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Laying a concrete workshop floor

It’s been a while since the last post with the little slideshow of our mad day-and-night floor laying mission, and since then we have had a rest in Berlin for some wedding related festivities, fixed some of the leaks in the dutch barn’s old roof, and helped with the official open day for the Golden Hill Centre. I should make clear, the open day excluded the FE workshop, which is far from finished, but we were in attendance with a display stand, and we gave a short ‘talk’ about Flowering Elbow – our plans, hopes and dreams, that type of thing – and got some encouraging feedback and interesting questions.

Anyway, we did promise some more details on the workshop progress and floor specification, and now it has seen some circle dancing and capoeira related action we best get on with it… The order of things and the way we ‘planned’ it:

1. Dig out organic matter and mud from the shed floor and roughly level. Clear the rubbish!

2. Research, research, research. Consider. Plan the floor from the bottom up thinking about: environmental considerations, insulation, price, practicality, time frame, durability, etc.

3. Submit written specification with diagrams to the contractors who worked on the other Golden Hill buildings.

4. Listen to builders scoff and propose a crazy high price, and decide to do it ourselves.

5. Order materials and begin!

The FE shop floor basics (bottom to top):

1. 32 tons of DTp type 1 stone for 150mm hardcore layer. DTp type 1 is a grade of stone used by the Department of transport (often still reffered to as ‘MOT1′ or ’40mm to dust’). It is basically a good mix of stones of various sizes, which key together well when compacted, to make a solid layer.

2. 3 tons of sand for 20-30mm sand blinding layer – this protects the damp proof layer.

3. 100% recycled 300mu plastic Damp Proof Membrain (DPM).

4. 100mm PIR insulation.

5. Covering of plastic (mostly made up from scraps found about the farm and discarded by builders during other building work). This is just there to prevent the concrete from reacting with the insulation.

6. A252 Steel reinforcement mesh, supported 50mm up from insulation by padstones, at intervals approximately 600mm apart to prevent the mesh sagging.

7. C40 concrete with polypropylene fibres. Including 50% ground granulated blast furnace slag (GGBS or GGBFS – a waste by-product from the iron and steel industry).


Where the slab joins the existing wall, we were going to have bitumen impregnated fibreboard as expansion joints, but instead we ended up just having it butt up against 100mm of PIR insulation, which ‘can’ theoretically compress if the force on it is great enough. As the slab in not ‘enclosed’ on all sides (ie there is only one block wall – the straw bale infill walls will go on top of the slab), we figured this would be sufficient.


Environmental considerations of the floor

Like everything else in the workshop build, the concrete floor is something of a compromise.  Lets face it, despite the big-corporate green-wash you get from well established concrete companies, cement is a bit of an environmental no-no. Environmental designers and architects go to great lengths to avoid using the material, specifying lime and rammed earth systems instead.  Problem is, when it comes to a floor that is going to take heavy vibrating machines and occasional vehicular traffic, concrete is unparalleled in its durability and performance. In many situations concrete is an unnecessary overkill, and a lower ’embodied energy’ option would excel – unfortunately for this application, it is about the only thing up to the job. At least we could get stuff with the inclusion of 50% GGBFS.

In order to reduce the energy needed to keep the place warm, or at least not totally freezing, we’re insulating from top to toe, so we put in 100mm of PIR insulation. Again, we had to weigh the long term benefits of PIR (the stuff often referred to as ‘celotex’) insulation against its embodied energy. We’d rather not rely on so heavily on the stuff, but its compatibility with the concrete floor system, ability to take the floor loading, availability in what turned out to be a short time-frame, won over on this occasion. The straw bale walls we are planning will make an interesting juxtaposition ;)

Having done the rounds with local builders merchants when trying to get hold of the recycled DPM, we had pretty much given up (most getting very confused the ‘odd’ request and answering in the negitive by default) and ordered their ‘standard’ 300mu one along with some other supplies so at least we could save on delivery trips. However, when it turned up it was 100% recycled anyway – just the builders merchant didn’t know what they were selling!

Preparing the Sub-base

General trash and “years of accumulated stuff barrowing removal service” under way. At this stage we do some leveling as well and try and dig away any obvious high points. This particular wheelbarrow is a magic one that fills and empties itself – it just needs very clear instructions or it gets carried away.

Two full 16ton loads of MOT1 have arrived! Time to appreciate the finer points of shovelling technique and equipment design. To Bongo’s dad’s apparently eternal annoyance, this was a digger free site, which added a few days to the process, but was a nice exercise and saved £150 or so on mini-digger rental, fuel and transport.

Spreading and raking the stone was done in layers of about 50mm. We used a very large piece of straight timber (discarded by the builders), and a long spirit level to keep the sub-base as flat and level as possible.


Each layer is compacted with a plate compactor/vibrator, before adding the next. With a total  of three layers (giving a depth of 100mm to 150mm), this leaves the surface very well compacted and solid. We hired the plate compactor from a local tool hire place (MPH), for a little over £20 for a week, which seemed ok. Unlike many other people in the building trade, these guys were actually good’n helpful, and didn’t try their best to rip us off – which was a nice surprise.


