How to install a new slab floor on top of an old slab floor – Laundry renovation: Part 4

The farm house is a hodge-podge mix of floor heights that will one day result in a broken leg for some unsuspecting visitor of this I am sure.

  • Green rooms are the highest and are level with the floor of the original structure.
  • Yellow rooms are about two inches lower than the green level.
  • Orange rooms are another two inches lower than yellow.
  • The red room is more than a foot lower than the yellow level.

Ultimately I want one single level throughout the house. I also learnt from the walk-in-robe project that a bad floor will make the build harder, cost more and take longer, and the existing floor in the laundry is about as bad as it gets.

For example when the tile is glued directly to dirt.

Actually the above picture is just before the laundry drops down to the toilet level, where I am building the new laundry there is at least a reasonable amount of concrete under the tiles, but it is still much too low and uneven, to counter this i am going to pour a new slab on top of the old slab.

Setting the slab height.
First I removed all remaining tiles and cleaned and removed dust as much as possible. so I now have an empty space to work with. I then had to mark the height I wanted the new floor to be at and I wanted it to be level with the kitchen. To do this I used the longest level I had and sat one end on the kitchen floor, and then worked around the room, drawing height markers on the wall in the dining room until I reached the laundry, then I drew solid lines at the height all the way around the space for the new laundry room.

Preparing for the slab. 
At the current time the laundry room only has three walls as the final wall will not be constructed until after the new slab is in. I did however dynabolt a 90mmx45mm timber base plate in where the wall was due to go so that the top of the timber is now level with the top of where I want the slab to be. I used some thin bits of masonite board to prop up the timber frame in places so that it is level. Once the dynabolts were in it was all rock solid and I now had “boxing” for the slab pour.

I wish I took more photos of this next step, but as I will be sure to be repeating this process in a future project I will document and link back here. Concrete slabs need to be reinforced with steel reinforcing mesh (rebar). I cut a sheet of reinforcing mesh roughly to the size of the slab leaving a gap of about 40mm on the sides. I needed to make sure that the two slabs would not be able to move independently of each other and so I needed to affix the mesh to the existing slab. I also needed the mesh to be sitting at about the centre of the new slab I was going to pour, meaning I needed to suspend the mesh somehow

Using a hammer drill I drilled 24 16mm wide holes about two inches into the slab at regular locations that would touch the mesh. I then got some 12mm wide lengths of rebar and hammered them into each hole. Before i did that however I blew the dust out of each hole using compressed air, and squirted some Chemset (a serious glue) down each hole and then followed with the rebar.

I then just used some scrap timber to raise the reo-mesh to the height I wanted and then welded the mesh to the rebar lengths. Once the welding was complete and I was able to jump up and down on the mesh without it moving, I just cut the extra length of the 12mm rebar verticals with an angle grinder.


Photo from the wedding venue build where I have done the same thing.

Mixing the concrete
As the new slab is not going to be very big I was going to be able to do the floor in one pour and just use a standard cement mixer. It is important that all the required cement is poured within a limited time frame so that it all goes off at the same time.

Mixing cement is very easy and for smaller jobs I usually just mix it up in a wheelbarrow, although in this case I am using a cement mixer. 
I just bough standard builders cement which I think cost about $6 for a 20kg bag, I also had a trailer load of conmix, which is a sand and stone aggregate that you mix with the concrete. I’ll do a post on what exactly is involved in the future, however I usually mix to a ration of 6:1 square mouth shovels of conmix to cement and add enough water to make sure everything is wet enough, but not too runny.

Pouring and leveling the slab
When I was ready to pour I just wheeled it through the house in a wheelbarrow (sorry El!) and dumped it on top of the mesh starting at the far back corner. I tried not to splash too much so that I wouldn’t get it on the walls.

Probably a little bit dry actually…

I also needed to screed the concrete, which is essentially just leveling and smoothing it out. This is done with a flat edge and you move it backwards and forwards and the concrete will fill in any gaps and level in the process.

Normally when pouring a slab the boxing is flat so you can screed off the boxing, however in this case I am using three walls as boxing so I needed to get a little creative.

Before I started pouring cement I affixed another length of timber on the back wall so that the bottom of the timber was at the line for the top of the slab. I then nailed two other bits of timber together as pictured and used that as a screed so I can level from the front and back walls evenly. Once the slab had dried I just unscrewed the extra timber from the wall and the result was a smooth, even slab.

Now the slab pour is complete. I hope you learned something. Share your own tips and tricks in the comments.

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