Advice needed re damp and DPC

[bfmonaghan]

Hello all

New to board and first post, but hopefully I can get some advice with a couple of problems I have with a 1907 farmhouse that I bought 6 months ago.

Problem 1:
The main house has a gable-end single floor extension which houses a large utility room and a play room. The floor of the utility room is concrete with no dpc and is level with the outside ground level. When we bought the house, the utility had lino. When we removed it there was a lot of damp beneath which dried up pretty quickly. I want to put slate flagstones down to improve the floor appearance, but am trying to work out the impact of no dpc. My original idea was to put in a dpc and a thin concrete layer, raising the floor level by about 1.5 inches. But the more I think about that, the more convinced I am that it will just result in the damp being pushed to the walls - since the floor is level with the ground outside and no opprtunity to install air bricks. So, I'm thinking that a better idea is to check out the breathability of the flagstones and go with a permeable means of fixing them to the concrete - but not sure what that fixing might be.

Problem 2:
The play room has developed a very damp lath and plaster wall where pointing at the corner of the room (where it joins the main house) has been washed away by overflow from a blocked gutter downpipe. The downpipe has been cleared. The pointing has been a nightmare to put right, since it's in very narrow horizontal gaps between square cut corner stones. So, I've had to push the pointing in with wires to fill the gaps. Given the narrow gaps being filled, the amount of mortar used has been very small, but it's taken a while. On the advice of a neighbour (a landscaper) I used cement pointing. So, the water ingress seems to have stopped affecting most areas of the wall and the plaster is drying out. The effervescence has stopped and I'm about to treat with salt neutraliser. However, there is one area, at waist height) that refuses to stay dried out. Running our dehumidifier results in a temporary drying, but it soon gets damp when there's any amount of rain. I've looked over the outside wall in detail and can't see any obvious remaining mortar problems. So. I'm thinking that water may be getting in through the stone, or that the cement mortar may have stopped water ingress, but be preventing natural evaporation. But the gaps pointed with cement are so thin that I'm not sure much water could have evaporated out of them (as opposed to the volumes running down the stone, then along the top of the unmortared gaps). Am not keen to take out the l&p to find the source, but can't do much with a wet wall that won't dry out.

Any thoughts or ideas appreciated.

Brian

 

[Flyfisher]

Regarding problem 2, how thick are the walls? There's a rule of thumb often invoked here that says one inch of thickness requires one month to dry out. Given that mortar has been washed away by the overflowing gutter it seems like a fair assumption that the overflow had been happening for quite some time and that the wall was therefore completely saturated.

You don't say when the gutter problem was fixed, but if we assume it was soon after you moved in then that's about five months, which over the recent bad winter may not have been enough time for the wall to thoroughly dry out. My inclination would be to give it more time through the summer and let the warmer, drier weather do its thing.

Meanwhile, I would go outside during the next downpour and have a good look at what's going on with the guttering just to be really certain there are no leaks, drips or splashing that could be slowing the drying out process.

Also, the cement pointing may be thin but it will certainly be restricting the drying out of the wall.

 

[MatthewC]

Hello Brian and welcome to the forum

Re Problem 1, my first instinct is to ask whether you can address the cause rather than the various symptoms: is it possible to lower the ground outside or install a French drain (http://en.wikipedia.org/wiki/French_drain)?

Photos are always welcome here!

Matthew
http://houseintheenchantedforest.blogspot.com/

 

[bfmonaghan]

Thanks Flyfisher

The wall has been saturated for some time, as far as I can tell. It has moss and algae growing which I plan to pressure wash off asap. The downpipe was only cleard a couple of months ago and the pointing's only been done for a few weeks. So, at 1 inch per month, we still hae over a year to go before we're dried out I guess.

Have thought further about the cement pointing. Think we'll just have to live with it for the moment. I've been told that the house is all cement - but at a 1907 build I'd thought that there would be a good chance it might be lime. Not sure how to tell the difference by inspection?

Thanks MatthewC also. Can't lower the ground outside unfortunately by any meaningful amount. There are drains outside to catch some ground water, but there'sma reasonable gradient all around, so I think it would be tought to capture all ground water before it reaches under the concrete. It's a good suggestion, but I'm not sure how do-able adressing the cause will be. Landscaper next door is sceptical that the DPC will casue any issues, given the volumes of water involved. But we're both guessing really!

 

[Penners]

I'm thinking that a better idea is to check out the breathability of the flagstones and go with a permeable means of fixing them to the concrete

Welcome, Brian. Slate has often been used as a damp-proof course in buildings. I mention this only to suggest that your slate flagstones are very unlikely to offer any real breathability. Using a lime mortar to bed and grout them might give a modicum of damp relief, but (in my amateur opinion) not much.

