Types of Damp Proof Course

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Dominic Carr

Types of Damp Proof Course

Post by Dominic Carr » Thu 31st Mar, 2005 12:23 pm

I know that there are 2 types of damp proof course out there - electro-osmosis and chemical, but which is the best and which should I go for? I have a 1911 terrace, and it has damp. Also, one contractor has advised I have special plaster which is anti-condensation at an extra 300 pounds. Is this worth it? What sorts of ball park prices should I expect? Help, Im a first time buyer!!!

Tony

Re: Types of Damp Proof Course

Post by Tony » Thu 31st Mar, 2005 2:17 pm

If this is a terrace, have a look at the other properties around you. See if they have a DPC (plack dots all around the walls about 20cm above the ground). Ask the neighbours if they have any damp problems.

See if there is any other reason for the damp first - blocked gutters and loose slates are the favourites.

Is there antthing resting against the walls which would trap water?

I would live with it for a few months - you might find that it is not required.

Good Luck

Tony

Johneds

Re: Types of Damp Proof Course

Post by Johneds » Thu 31st Mar, 2005 2:26 pm

Well Dominic, you'll get some mixed responses to that question. Several will say you don't need a damp proof course because there isn't any such thing as rising damp. Others will tell you that the damp is caused by something else and if you remove the something else the damp will go away. Others will tell you that a spot of damp comes with a period property and if you don't like it don't buy it. And others will tell you that walls need to breath and if you inject a chemical damp proof course you will stop it breathing and that will cause all sorts of damp elsewhere worse than the damp you had originally.

Now, it's not that any of that advice is wrong, in fact it is all correct - to a certain extent, some of the time. (no simple answers here) What you need to do is work out to what applies to your situation and eliminate all other suspects.

First thing to check is that the external ground level is at least 150mm below internal ground level. If it isn't, no amount of injection or electrocution will stop the damp.

Second thing to check is that the underfloor void is well ventilated. You should have vent grills at the front and rear of the house and these should be clear of obstructions.

Third thing to check is that you don't have excessive wet caused by external sources - broken drains, incorrect falls of paths, leaking or blocked gutters and downpipes, etc.

Then we need a bit more info - a photo would be ideal.

PS - I have seen chemical injection work well, but I have never seen electro osmosis work well.

PPS - Forget the 'special' plaster. The only thing special about it is the price.

Geoff

Re: Types of Damp Proof Course

Post by Geoff » Thu 31st Mar, 2005 8:08 pm

I bet the 'special plaster' is tanking cement, which is pretty pricey. It can be useful in some instances, e.g. where interior floor levels are lower than outside ground levels and can't be raised for one reason or another, but even that depends on wall structure and materials. Go through the steps Johneds suggests first.

cammy

Re: Types of Damp Proof Course

Post by cammy » Thu 16th Mar, 2006 5:12 pm

The Plaster will no doubt be a lime light plaster which is the only type of plaster that should be used on walls that have suffered from rising damp. If a new DPC in inserted regardless of type and it works as it should the the walls will begi to dry out and when that happens the evaporating moisture will leave behind contaminates draged up from ground level.

If this is not done no matter how much painting is done salt crystals will form on the newly decorated wall

Evelyn

Re: Types of Damp Proof Course

Post by Evelyn » Thu 16th Mar, 2006 5:38 pm

Limelite has nowt to do with lime - which would allow walls to breathe. It's not really what I'd use in a period building.

There again I wouldn't use chemical injection either in most period buildings.

Matt Green

Re: Types of Damp Proof Course

Post by Matt Green » Thu 16th Mar, 2006 8:28 pm

"If this is not done no matter how much painting is done salt crystals will form on the newly decorated wall "

Salts will only deposit on the new wall if they move from the masonry to the interior surface to evaporate. To do that they must move there in solution.

Now if the wall is dry, there's no water for the salts to disolve into and they will not move. Any salt retardant plaster additive will remove salts from solution if the wall is still drying. You can add such additives to lime plasters- and they wreck the surface of the walls when they are removed like anything with cement.

Now, Limelite is better than most in that you get a smooth surface, no salt deposition and a reduced amount of surface damage on removal; but to say it's the only finish to use on hygroscopic walls is rubbish.

Matt Green

Matt Green

Re: Types of Damp Proof Course

Post by Matt Green » Thu 16th Mar, 2006 8:32 pm

ug.

"-and they *don't* wreck the surface....

Biff Vernon

Re: Types of Damp Proof Course

Post by Biff Vernon » Thu 16th Mar, 2006 8:59 pm

Limelite is made by Tarmac

H Gander

Re: Types of Damp Proof Course

Post by H Gander » Fri 17th Mar, 2006 7:52 am

If rising damp is definitely the problem, then I'd go for injection over electro osmosis. I have never seen electro osmosis work.

The main problem, as has been highlighted in previous posts is the replastering. The options offered by UK damp-proofing companies are somewhat limited by their requirement to offer 30-year guarantees on their work. In Europe special lime plasters are use which contain voids for the salt crystals to evaporate into. These plasters only last 10-15 years before they need to be replaced due to all the salts that build up in them. But they have the advantage of drawing harmful salts out of the wall, rather than trapping them in.

