Laying a Garage Concrete Floor

Laying a concrete garage floor involves digging out any soil, compacting 150mm hardcore, a 50mm layer of sand, a damp proof membrane and a final concrete slab. Adding a reinforcement mesh will add strength. To get it completely level I used timber guide battens and tampered the concrete using a timber straight edge. Building regulations require and 100m step to prevent any fuel spillage running into a habital room in case of fire. Adding a slight slope towards the outside will allow any liquid to flow out. Keeping in mind that the floor level will be under the main damp proof course of the property, in the event of a flood the garage will most likely flood before any other rooms. Adding a maintenance pit with a water pump again in case of a flood can be also added if required however I opted not to have one as I will make when when I rebuild the shed into a garage in the future.

As this concrete floor is for a new extension, air vents were extended from the main house to go under the new garage concrete floor using 100mm pipe, telescopic risers with air bricks on the outside. Before the facing brickwork was started on the extension the hardcore was compacted underneath. This is what i did…


Final layer of hardcore down and ready for final compacting. In some places it was 200mm deep. All air vents were connected to telescopic risers to allow air flow from the outside to under the inside timber floors. Any gaps around the riser were filled with concrete to prevent rodent infestations. You can see the pipes will be just about covered by the 50mm layer of building sand.


To assist in the levelling I marked a black line using the laser level around the walls. Bear in mind that there will be a slop outwards which I will set using the guide battens later.


I kept the line 5mm lower than the bottom of the garage doors. This will allow the doors to open freely if any settling occurs in the future.


I filled in any visable gaps with concrete as I did not want these vents to move out-of-place once the floor is in.


I needed a level edge to work from so I made a concrete edge on the inside of the garage. I placed some heavy lintels on the outside to keep the shuttering wood in place.  I had a spare re-bar so I put that in too. I mixed 1 cement to 4 ballast with liquid waterproofer. Rain splashing on the garage doors will make its way in so I planned to keep the main slab damp proofed with membrane.


The day after I removed the stuttering a gave it a light brush to smooth it out. I noticed the colour was slightly browner than normal. My concrete should be more of a grey. Perhaps I missed counted the buckets and got the mix wrong, or using red sand with a premium cement gave it this colour. I will know if it doesn’t harden fully in a few days. Fingers crossed! [UPDATE: after a month the concrete did reach full strength. It was the colour of the materials!]


I found by wetting the hardcore allowed better compaction with the compactor/wacker plate. I must have done at least 5 runs to make it solid. On top I wacked a thin layer of sand and compacted that before laying the final sand layer to bring it up to 50mm. The sand was slightly wet from being outside which made it difficult to level off. I’ll wait a few days before running the compactor all over.


The damp proof membrane came came only in 4m widths rolls. I could not find any 5m widths anywhere so I had to join them up.

garagefloor-damp-proof-membrane-laying-chimmey Around the chimney breast the DPM would not stretch so it had to put in a cut and join.


I made a start by cutting a 50cm strip from another sheet, using jointing tape I planned to make not a double sealed but in fact a triple seal. Double side extra strong tape in the middle to connect the layers and 70mm tape on the top and 50mm tape underneath. Triple seal! I found if the DPM tape was stretched when pulling of the roll it would shrink the day after so pull it off very slowly of the roll.


Little creases  in the DPM were difficult to seal. I had to make sure the tape was pushed in any voids firmly.


Around the chimney I had to cut the DPM.


I placed a piece of DPM under and one over. Again I sealed the layers with a triple seal.


The next day I placed straight timber battens under peaks of concrete. Using string line 30mm above the guide line at the rear of the garage I levelled the battens. This would give me enough slope to brush away any water if there was a flood inside. The batten across the guides will be dragged across to level the concrete. When it’s almost level and starting to go off, the battens will be removed and filled.


I checked the slope using the laser just to be sure of the fall towards the outside.

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I noticed the DPM was not flat so I had to stretch it into place.


My first attempt at the concrete. It’s was such a busy one hour I completely forgot to take pictures at the start. The concrete arrived on Friday afternoon 1pm. It was the mix on-site type so I only paid for what was needed. They parked on the drive and unloaded 2 wheel barrows.  I asked if they were going to put a plastic sheets down and was not told all they do is concrete.  I did not even have a chance to see what was going on as I had to quickly cut some damp proof membrane to cover the block paving. The first barrow came in and the 2nd chap followed something like 30 seconds after.  It happened so quick I was down on my knees compacting the concrete. I got the rake to move it into place. The first 5 minutes I was shattered.  It was getting dumped so fast I could not keep up so I let them dump it all in. The levelling battens I put down the day before helped however the concrete was just too high so it made it difficult to level a section and drag the leveling timber across. Perhaps it was Friday and the guys wanted to finish on time and get home??


Walking with the wellies just destroyed anything I levelled. At the top end I had remove the battens, filled in the gaps, levelled using a trowel and moved further down.


Without the bull float I don’t think I would be able to get it flat. It made it so easy. Just a few runs flatted the surface. Any dips or holes my wellington boots left I threw a shovel of concrete on top and troweled on top. A further run with the bull float done wonders.


The left hand side had way too much concrete. I did say to the guys stop however he insisted once I removed the battens it will fill up. Later I removed 3-4 barrows and used the concrete to make a quick path on the outside side of the garage. I think this extra cost me a unnessary £50.


You can see how long the bull float is. It has a gear system which allows you to control the angle of the float by twisting the handles clockwise or anti-clockwise.


The day after… Next time I must have the concrete delivered first thing in the morning. This will give me more time to polish it in the evening.  Late that evening the concrete was still too wet to trowel over.


I had to remove the peaks the bull float left the night before by grinding the trowel on top.


With all the peaks removed I brushed the surface and left it to dry for a few days. My next step would be to put down a thin layer of self-levelling compound, not to level the floor but to seal the floor and to provide a better finish.

The cost of the concrete for 2.6 cubic metres came in at £350. Next time I will try some one else.