Concrete floor construction

[trevorbayliss]

What is the typical construction for a concrete floor.

Would these figures be ok:

75mm hardcore
25mm sand
membrane

100mm insulation

60mm concrete
40mm screed

It's an extension for a new sitting room.

 

[pilsbury]

I'm doing the same make up as you but 100 MOT, 20 sand blinding, DPM, 100 Insulation, 100 concrete, then a thin self leveling compound.

Maybe a bit more MOT sub base? Not sure about your 60 concrete which seems a little thin, but you're making it up with the screed....

 

[trevorbayliss]

I just thought a concrete and screed of 100mm would be ok, just depends on the ratio.

Another point, does the floor have to have concrete in it, or could you just do a screed of sharp sand and cement say at 100mm ?

 

[pilsbury]

Not sure on that - and also not 100% on whether 60 concrete and 40 screed counts.... let someone with more knowledge advise. I added my soon to be floor makeup as I think from my research this is fairly standard, tried and tested. Not that other iterations are not as good.

 

[r896neo]

Building control will never pass 60mm of concrete. A normal make up would have 100mm of hardcore, 20mm of blinding sand, dpm, 100mm of concrete, insulation, then 65mm of screed.

You can omit the screed and put the insulation under the concrete and use the concrete as the floor but it is more difficult to get right If you are matching up to an existing floor and in that case its much better to use a screed.

Anything less than 75mm of concrete is almost guaranteed to crack at some point in the future.

 

[trevorbayliss]

Ok, so i am matching up to a concrete floor. So a screed will be needed i think. Your floor is 385mm thick (inc 100 insulation).

I need to do some more reading because i think i have seen screeds thinner than 65mm.

What about concrete WITH fibres added. Doesn't that mean you can have a thinner concrete screed.

Just working out at the moment how deep to dig and how much waste to get rid of.

 

[vinn]

You work to your FFL - meaning: room must be allowed for in your total depth dimension for say a tiled or wood finished floor that flows into the adjacent room(s).

For the average extension, or solid interior floor, we hardly ever use a screed/SLC on top of the typical 150mm of concrete.

However, you might be best off using SLC - problem is you still have to float it accurately for depth and flatness on top of the correctly cured, primed and levelled concrete.

 

[pilsbury]

Screed is optional - you can have no screed if you like, but to get the concrete bang on will be a real mission. Hence why I am going with concrete as close to the floor level I am matching in as I dare, the a very thin layer of self levelling compound to get spot on.

 

[garyo]

For the average extension, or solid interior floor, we hardly ever use a screed/SLC on top of the typical 150mm of concrete.

Interesting. When do you pour it? At the end after the knock through?

 

[garyo]

Hence why I am going with concrete as close to the floor level I am matching in as I dare

Haha me too. It's not fun is it. I especially don't like having to work on a site where I'm battering my beautifully trowelled FFL concrete every time I move a trestle or stack some blocks!

 

[trevorbayliss]

Ok so maybe a concrete screed is best and then finish with self levelling as Pilsbury mentions.

Do you think a 100mm concrete base will be fine then ? Is 75mm hardcore base too thin ?

 

[pilsbury]

garyo- Not done a concrete floor before but I have a sneaking suspicion I'll be fighting against time to get it reasonably level, then be cursing as I push ripples in it with a crap job bull floating! SLC to the rescue once all the work has been done on the rest.

 

[pilsbury]

Ok so maybe a concrete screed is best and then finish with self levelling as Pilsbury mentions.

Do you think a 100mm concrete base will be fine then ? Is 75mm hardcore base too thin ?

I am no expert, just read up on what I need to do for my extension. I would guess poured concrete is going to give you the best solid floor. 100mm will be ok too - what I'm doing. As for the 75mm of MOT, depends how firm your ground is. I'm going to wacker the sh1t out of 100mm dumped in in several thinner layers to make sure its solid.

 

[chappers]

Ok, so i am matching up to a concrete floor. So a screed will be needed i think. Your floor is 385mm thick (inc 100 insulation).

I need to do some more reading because i think i have seen screeds thinner than 65mm.

What about concrete WITH fibres added. Doesn't that mean you can have a thinner concrete screed.

Just working out at the moment how deep to dig and how much waste to get rid of.

For a bonded screed you can reduce to about 45mm
a normal make up would be;
10-150mm compacted hardcore
20-25 sand blinding
DPM
100mm insulation
100-150mm concrete
50mm screed

an unbonded screed over insulation should be around 75mm

 

[John D v2.0]

a normal make up would be;
10-150mm compacted hardcore

I guess that's a typo for 100-150mm as 10mm wouldn't do anything