More Concrete floor questions!

[tumbledown]

I know this subject has been covered quite a few times, but I've been given a slightly different theory from a builder advocating concrete floors, and I'd appreciate people's opinions on it.

This builder is quite experienced in timber frame repairs, and I've seen examples of his work, which looks good. He is also experienced in using lime products, and repairing wattle and daub. I was therefore expecting him to advocate the traditional method of bricks/flags straight onto earth/chalk etc. especially as he had already commented on removing cement renders and lowering ground levels. Anyway, his comments on floors were:

"If you don't have a floor with a DPM, any mats you lay will constantly suffer from mould and damp, as will the bottom of any heavy furniture. The best thing to do is to lay a concrete floor with a DPM, but do not continue it right up to the walls. There should be a small strip of breathable material between the floor and the walls. In this way damp is not driven into the walls because the walls are isolated from the floor.”

So there you have the theory. I have heard the ‘damp mats’ theory before. But if there is enough moisture coming out of the floor to make your mats go green, then why would this moisture be able to escape harmlessly through a small gap around the edge of the floor? Surely the moisture would simply be concentrated and you get a damp strip around the edge of your floor. and after all, most people place there be furniture around the edge of the room, so wouldn’t the ‘trapped moisture’ problem be worse not better? I’d have thought that the only way this builders theory could work was if the DMP was continued through the walls (into the brick plinth below the soleplate), so that the moisture could evaporate outside the building and not raise up through the floor or the walls?

Anyway, I’d appreciate peoples opinions. I’d just about convinced myself that the fully breathable floor was the way forward, but this has made me wonder. Oh, the other thing he said was that a concrete floor was much better for fixing new stairs and internal walls, which was more difficult otherwise

 

[biffvernon]

This is really a continuation of Harkerhill's sub-basement floor thread.

It's an interesting idea, tumbledown, but I think your conclusion has a lot of merit.

On Saturday I went to the SPAB visit to Geoffrey's (#51) Knedlington Hall. He had layed a dpm + concrete screed in the kitchen and then relayed the original pammetts on top of that. BUT, Geoffrey told us that he did that early on in the project and would not have done it had he known what he knows now. The general SPAB advice is not to use a DPM.

However, it is doubtful whether the benefits are so great that it is worth the effort of removing a concrete and DPM floor that already exists.

You can see Geoffrey's gallery of renovation pics at http://www.oldhallknedlington.co.uk/renovationgallery

 

[Ian Bond]

Sorry, I don't buy his theory at all.

After all, how come there's so much antique furniture about (unless it is all 1st floor stuff) - it hasn't rotted away from rising damp. A bit of woodworm perhaps!

Plus, concrete is a real no-no in environmental terms. How about limecrete instead? Or a stone flag, slate or brick floor?

I've got a solid ground floor in my house, and no DPC. Just flags laid direct to earth. The only damp is in the cellar, in one corner and that dries out from time to time.

I reckon that laying a concrete floor would be quicker and easier than the alternatives, and probably cheaper. That's all.

 

[GK]

Hi Biff, it was good to see you again and all the other devotees to The Cause.

I've spent the night recounting what I said, what I forgot to say and what I wish I'd explained more clearly!

I've got to be somewhere else this morning but when I get back I'll go and run my fingers around the outside edge of the bit of dpm sticking up and report back.

Geoffrey

 

[tumbledown]

Biff, I won't exactly be removing a concrete fllor with a DMP. The exisiting floor is conctete, but doesn't have a DMP. Its been poorly laid, is uneven, and is breaking up around the edges. Everyone i've had around has said they would remove the floor and start again - its just some say lay a new concrete floor with DMP and others say just lay brick/flags on the earth. Either way, everyone seems to be saying the existing flor should go!

I'm having quite a lot of timber-frame repairs. Basically, all the soleplate and bottoms of studs/post need replacing. Along one wall a new brick plinth and soleplate need to be introduced. as I'm having al this work done and the floor needs to come up anyway, I want to get it right - it was just that this latest builders idea was a slight variation on a theme.

Biff, I was suprised to here you say removing an existing concrete floor with DMP would not gain much advantage - I thought the theory was that it would give big advantages if it went right to the walls?

 

[GK]

OK, here's what I found: Tiles on screeded floor on dpm, against 2 external walls comprising stone plinth. One external wall continues to a depth of 5 metres or so where it forms one side of a square well where the water level is a couple of feet below the interior floor level. So it might be reasonable to presume that the ground at the other side of the well wall, which is under the interior floor, will be saturated. We're on low lying ground.

Poking around inside where the dpm meets the wall, it doesn't seem particularly damp, there are some battens fixed to the wall at floor level as grounds for skirting, and they are dry. The wall above is lime plastered above the batten line and is not showing any sign of dampness.

Although I wouldn't lay a dpm or a cement based screed again in this situation, neither would I lay tiles or flags on earth. I would (and have elsewhere) lay a base of crushed limestone, sharp sand mixed with a bit of hydrated lime and bed the tiles on that.