Here we are (below) with the floor’s stone sub-base almost complete – the magic barrow is getting over excited again!


The sand arrives – three bags, what people call three tons. Each ‘one ton’ bag actually contains about 750kg, but are named ‘one ton’ so that delivery vehicles and the like will not go over their rated maximum load carrying capacity (or that is the common excuse anyway, not that I am at all sceptical).


With the sand arrival we enjoy some more shovelling and  raking antics…  This sand blinding layer will protect the damp proof membrain from the stones below, and give us the last chance to achieve absolute flatness.


As you can see, at this stage we have put up the lower layer of vertical insulation, ready, so that the slab with be thermally isolated from the outside walls. Notice also the real and ultimate flatness of this sand layer, which is not at all looking like a busy beach area.



After yet another levelling and raking exercise, the compacter is back to work. It is hard to avoid little ridges at the edges, especially when you turn the bends, but generally the floor is ready for a DPM and some insulation. We have also started constructing the shuttering…


Ok, so here is where the timer really started ticking, as Sam and I both had to leave for Berlin in a few days and the concrete readymix was all arranged. As a consequence there are no photos of the DPM that went on top of the sand blinding layer, or the 100mm insluating sheets that were on top of that. The next photo was from the day of the concrete pour. The morning was spent laying the last of the insulation, scavenging together enough scraps of plastic to go on top of that (so the concrete wouldn’t degrade the insulation), and cutting and arranging the A252 steel reinforcement.



It looks quite like a paddling pool at this point (aside from the sharp steel bits of course). The mesh is held up with odd bits and pieces of old tiles we could scavenge from around the farm. The trick was to support it enough that we wouldn’t dislodge it completely when we were stomping all over it barrowing the concrete.


The Concrete Readymix Pour

After wire tying the sheets of mesh together, apart from some gaps for the crack control joints, and enlarging the ‘barrow highway’ we are just about set for the readymix to arrive. And it does. And the barrowing begins!  Dave, Alex and me, become the barrowing force, while my dad uses the petrol powered ‘concrete poker’ to get the air out of the mix and keep it from curing prematurely. We also have Steve helping to scrape out the cement from the upturned wheelbarrows and spread and even out the mix with a shovel.


After many wheelbarrow loads, in not much time at all, we are getting towards the end of the first readymix tanker’s 16 cubic meter load.

After the last barrow is filled and pored, we waste no time in taking up the long tamp we prepared earlier. It is basically the long straight timber, used previously for levelling the sub-base, with some handles screwed on, and two spirit levels attached either side. With this Dave and I, move slowly along levelling the wet concrete. We work the tamp across the the carefully levelled shuttering, ensuring it stays level with the spirit levels. Alex is on hand to help rake away excess.


Despite the driver telling us we would get a good while to ‘sort things out’ before the next load arrives, it appeared, chomping at the bit, before we had even finished tamping the first bay. Once again ‘barrow frenzy’ commenced. This time we were being loaded up at the front of the shed and barrowing to the back. When we got about 2/3 the way across the shed the remainder was ‘chuted’ in.

Sam arrived bringing cake, just as we were emerging from the barrow frenzy – and in time to help with the raking and tamping. At this stage we all breath a sigh of relief, as the petrol powered concrete poker/vibrator is turned off, and the ear ringing slowly subsides – that thing did a good job, but was loud enough to make communication difficult.


Flattening these two bays took a bit longer – seems we filled them a bit more and had more ‘shovel pushing’ and raking to do, to get the right level with the tamp. You can see, if you mouse over the image, where the excess is going – to form a little extra bit of floor at the front of the barn.


And then time for a quick break while it hardens up a bit… There are a few ‘tamp marks’, but we’re not too worried at this point because the powerfloat should take care of these.



And then, just as it is starting to get dark, it starts to rain. Heavily enough that the old curved section of the Dutch-barn reveals its holy patches, and we have to arrange some concrete protection from drips.


The Powerfloat

A little bit later and the floor is firming up, the sun is well set, and we begin the powerfloating! Judging when to start the process is tricky – too late and the machine will not be able to fill the dips and trim off the high points because the concrete will be too hard. Too early and the machine will tear up the surface and gouge big inconsistencies into it. For this reason people say to stay with slab while it cures, so you can test it periodically, and start at just the right time.

Looking back we should have waited just a touch longer, before the first powerfloat. We went off what the powerfloat manufacturers recommend: when your heels only sink in about 3mm when standing on the concrete it is time to begin (we didn’t factor in that some of the first concrete to go into the floor had been curing longer). The instructions say this ‘first floating’ will leave ‘circular marks’, which are removed later in the ‘finishing float’  – but I think our marks were a bit too pronounced in areas.

