Replacing(and lowering) existing timber floor with concrete floor

[DeanAlan]

Hi All,

 

My first post here - be gentle with me ?

 

I am about to embark on a significant renovation and extension project. I have used an online architect (which are so so but not full service .. but anyway) and a great structural engineer on board. I'm going to managed the trades myself and do a little of the work myself.

 

One of my first tasks and the question here is around removed an existing rotten timber floor, that is 600mm above ground level) and replacing it with a solid floor at ground level. Solid so I can get good UFH system in (and consistent with the extension) and lower so that it is consistent with the intended floor level of the extension.

 

Structural engineer has looked at foundations (1930s semi) and says they have enough depth for me to do that on the basis that a concrete slab would be circa 300mm (50mm hardcore, 100mm slab, 100mm insulation, 50mm screed, blinding and DPM). I'll need to break up the existing oversite concrete that I know is there and excavate out another 250mm say. I'm not going below the level of the existing footings obviously.

 

I'm attaching some diagrams.

 

Q1: any issue with this 'logically' and is my 'circa 300mm' right for current building regs on concrete floor?

 

Q2: current DPC is a badly compromised slate DPC (was checked by a damp specialist and visual inspection tells you all you need to know) and 600mm or so above ground level. I will need to bring the new DPM from under the new concrete slab up the walls to lap into the wall above or below a new chemical DPC? When they say 'lap in' does that mean a shallow horizontal trench (10mm?) cut into the masonry (or mortar)

 

Q3: as the new floor level will be inline with ground level (and below original DPC) are there any other 'damp' concerns and how would I protect against them? The concrete floor will be below ground level - finishing at ground level. You could consider what I am doing as a shallow basement - but water table is a good 1m below where I am. Do I need to 'tank' this more substantially, at least up to 1m above ground level or something?

 

Q4: from the diagram you will see there are 4 sections to the house. The two yellow are timber. The blue are concrete and I need to break that up and replace (lower, insulated, UFH etc). I would ideally like to do them one at a time. I assume this is ok but i need to make sure the DPM of one segment then overlaps (150mm I believe) the next segment. Any issues with segments of concrete? Do I need to tie them together (reinforcement bars?)

 

Finally (thanks for reading this far)

 

Q5: I hope to put in ASHP and will need to run power and flow/return/control cables from what will become a plant room/cupboard. What trunking products would I use to give watertight, airtight solution?

 

Thanks in advance for any advice.

 

cheers,

-Dean

[PeterW]

If this was me, I would not bother with the concrete and screed but back fill with MoT1 to 275mm below FFL, compacted in 100mm layers. Then lay 25mm EPS as a blinding layer, DPM lapped 600mm up the wall (well past current DPC) and then use 130mm PIR with 30mm to all walls. UFH pipes into the bottom of this and then self compacting concrete, leaving a 5-8mm layer for SLC to even off the slab when everything else is complete as you want to do this in sections. 
 

I would ensure your blue/yellow sections break at doorways and I wouldn’t worry about securing them together - in fact I would insert 10mm foam to give and expansion gap and allow movement. 
 

Injected DPC is a waste of time - they don’t work. Unless someone is insisting for a mortgage (which is very shady practice) then lap the DPC up the wall, bond it on with a PU Adhesive and then board and skim as normal. As long as the internal DPC goes above the external ground by 2-300mm you will be fine. 
 

If you are doing EWI also consider digging down and putting a French drain around the property as it will help with reducing localised moisture levels. 

[DeanAlan]

@PeterW - thanks for your reply.

 

EWI - yes, I had considered a french drain myself - a belt and braces approach.

 

Injected DPC a waste of time. Ah ok. Not required as part of mortgage (already have that). Walls are very damp to 1m ish up. By 'internal DPC' do you mean the DPM going up the wall? If this is the case then there would be no real DPC in place - beyond the current compromised one

Expansion gap - got you.

 

For concrete you have (if I understand correctly) popped the insulation above the MOT/DPM and below the 'self compacting' and of Self Levelling Screed (SLC?) to level across all sections.

 

Helpful feedback. The Chemical DPC being a waste of time is interesting comment, wonder if that is a general feeling amongst others here. I have no basis for an opinion so all ears.