If you're really concerned about sideways migration of the damp exacerbating the dampness of the walls, then I'm afraid you may have to consider digging out the concrete.

the floor is level with the ground outside and no opprtunity to install air bricks

I should also mention that air bricks are only of any help when they ventilate the void under a floating floor. Even if your outside ground level was significantly lower than the interior floor, air bricks won't offer any help with damp concrete.

Sorry - that's all very negative! But I hope it might help your decision-making process.

 

[Flyfisher]

So, at 1 inch per month, we still hae over a year to go before we're dried out I guess.

It's only a rule of thumb and wall materials and immediate environment will obviously affect the speed of drying out, for better or worse, but it certainly sounds as if the wall will need more than just a couple of months to completely dry out. The good news is that summer will soon be upon us. The main thing is that the source of the problem is now fixed so the wall just needs some time to 'heal' back to its normal state.

 

[middi]

Hi Brian

If you are presented with limestone walls, cement floors, no dpm's, not much heating, poor ventilation, poor pointing and (perhaps ?) leaking gutters in a north or east facing corner, there not much more to go wrong exept a hole in the roof and Noah's flood.

On a more serious note I would concentrate ( as you have) on the condition of stones, guttering and pointing being in good condition. Make sure that in cold damp weather there is enough ventilation and gentle heat to dissipate moisture which naturally tends to condensate on cold surfaces such as cement and stone.

Short of knocking the lot down and starting off with a " modern" insulated building there is very little you can do and you might as well put you flagstones on the existing cement with a weak mixture of sharp sand/cement mix or lime if you wish.

You have a naturally " cold " building structure and it is a question of learning to use it for what it is.

Middi

 

[Penners]

Make sure that in cold damp weather there is enough ventilation and gentle heat to dissipate moisture which naturally tends to condensate on cold surfaces such as cement and stone.

I'm sure Middi won't mind if I add the words "...and ventilation..." to that sage advice. The reduction of condensation requires warmth and ventilation (to carry away the moist warm air).

 

[bfmonaghan]

Thanks for all your replies. They have been very helpful.

I'm encouraged that my damp wall will hopefully get better as we move into spring and summer. Have now taken off algae to reveal areas of damp in outside wall - so there's not much more to do other than wait and watch.

On the utility room floor, though, I'm still undecided about the best course of action. I could:
- just lay a weak mortar mix and flags on the existing floor (without dpc) and hope that the moisture naturally rising through the cement doesn't loosen the flags; or
- put down a dpc and a thin pedestrian concrete layer then tile cement and flags, and hope that the moisture that currently evaporates through the floor doesn't migrate to the walls and give me rising damp.

Can't say that digging out the existing concrete floor and starting again with a suspended floor appeals.

Any strong views on which is the better risk to take?

Thanks

Brian

 

[andrew gayton]

you could always remove a section of the existing concrete floor adjacant to the walling and install limecrete in its place to provide an evaporation zone. it works.

 

[robgil]

I am contemplating this my self , have you done this Andrew? Did the floor crack or move much?

 

[andrew gayton]

Yes, both in practice and specifying it for other projects; it is a good compromise approach in both existing situations and in conversion works where the budget will not stretch to a full limecrete floor. The more permeable the limecrete the better - I have used non hydraulic lime, aggregate and broken soft red brick to good effect. No cracking at all.

A current cottage restoration I am involved with is having a chalk floor installed. A very cheap alternative to limecrete. A compacted chalk slab to take a lime screed and clay tiles over.

 

[bfmonaghan]

Thanks Andrew

Let me see if I've got this right.

Put down the dpc. But don't dpc at the wall edge - instead, take up the existing concrete and then put down the limecrete.

The limecrete (as with the concrete) would then be covered by the final falg floor and breathable mortar (if I can find some!)

So moisture from beneath the dpc will hit the limecrete before the wall and evaporate. Will the limecrete still be effective if covered by mortar and flags though?

Appreciate your help.

 

[tobydog]

Andrew - how deep and wide would you recommend the limecrete gap should be between the wall and the concrete slab?. Have similar issue coming up shortly.

 

[robgil]

Chalk floors? that sounds interesting , can you give some specifics, depth of chalk and screed etc?
Sorry to barge in on your thread Brian.

 

[andrew gayton]

take up existing concrete slab, say 300-450mm from edge of walling. If there is a dpm in place, turn it up vertical edge of cut back conc. slab. No new dpm's or dpc's. install limecrete. Use a lime and sand mix to bed clay pamments/tiles/floor bricks; only seal with 50/50 mix of raw linseed oil and real turps.

Take the limecrete down to form at least a square in section (300-450mm x 300-450mm) or deeper if founding etc allows it.

For the chalk slab, 250-300mm is best. The chalk can be bought as 'waste' in ton bags. This is then whack plated down to form a good base for flooring as above.

permeable flooring is, in my opinion, as important as permeable walling.