Controlling the beast took a bit of getting used to, but once you figure out you push up or down to go side to side, it wasn’t so bad. After the first pass everyone apart from Dave and I had hit the sack. We were pretty tired too, and had to rely on a periodic alarm to ensure we didn’t sleep and miss the small window of opportunity for the second, finishing pass with the powerfloat. The sun is just coming up when we get going, taking it in turns to do the final finish powerfloat.

It came up quite nice in the end, though I know if we were to do it again we could produce a better finish. I think I would probably use a long handled bull-nose float for the first float (instead of the first pass with the powerfloat), because the timing doesn’t have to be so perfect… Then you can finish with the powerfloat, for that super hard & smooth surface.


Grinding the Crack Control Joints

Sometime later we test out the big angle grinder with a diamond tipped blade, on an inconspicuous patch, to see if it is hard enough. It was, so all systems are go for making the crack control joints.

Desperately trying not to mar the nice powerfloat finish we move on big bits of insulation to spread the load, and use a long plank as a straight edge to grind along.

As with the power-floating, I think we went a touch too early with the grinding. One or two stones were pulled up, not a big problem, but not perfection… The joints will be filled with appropriate sealant later.


As the rest of the crew were beginning to wake up, I took the tools back to the hire shop in the van, and the others continued to cover the concrete with plastic, to prevent it drying out. Before the plastic went down we gave the floor a misting over with the garden hose.

As we were going away for a few weeks, we needed to make sure that the slab would stay safe, and covered up, so we recruited all kinds of bits and bobs to weight it down, especially along where the plastic overlaps.


So yeah, we have a reasonably nice, insulated workshop floor. Plenty left to do on this, but a bit of a milestone is achieved! Here the plastic is off and we are getting our first look at it after three weeks of curing.

The crack control joints are raked out and hovered, and then sealed up with special (non-silicone) sealant. We used a medium modulus sealant – which basically means it flexes a bit so shouldn’t crack, but not so much that has no compressive strength. It needs to be a bit strong to help support the edges of the concrete, which would otherwise chip off with heavy wheeled things driving over them.


Now we just need to choose the final finishing sealer/paint – any recommendations/opinions?



Responses to Making an Insulated Workshop Floor

  1. Jorge Santiago

    Hey guys,

    Your workshop is looking great! Congratulations!

    I plan to build mine soon so these posts have been really helpful for planning.

    I have one question about the slab. I understand this is a floating slab since the footings and walls were poured first. I was planning to build a monolithic slab, then lay one or to courses of concrete block around the perimeter.

    I don’t see any clear advantage for me to build the floating slab vs the monolithic one. Well maybe one, the floating slab pour can be more easily protected from the elements and have a more uniform curing due to the roof built first.

    Do you see any other advantages of one method over the other? Which one is more prone to cracking due to differential settling? The soil under my future slab has been manually compacted so this is a concern for me.


  2. Bongo

    Hay Jorge,
    Thanks for the comment. It is hard to say, without knowing the particulars of your situation. How big will it be?
    Will the floor be insulated? If so, how are you planning to detail the insulation round the perimeter?

    What exactly do you mean by ‘manually compacted’? For the sub-floor, I would personally want a decent amount of stone and a plate compacter, as a minimum, unless you are down to bedrock.

    In terms of which one is best, monolithic or floating floor, answer is, I don’t know. As far as I can tell, they both have advantages depending on the environmental conditions etc. Our research was focused round our particular application I am afraid – we didn’t have to make that decision…

  3. Serene

    How about some funky patterns? , or but in some warm terracotta colour?

  4. Bongo

    OOO OOO. Loving that last link Serene – with the ‘greenery’ round the edges. I think if we do do something like that, it will have to be later, when we can spare the time to make stencils and all that jazz. Also, it would be a shame to put all that effort in and find that the patterns are covered up by heavy machines anyway…
    So yeah, a simple non slip, super heavy duty, abrasion resistant single colour is what we are going for. Not quite sure how it will turn out yet – it is called Alpine – quite light in colour (which we need to keep artificial lighting use low). Looks nice on the computer screen, bit nervous about seeing it on the floor… We will have to see how it goes.

  5. PaulF

    Hey it is looking great. My vote is square vinyl tiles on the floor. Maybe asphalt tiles. Something that reduces fatigue a little. If they get scarred up and you really care you can even pop them out and replace them. But come on its a workshop! They can take a pretty bad beating and still look OK.

  6. Jack

    I was hopeing you would put PEX tubing in the concrete, but that part is to late. The top of the PEX needs to be 1″ (approx 3cm) below the surface if someone else is getting into it.

    The pex tubing can be used for helping to heat/cool the slab but doesn’t have to be used at all. It just adds more options for the future.

    In areas where you are walking (not moving equipment), local farm supply places have 2 to 5mm thick recycled rubber matts. They are often used in livestock trailers or even in stalls. The matts really help on standing areas.

    Optionally I have heard of using just plywood over concrete to help with the standing/waling areas.

    Both of these options help when I (you are more coordinated that I am, I’m sure :) drop a tool accidently. At least they are less likely to break or bend badly.

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