 

cheers,

- Dean

Edited January 15, 2021 by DeanAlan

[PeterW]

58 minutes ago, DeanAlan said:

The Chemical DPC being a waste of time is interesting comment, wonder if that is a general feeling amongst others here. I have no basis for an opinion so all ears.

Make a brew and get a pack of digestives out and have a long read of Pete Wards website. He’s incredibly knowledgeable about old houses and buildings and his explaination of “rising damp” is well worth a read. 
 

https://www.heritage-house.org/damp-and-condensation/types-of-damp-what-have-i-got/what-is-rising-damp.html

[DeanAlan]

@PeterW I like the idea of looking at a website called "heritage house" makes me sound like Lord of the Manor.

 

Opening sentence:

 

Quote

Rising Damp is a term coined by the British damp industry to help them sell chemicals.

 

Ta,

-Dean

Edited January 15, 2021 by DeanAlan

[Roger440]

Once you have read the heritage house website, you will, hopefully realise, that with a compromised DPC, you are not fixing your damp issue, merely covering it up. With a DPC on the inside, and EWI on the outside, your wall will be wet forever. 

 

As one of the videos says, if you wash your clothes, put them in a plastic bag, and hang them on the washing line, what do you think will happen. This is, effectively, what your proposal does.

 

Either you need to fix the dpc properly, or treat the building as though it doesnt have one.

 

The fact the floor was rotten says you have a problem with moisture, Dumping a load of concrete in there wont fix it. You need to understand the reason why first and work from there.

[Roger440]

OOh, and a chemical DPC us just a scam, as already pointed out. Dont waste your time or money.

[Gus Potter]

Hello Dean.

 

Welcome to BH, it's a great resource.

 

"Q2: current DPC is a badly compromised slate DPC (was checked by a damp specialist and visual inspection tells you all you need to know) and 600mm or so above ground level. I will need to bring the new DPM from under the new concrete slab up the walls to lap into the wall above or below a new chemical DPC? When they say 'lap in' does that mean a shallow horizontal trench (10mm?) cut into the masonry (or mortar) "

 

Generally slate DPC's last a very long time. Yes, they may get the odd crack in them as an old house moves about but the odd crack in it's self will transmit very little moisture by way of capillary action if it is small, if large then then even less so if not bridged. You'll also often find that the DPC is roughly level with the floor and any solum vents. If you wish post some photos you get additional feedback.

 

One thing worth considering is that if you are breaking up the concrete floor you can recycle this if you keep the lumps under say 75mm and mix this up with the fines. You'll often find you can whack this in and get it quite "tight" then top it off with a little MOT type one if need be and your blinding. Saves on disposal costs. Have a look on the net at what is called 6F2 / 6F5 recycled aggregate and compare this with what you might end up when you break out the floor.. If you are breaking the floor up by hand as opposed to getting a mini digger in with a pecker (maybe not so good for the old house) then you'll be surprised at how much good material you can recover... and save on skips etc.

[DeanAlan]

@Roger440 and @Gus Potter thanks so much. I clearly need to read more and bounce ideas of the people here. 

 

I'll be back.

 

cheers,

-Dean

[Adsibob]

I’m in a very similar situation to @DeanAlan. Except mine is possibly worse, as when we did our trial holes earlier this week to expose the foundations we discovered the water table was about 1m below our finished floor level! (See photo.)  It’s been raining in London quite a bit these last couple of weeks, but not crazy amounts. I suppose it’s a result of deforestation and climate change. Water has seeped up the bricks and, yes as the 1930s semi is now 90 years old, there is a lot of damp. I too need to drop my FFL to accommodate a new extension (in my case by about 40cm) and I’m replacing the timber subfloor with a concrete slab. My structural engineer and architect are working on a solution, which I will post here before deciding, but they did say they would include injecting a chemical DPM so curious to hear from people such as @PeterW as to why they think injecting chemicals is a bad idea / scam by the damp treatment industry. 
 

 

[Roger440]

You could look at it another way.  And some evidence it actually works?

 

Think about it. A man will drill some holes in you wall and inject "something". This something needs to permeate through the bricks and mortar to such an extent that they form a barrier impervioius to water. Its simply not credible.

 

Get a brick drop it in a bucket of water for a week, then cut it in half. I bet you its dry in the middle. Thats a week in water. But a bloke with a mastic gun is going to squirt something into you brick to make it completely waterproof in a few hours.

