Concrete slab query......

[merlecollins]

Hi all,

Just joined the forum and am looking forward to a bit of clarity to a few problems I currently have with a house we have just bought

We knew before we bought the house that it had damp issues - it is 200years old and built on the sea front, also the dining room internal floor in the rear of the house is approx 300mm below external ground level.

I have started by completely stripping the plasterboard walls and their timbers in the dining room back to the stonework and lifting the timber floor, which it turns out only covered an original flagstone floor built up from the earth.

The walls are damp in places (mainly where the outside ground level is higher) and the floor is particularly damp.

I anticipate having to lift the flagstones and am thinking of laying a new concrete slab with membrane. Then I was thinking of tanking the walls with the Sovereign Hey'di Tanking System.

My questions are this:

1. What sort of slab build up thicknesses do I need to be looking for?
2. Do I need to install mesh reinforcement?
3. Do I need to install insulation in the floor?
4. What grade DPM is best?
5. Has anyone used the Hey'di system and is it any good?
6. How do I tackle the interface between the floor DPM and the Hey'di tanking?

Hope I have not asked too much for my first post......



Cheers,

MC

 

[r896neo]

First off welcome.

Secondly, and this is just a general warning i'm sure others with specific experience can clarify but you need to bare in mind the floor and wall construction of your house because of it's age it may not have much in the way of foundations and removing a lot of material from the floor can affect the strength of the walls/ foundations. But as i said don't panic just something which you should take some advise on board about.

A standard solid floor make up would be

Sub-base
sand blinding
1200 gauge DPM
100mm insulation
100-150mm concrete, (Seeing your near the seaside and perhaps a sandy sub-soil mesh would probably be a good idea)

Depedning on your heating system you can have the insulation above the concrete and then a screed over the top of that.

I can't offer any specific advise on that tanking product but as with all those products you need to follow the MI's to the letter anyway to make sure your guarantee is valid.

 

[merlecollins]

First off welcome.

Secondly, and this is just a general warning i'm sure others with specific experience can clarify but you need to bare in mind the floor and wall construction of your house because of it's age it may not have much in the way of foundations and removing a lot of material from the floor can affect the strength of the walls/ foundations. But as i said don't panic just something which you should take some advise on board about.

A standard solid floor make up would be

Sub-base
sand blinding
1200 gauge DPM
100mm insulation
100-150mm concrete, (Seeing your near the seaside and perhaps a sandy sub-soil mesh would probably be a good idea)

Depedning on your heating system you can have the insulation above the concrete and then a screed over the top of that.

I can't offer any specific advise on that tanking product but as with all those products you need to follow the MI's to the letter anyway to make sure your guarantee is valid.

Thanks for that

Once i've dug down the relevant 200mm or so, do I need to wack the base before putting in the sand blinding?? Do I need to keep digging down till I hit a solid sub base as with strip foundations?

We are installing a new oil fired central heating system with radiators - not underfloor pipes. Is it possible to run copper pipes within the slab, if so is there a protocol I need to follow?

Cheers,

MC.

 

[merlecollins]

Just been along to the hose this afternoon and lifted a couple of flagstones.

It seems to be like a seaside shale underneath for quite a depth - over 300mm at least

Do I need to dig all of this out??

 

[r896neo]

to be honest it's been under there for a long long time so any settling will have happened already. So dig anything out that you disturb but after that you only really need to take maybe 150mm out but if the ceilings are already pretty low consider the extra 9-10 inches you will be putting on top.

You can run heating, cold and DHW under the concrete and usual practise would be to run them in the insulation layer, if you are doing this i would recommend laying a second dpm on top of the whole lot to keep the concrete from seeping in to the joints.

Your best bet would be to give your local building control a call, if they exist in orkney? It sounds like you are doing pretty major works so a lot of it may be notifiable and he will also be a good source of knowledge of local building methods sub-soil types etc.

 

[merlecollins]

OK.....

....Spent the day lifting all the flags and digging out to a depth of 260mm.



I am thinking of the following two build up options:

1.

100mm sub base (compacted)
30mm sand blinding
Visqueen - 1200 grade
30mm insulation boards
100mm reinforced concrete

2.

100mm sub base (compacted)
30mm sand blinding
Visqueen - 1200 grade
130mm reinforced concrete

Which would you guys say was the best option?

I've no underfloor heating and the rest of the house doesn't have insulation, so not sure if it's necessary....

MC

 

[Deluks]

Insulation is a must have. Use 80mm of foil backed foam like kingspan/celotex etc.
4" concrete will be enough, reinforcement not really necessary, but is a nice touch. If working to a budget use more inso over reinforcement.

So you will need to dig down a lickle bit more.