Matt Green

Re: Types of Damp Proof Course

Post by Matt Green » Fri 17th Mar, 2006 9:43 am

"If rising damp is definitely the problem, then I'd go for injection over electro osmosis. I have never seen electro osmosis work. "

Rising damp very rarely ever is the problem though is it? Once high ground levels, gutters and bad rendering have been eliminated- so has the damp problem in 99.9% of cases.

"The main problem, as has been highlighted in previous posts is the replastering. The options offered by UK damp-proofing companies are somewhat limited by their requirement to offer 30-year guarantees on their work. "

These days it's 20 years and the plastering is not included in that gaurentee, just the effectiveness of the DPC. The usual modus operandi when dealing with a guarentee claim is then:

-check that the ground levels have been lowered (as would likely have been mentioned in the report). If not, walk away.

-check the building is well maintained and in a watertight condition (check the small print). If not, walk away

-test the wall with a calcium carbide meter. If it registers less than 5%, walk away proclaiming the wall dry and the injected DPC working. Hope the client doesn't suddenly realise that the wall may never have been damp if such a test had been performed in the first place.

-if it's less than 5% it off to the hygroscopic chambers to show the wall is salt encrusted and sucking water in from the atmosphere. Anything over 3% using the oven balance method and you can walk away.

Now the way has been cleared to blame the plastering which only has a statutory 6 year (?) fit-for-purpose guarentee. Cement-based plasters can usually hold out for that long.

Use breathable finishes and make sure the causes of the water ingress are dealt with. It's the best option for the building, regardless of who happens to own it at this point in time.

Matt Green

Evelyn

Re: Types of Damp Proof Course

Post by Evelyn » Fri 17th Mar, 2006 10:41 am

There was an item on TV last week where a shop owner had a 'consultant' testing his underfloor for damp.

This was in a shop which sold flooring, and it had UFH.

The heating had apparently been leaking in a small area. The contractor therefore spent ages with a hand-held damp meter going over adjacent pieces of floor, ripping it up, and sticking his damp meter into the UFH plastic membrane.

He declared that any reading over 15 would mean that the damp had spread and that the whole floor had to come up.

Naturally, the shop owner was devastated, but all the floor eventually was lifted - a large area - and the meter duly stuck into this plastic and a reading of 'over 15' seemed to be arrived at. The underfloor didn't look awash I have to say.

I have no idea if these meters can meaningfully test anything very much.

I still recall the off the meter reading in my house. This gave rise to a report recommending it was the most damp house ever inspected, and all (200 year old) plaster had to be hacked off to ceiling height and cement tanking installed, along with the obligatory chemical injection (in rubble stone).

This meter reading ignored the foil wallpaper underneath the existing wall covering, which may have skewed the readings somewhat...

Matt Green

Re: Types of Damp Proof Course

Post by Matt Green » Fri 17th Mar, 2006 11:16 am

Resistance damp meters are non-destructive, at least. They are best used for pattern recognition rather than absolute values, as Graham Coleman's book goes into some detail about.

A good tool that is frequenty mis-used and that produces results that are frequently mis-interpretted.

Matt Green

GRC

Re: Types of Damp Proof Course

Post by GRC » Fri 17th Mar, 2006 2:46 pm

Matt,

You may be intersted in the following:

I've reviewed the last 1000 samples I have analysed for groundwater salts; these have been supplied by assorted authorities - I have left out the ones that I have removed during my investigations.

75% had clearly levels of salts well above that one may expect 'naturally' in building materials. Most likely origin in most of the cases - RD (some may have been around chimney flues where they have arisen from burning fossil fuels - many didn't describe the precise position from where removed (Chimneys look for massive Cl to N03 ratio- often a give-away).

The figure is interesting in that it it close to that of BRE where they found that 82% of the properties they investigated in Cardiff had rising damp to a greater or lesser degree.

Haven't got much faith in renovating plasters - over past 20 years have received a number that have failed in their required design function (heavily contamiated with groundwater salts) BUT dare I say it - I have RARELY received strong sand cement mixes (1 : 3) that have been contaminated; suhc materials do work well but as appreciated, unsuitable for some properties.

Try Robbie for pics of dpc's in older properties

Matt Green

Re: Types of Damp Proof Course

Post by Matt Green » Fri 17th Mar, 2006 4:11 pm

"75% had clearly levels of salts well above that one may expect 'naturally' in building materials. Most likely origin in most of the cases - RD "

Define: rising damp. Damp rising in walls as a result of high ground levels? I believe you. True, 'real', classical, textbook rising damp? No way.

We're probably agreeing here, just with a difference in definitions.

"(Chimneys look for massive Cl to N03 ratio- often a give-away). "

That's interesting. I knew the salt assemblages differed but hadn't twigged the ratio relationship.

"The figure is interesting in that it it close to that of BRE where they found that 82% of the properties they investigated in Cardiff had rising damp to a greater or lesser degree."

Had rising damp? Or had HAD rising damp.

I don't quite know what you're getting at. Water rises in walls and soluble ground salts are deposited at the point where water movement into the wall is equal to water loss through it's surfaces from evapouration. I agree. That can be demonstrated.

Did they get there by rising damp? Well, that depends on the definition. They certainly got there through water rising into the wall for whatever reason.

These are the important questions:

Would the damp have all been solved by good maintenance and building practices rather than injected DPCs and cement plastering?

Was there even a problem with damp at the time the samples were taken?

Matt Green

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