 

Doesnt look like the damp comes up far enough to be a problem?

[Adsibob]

1 minute ago, Roger440 said:

Doesnt look like the damp comes up far enough to be a problem?

The issue is that we need to drop the finished floor level by 420mm in that room. So we are getting the FFL closer to the damp. 

[TonyT]

+ infinity to the above

 

dont inject into the brickwork- 

architect and SE clueless..

[Roger440]

3 minutes ago, Adsibob said:

The issue is that we need to drop the finished floor level by 420mm in that room. So we are getting the FFL closer to the damp. 

 

420 is rather a lot. 

 

Solutions aside, thats the water table now. Id suggest that at times it will be higher.

[Adsibob]

6 minutes ago, Roger440 said:

Solutions aside, thats the water table now. Id suggest that at times it will be higher.

Exactly!

[PeterW]

Water hasn’t seeped up the bricks. What has happened is the ground below has got wet and that will have soaked into the bricks from the sides. What is the ground level to the other side of the wall..? You will need to put some decent drainage all round - a gravel filled French drain as a minimum. 

[Adsibob]

10 minutes ago, TonyT said:

architect and SE clueless..

You may be right. I simply don’t know. But I need to get a trustworthy surveyor to have a look. I have a RICS surveyor in mind who inspected a damp issue for me many years ago and fixed it without recommending injections, so hopefully he will give an informed view. 

[Adsibob]

4 minutes ago, PeterW said:

Water hasn’t seeped up the bricks.

How can one be sure about this?

[TonyT]

https://www.michaelparrett.co.uk/publications
 

 

buy this. It will save you money in the long run

[PeterW]

Just now, Adsibob said:

How can one be sure about this?

Rising damp is a myth. Read what @Roger440 has said - get one of those bricks cut out and chop it in half and it will be dry in the middle

 

Start here for some decent information about old houses

 

https://www.heritage-house.org

 

[Roger440]

1 minute ago, PeterW said:

Rising damp is a myth. Read what @Roger440 has said - get one of those bricks cut out and chop it in half and it will be dry in the middle

 

Start here for some decent information about old houses

 

https://www.heritage-house.org

 

 

Yes, this website^^^^.

 

And read all of it.

[Adsibob]

9 minutes ago, TonyT said:

https://www.michaelparrett.co.uk/publications
 

 

buy this. It will save you money in the long run

Thanks. This appears to be the updated version of that book, albeit only written by the co-author to the original:

https://www.amazon.co.uk/How-Investigate-Damp-Practical-Inspection/dp/0367434954/ref=mp_s_a_1_1?dchild=1&keywords=ralph+burkinshaw&qid=1610924359&sprefix=ralph+bur&sr=8-1

 

 

[TonyT]

Stick to Mike’s book he’s the expert

 

no rising damp found even in Venice!

[Gus Potter]

14 minutes ago, TonyT said:

+ infinity to the above

 

dont inject into the brickwork- 

architect and SE clueless..

Not aiming to miss here TonyT!

 

Anyway, in the spirit of BH a bit of banter does no harm and is fun.

 

One good reference document I use is "Rising damp in walls - diagnosis and treatment" published by the BRE. I have a copy but can't post due to copy write etc. Current price seems like £15 quid.  It's worth the money compared with a say "Domino's pizza".. of course if you are hungry then a Dominos wins.

 

The guidance it pretty user friendly so you don't need to be an SE to understand it.

 

Here is the link: https://www.brebookshop.com/details.jsp?id=287528

 

@Adsibob don't panic if your founds are sitting in water, it's quite common.

 

@DeanAlan I think this £15 may help give you some confidence how to progress, marry that up with the comments on BH and the cost effective / practical solution may be there for you.

 

If you think about it you will pay a Surveyor a good few quid, inform yourself and talk to them. They will then know that you know a little about it and will often put more effort into the report as they may well be more invested in your job, over and above what they normally do. It's a people thing.. funnily, folk like passing on knowledge as you often see on BH.

 

 

 

[TonyT]

https://www.isse.org.uk/sites/default/files/2018-08/3. Rising Damp in walls - Bre.pdf
here it is for free 

 

not all SE and Architects are useless, just the pair discussing injection unicorn cream into walls.