecrete” (I’m using the term limecrete to refer to lime-based concrete i.e. any sand/aggregate/lime mix rather than a specific branded product).

It’s generally understood that old, solid-wall houses require the use of breathable materials. That means no use of cement anywhere. However, I did come across some compelling arguments why this is not always applicable to floors. Without going into too much detail, the conventional wisdom is that a concrete floor (especially with a damp-proof membrane will simply force moisture outwards and into the walls. However if your floor level is well above the outside ground level and you have a good insulation/drainage layer, and your walls are breathable, then concrete floors could be ok. I also found examples where people used a hybrid system where the outer perimeter of the floor slab (20-30cm) was limecrete and the centre is concrete. The logic behind this being that concrete is quicker (to lay and set), cheaper and easier (especially if you can get premix in a lorry). We chose not to take this route and instead opted for a full limecrete option.

Structure of a lime based floor

Our next decision was what materials to use and the composition and thickness of the different layers. The different approaches a I came across were either:

1. A single layer slab/screed. Typically a slab is structural and uses coarser aggregate (sand and stone) while a screed is non-structural and uses finer aggregates (sand) but when you add alternative materials like clayballs into the mix (pardon the pun), the terminology becomes a little interchangeable. Depending on its composition and what’s beneath it, a slab should be between 10cm and 15cm thick

2. A separate slab and screed. Again, depending on composition, the slab should be around 10cm and the screed 5cm to 8cm thick. A screed allows a smoother finish on which to lay tiles and is also useful if you are laying underfloor heating.

Either of the above options can be used with or without an insulation/drainage layer below. This is what’s referred to in France as a hérisson (hedgehog). The importance, material and thickness of this depends on your situation. If your floors are below the outside ground level it’s definitely advisable. You can simply use washed stone - typically 20/40 - or coated clayballs (billes d’argile) or expanded foam glass (e.g. Geocell) both of which offer better insulation than stone, are lighter (and therefore less labour intensive) but cost a lot more. If you are installing a hedgehog, it should be wrapped in geotextile (the felt type not the stuff you use in flowerbeds) to prevent it silting up. Depth should be 10cm to 30cm.