What will be the final floor finish?


Also I hope you have a plan for the walls, cos it will probably be better tying the floor dpm in with the tanking.

 

[merlecollins]

Insulation is a must have. Use 80mm of foil backed foam like kingspan/celotex etc.
4" concrete will be enough, reinforcement not really necessary, but is a nice touch. If working to a budget use more inso over reinforcement.

So you will need to dig down a lickle bit more.

What will be the final floor finish?


Also I hope you have a plan for the walls, cos it will probably be better tying the floor dpm in with the tanking.

Cheers for that mate

Will 50mm insulation not be enough? I have a price for a product called Quinntherm, which I am reliably informed is exactly the same a Celotex/Kingspan.

If the 50mm is OK i'm laughing, so to speak.

I have had the rep out from Sovereign advising me on the joint between slab and tanking and I need to install a 'Barrier Mortar' fillet between wall and floor before the final coat of tanking.

I will be tiling the finished floor (i'm a tiler to trade)

So do you really think that the reinforcement mesh would be overkill?

 

[Deluks]

The regs are about 70 - 80mm. Quinntherm's ok, yeh pretty much the same stuff. It should only be a few more £20 notes for the extra inso so why skimp? Go to 100mm if you like, will save your heating bills over time and keep yer feet toasty. Peeps think that floor inso is a waste because 'heat rises' But it also gets drawn away by something colder, ie: the ground. An uninsulated slab will suck the heat out of a room.

You will need edge insulation too, pick up a 20 or 25mm board from the suppliers and run it around the edge of your concrete slab. It can be flush with the surface, you can just tile straight over it, plus skirting will help cover it.

Mesh isn't overkill, bit it's not strictly necessary either. I would always use it simply because it's cheap enough round these parts. Under a score for a 3.6 x 2m sheet and may help prevent cracking of the slab. Having said that a mesh is no substitute for good concreting practice and a sheet of poly on top for a few days after laying.

 

[merlecollins]

The regs are about 70 - 80mm. Quinntherm's ok, yeh pretty much the same stuff. It should only be a few more £20 notes for the extra inso so why skimp? Go to 100mm if you like, will save your heating bills over time and keep yer feet toasty. Peeps think that floor inso is a waste because 'heat rises' But it also gets drawn away by something colder, ie: the ground. An uninsulated slab will suck the heat out of a room.

You will need edge insulation too, pick up a 20 or 25mm board from the suppliers and run it around the edge of your concrete slab. It can be flush with the surface, you can just tile straight over it, plus skirting will help cover it.

Mesh isn't overkill, bit it's not strictly necessary either. I would always use it simply because it's cheap enough round these parts. Under a score for a 3.6 x 2m sheet and may help prevent cracking of the slab. Having said that a mesh is no substitute for good concreting practice and a sheet of poly on top for a few days after laying.

OK, so what if I take 20mm out of the sub base(making it 80mm) and 10mm out of the sand blinding (making it 20mm)?

That would give me the required depth to put 80mm Quinntherm in?

 

[Deluks]

I was gonna mention that 20mm blinding will be plenty, shouldn't really skimp on the sub base though. Can you not dig out another 20mm? or have you now got it all level and ready to roll?

you could nick 10mm out of the sub and go down to 90mm concrete if using mesh, You still need a bit more than 260mm though.

 

[merlecollins]

I was gonna mention that 20mm blinding will be plenty, shouldn't really skimp on the sub base though. Can you not dig out another 20mm? or have you now got it all level and ready to roll?

you could nick 10mm out of the sub and go down to 90mm concrete if using mesh, You still need a bit more than 260mm though.

No i'm not completely finished digging just yet, so reckon I could take another skim off the surface

So by the looks of it I would need to go down to 300mm??

Does the sub base not compact down once I've wacked it? I've got a nett floor area of 14.6m2 - what quantity of sub base would you order working on 100mm finished thickness?

Thanks for all your help here mate!

 

[Deluks]

The stated depth should be after it's been whacked. 300mm sounds about right.

Broken brick and slate etc, covered with a couple of bulk bags of Type1 MOT.

 

[merlecollins]

The stated depth should be after it's been whacked. 300mm sounds about right.

Broken brick and slate etc, covered with a couple of bulk bags of Type1 MOT.

I've dug out loads of broken slate, but was just going to replace with solid Type 1 - is this OK?

 

[Deluks]

Broken slate is alright for sub tbh. If you're still at 260mm, you can just chuck a couple of bags of mot down as is. I wouldn't want to blind off over slate as it may still puncture the dpm, you've probably just about stopped digging in the right place.

If you've a whacker plate then this is on the way to being a top notch job. You getting concrete delivered?