 

[biffvernon]

Biff, I was suprised to here you say removing an existing concrete floor with DMP would not gain much advantage - I thought the theory was that it would give big advantages if it went right to the walls?

Actually I said

it is doubtful whether the benefits are so great that it is worth the effort of removing a concrete

which isn't quite the same thing.

GK, your wall with no dpc standing above the well with water so close to ground level is a fine example of how damp does not rise. We don't need to go to Venice.

But I was puzzled that the water didn't flow away to the main level of the lower yard. Is it more a tank, with its walls set in clay, rather than a well that relies ofn the local water table?

 

[tumbledown]

Biff, surely damp must rise to a certain extent? How do soleplates rot if damp is not rising up the brick plinth they sit on? Be gentle with my if I'm being a bit thick - just trying to get my head around all this damp malarky!

 

[GK]

But I was puzzled that the water didn't flow away to the main level of the lower yard. Is it more a tank, with its walls set in clay, rather than a well that relies ofn the local water table?

The round one is surrounded by clay, I don't know about the square one.

A tenant who lived here in 1748 complained that the well at the back door did not half fill up with water, so another well was dug.

 

[biffvernon]

surely damp must rise to a certain extent?

It's a fair question but the answer is controversial, evidence is contradictory and the truth is illusive. The movement of water and water vapour through the fabric of an old building is complex and difficult to determine. The view that a house without a damp course will always be damp is far too simplistic.

It is often stated that a dpm in the floor but no dpc in the walls will lead to 'rising damp' as the water trapped under the floor has nowhere to go but up the walls. I find this explanation somewhat unsatisfactory.

GK, I assumed the two 'wells' were connected as the water levels seemed to be the same.

 

[tumbledown]

Biff, would you say that the damp in the walls and the decaying soleplate had far more to do with the cement render inside and out, and the high external ground levels? Actually, the external ground levels are not terrible - they are about level. just wondered that if I made sure only lime products were used on the walls, and the ground levels lowered a little, I could get away without doing too much to the internal floor?

 

[biffvernon]

I guess the state of the art advice would be to remove the cement render, reduce external ground level, clear vegetation, prevent rain-splash from hard surfaces, ensure water drains away from wall, provide good ventilation indoors and sit back and see what happens over a couple of seasons. That might be enough. Or it might not.

 

[tumbledown]

so far, I've chipped off the internal cement render, but not done anything else yet. I have to say that the brick plinth has dried out remarkable just by doing this. however, once I get my nice new soleplate in, I want to make sure this one doesn't go the ame way as the last .......

 

[peter-300]

In reference to antique furnture, you find some chairs that have been cut down due to damp from floors in the past. I have a set of four chairs - two I bought at a different time, - and the last ones I got are nearly an inch lower!

In relation to my origina kitchen quarry tile floor, I have place discreet studs on the bottom of my antique dresser and units to prevent damp form rotting the bottom. I also find that once the floor has 'breathed' a bit damp on the door mat is not a huge problem.

Thanks

 

[biffvernon]

In reference to antique furnture, you find some chairs that have been cut down due to damp from floors in the past. I have a set of four chairs - two I bought at a different time, - and the last ones I got are nearly an inch lower!

No, not damp floors. There's a Roald Dahl story that explains it.

 

[tumbledown]

I guess the state of the art advice would be to remove the cement render, reduce external ground level, clear vegetation, prevent rain-splash from hard surfaces, ensure water drains away from wall, provide good ventilation indoors and sit back and see what happens over a couple of seasons. That might be enough. Or it might not.

biff, from the way you phrase your comments, can I take it that you do not altogether agree with "the state of the art advice"?

 

[biffvernon]

Not at all. I think the advice is good. I just don't want you to do all that good conservation practice, find the wall is still damp and blame me when you forgot to mention the artesian fountain / leaking water main / cat that keeps widdling in the corner etc.

 

[NT]

GK imho your situation is one in which concrete and dpm would be most effective and cheapest. Because the ground is saturated, a breathable floor wont dry it out to any extent, and will only have the effect of adding water evaporation into the interior. I'm assuming here that the floor will be below the soleplate.

NT

 

[NT]

Water can and does rise by capillary action to a limited extent. This is well knwon physics and is confirmed by the experiments in which walls are stood in water. Note I did say limited.

But it will stop rising where it meets coarser interstitial spaces, as there is no upward capillary action at that junction. Lime mortar and brick have different pore sizes, and water rising stoppage is seen at the juntion between the 2 in the wall-in-water experiments.

Presumably there will be some evaporation from this capillarily risen water, causing dampness to a limited extent above the area of saturated wall. But with breathable inner and outer skins this will not travel up far at all.

The injectable dpc companies seem to be claiming rising damp as something that rises far beyond the limits suggested by the above mechanism, which would normally mean water not reaching as far up as floor level. As ever, reality depends on the numbers.


NT

 

[Me!]

Sorry if this is a stupid question, but if portland cement mortar and concrete are as impervious to water as many previous discussions on this forum have suggested, then why would it ever be necessary to lay a dpm under a floor slab?