Soil Practically At Floor Level!

[Tozzy]

Hi guys,

Starting some work on my downstairs loo. The floor is, from what I can make out, concrete... very dark concrete and is roughly 1 inch thick. Directly underneath is soil . I'm thinking of breaking the old floor up and relaying it. The key question is, do I lay it directly over the bare soil? Should I add a layer of ballast before laying the new concrete?

My plan:
Break up old concrete.
Lay ballast over soil (only if absolutely necessary).
Lay sand and cement as advised in book (without agregate).
Paint on a 'damp proofing sealer'.
Lay screed with a depth of 1/4 inch and bring to level of skirting.

I would have thought that agregate would have been essential to the mix though .

Thoughts regarding my plan are very much appreciated.

Thank you.

 

[Softus]

Where is the wall DPC in all of this?

Basically, you're heading for damp problem unless you do the job properly.

You need to dig down at least 4", lay at least 2" of concrete (i.e. just not sand & cement) base, and lay a damp proof membrane over the top of that, joining it up with the wall DPC. Then 2" screed on top.

When you put the WC pan back, bear in mind that the screws mustn't go more than 2" into the floor or else they'll puncture the DPM.

 

[snozzer]

Well its up to you, but I would recommend taking the floor up and re concreting.


Take out 6" of the old concrete/earth below your damp proof course (DPC) height.

Put down a thin 10" layer of sharp sand directly on the soil.
The get some damp proof membrane (DPM) and lay it on the sand and lap it up at the sides past the height of the DPC
Mix some concrete and fill in the hole, leaving it 2" from the finished floor level, tamp it with a piece of timber to fill and settle the voids and get it level, or if your tiling on top you could bring it all the way to level and then use a steel float to polish it smooth, then wait an house an polish again.

Jobs good, floors solid, no damp to worry about.

The book gave you some serious errors

1. You need aggragate in the mix or it will not be strong enough
2. Painting on the DPM, whilst not totally wrong, is not entirely right, the best method is a sheet of DPM.
3. 1/4" screed, erm no 2" is the normal, 1/4" is just 6mm, it would crumble within weeks, it needs more depth to form a homogenous block of screed, unless you mix it with grano dust (granite dust), however if your tiling, skip the screed and lay tiles on ploished concrete is fine, BUT wait 6-8 weeks to let the concrete dry out a bit (or use a dehumdifier for 48hrs)

 

[Tozzy]

Thanks very much for your replies guys.

This is a diagram of the room showing that the DPC level is too high to join my new DPC . However, I could just make sure it overlaps the wall about 2 inches and then the screed (being 2 inches) will submerge it. Please let me know if this is not a good idea. It's all I can think of really lol.

Oh and there is my final plan of how the floor will be constructed .

Well, I'm going to get started on lowering the floor. I don't know if I'll be laying DPC today though because I will have to wait for concrete to dry out. I don't even have any DPC membranes so I need to go to homebase to buy some.

Put down a thin 10" layer of sharp sand directly on the soil.

LOL I didn't know the 10" was a typo. I was about go digging down 10 inches!! hehe

The book gave you some serious errors

The book I'm reading I believe is dated back to the 70s-80s so, there will be some newer methods/ materials. I think the book may even cover fitting asbestos slates in there somewhere lol.

Thanks again for replies!

 

[snozzer]

Sorry 10mm sharp sand not 10inches what was I thinking.

As for the internal finished floor level (FFL) being lower then ground level (GL) outside, you have a problem.

Damp from outside will be able to penetrate through the walls and under the existing DPC. The DPC is always meant to be higher than the external ground level and lower than the internal FFL.

So, what you need to do is tank the wall internally from the new floor to above the existing DPC.

Knowing this I would scrap the plastic DPM and use synthaprufe or similar as follows

lay the sand, pour the concrete and let it cure.

Then paint the synthaprufe on the new concrete and up the wall to about 6" above the existing DPC having taken the exisiting plaster off to give a 6" clearance above the exisiting DPC. At the bottom where the new concrete meets the wall ensure you get a really good filling of syntha, you need to form a little triangle fillet in that corner. Let the syntha dry, then do 1 or 2 more coats, some people sprinkle sharp sand on the dry syntha before adding the next coat, I do if I am having plaster put straight on top of it (walls only of course). Let the syntha dry, then put your screed down, the plaster the wall and fit skirtings.

This process is known as tanking.

 

[JohnD]

As I understand it, Snozzer's suggestion gives you a "tray" of polythene DPM under all the concrete, and between the concrete and the wall, this will stop damp transmission through the slab. But you can expect the side wall to be damp all the way up to the dpc in the wall, and you do not want your screed touching this damp wall.

You might think about brushing on Synthaprufe or some other rubbery black stuff onto the wall all the way up to the dpc, you can hide this with some kind of skirting.

Edited: While I was thinking and typing, Snozzer had already covered it.

 

[masona]

Tozzy,

Can you not carry on by extending the suspended floor by putting in new sleeper wall for the floor joists then add brick air-vent to the outer wall under the dpc level?

Or is there's a height problem?

 

[Tozzy]

snozzer,

Am I glad I checked back here before starting the work!

I went to homebase today and bought everything I needed, concrete, sharp sand, DPC polythene. From what you've pointed out, I now understand the issue regarding the DPC levels. So, I need to use synthaprufe liquid based DPC membrane because if I use polythene and take it up to the GL DPC, I will run into problems when I come to plaster the wall? Therefore, liquid based will provide more of a good key?

Ok, I'll dig old concrete out, lower the floor level by digging out soil, lay sand and the concrete. Whilst, I wait for this to dry, I will go to homebase and exchange the polythene DPC for synthaprufe. Then, I'll coat the wall above and beyond the existing GL DPC. Give more coats when dry and sprinkle on the sharp sand /where when needed (thanks, good tip by the way ). Lay screed as normal.

Question: Is it possible to lay screed before the last coat of synthaprufe dries or do I have to wait until it is dry?

taken the exisiting plaster off

I'm glad the wall was never plastered by the council hehe. No plaster to take off.

I'm absolutely appalled at the state of the room knowing that the council never used DPC on the floor! A total bodge! Ah well, that's the council I guess . Maybe they used liquid based though so I can't tell for sure and the walls have been painted white since then.

JohnD:

You might think about brushing on Synthaprufe or some other rubbery black stuff onto the wall all the way up to the dpc, you can hide this with some kind of skirting.

Thanks, I'll hide this with plaster .

masona:

Or is there's a height problem?

Height is a major problem if I did this . There would only be about 5 foot in height. This is due to the wall sloping down . Thanks for the suggestion though.

Well, I'm going to get started. Not done a thing yet. I really appreciate all of your help. I want to do this as best I can and properly. I don't see the point of doing anything unless it's done properly because it may cause problems later on.

Cheers!

JohnD:

As you've already bought the polythene, you could still use it under the concrete slab, and folded up along the walls into a tray (trim it off after screed finished). It wll give further protection in case anyone drills through the screed too far as mentioned.

Great idea. Will do. Thanks.

 

[JohnD]

As you've already bought the polythene, you could still use it under the concrete slab, and folded up along the walls into a tray (trim it off after screed finished). It wll give further protection in case anyone drills through the screed too far as mentioned.

 

[masona]

Height is a major problem if I did this . There would only be about 5 foot in height. This is due to the wall sloping down . Thanks for the suggestion though.

Okay, how high is the suspended floor? Could that not be lowered then you have a one floor level?

 

[Tozzy]

Height is a major problem if I did this . There would only be about 5 foot in height. This is due to the wall sloping down . Thanks for the suggestion though.

Okay, how high is the suspended floor? Could that not be lowered then you have a one floor level?

I hope/presume you are joking masona .

Btw, just to update, half the concrete floor is up. I've not been working as much as I should have done today so I hope to work through til about 10pm to make up for lost time.

Update: All of the concrete is up. All of the soil is revealed. Finished this around 18:30. Took an hour that's all .

 

[Tozzy]

Hey sorry to dig this thread back up, but I just needed a little advice. I have nearly got the concrete base done. It's taken a long time as I'm working alone and mixing concrete by hand. Yep, a nightmare lol !

Anyway, I was a bit confused about the difference between screed and self levelling compound. I thought screed was self levelling compound! I was going to use a self levelling screed of 2 inches, but have discovered that this is wrong. To use self levelling compound, you can only use on a floor that slopes 1/4 or something. This is what the book advised, but I confused you guys by referring to it as screed.

So, can I use a mortar screed (4:1 sand and cement) of 2 inches over my concrete base and use self levelling compound (1/4 inch) on top of this when dry incase there are any undulations? I'm a total beginner when it comes to laying screed and to be honest, the concrete base was hard (difficult) enough (even with levelling pegs!!).

Is the following method possible to apply when laying mortar screed?:

You see the battens fixed to the wall at final floor level? I could use this to help give a smooth and straight finish and no need to remove them afterwards because they will just be sunk into the screed. I'm laying tiles over it anyway . Do you think it's a good idea or?

Will be really grateful for any advice/ confirmation.