house sale fallen through Rising damp in survey help!

Fire_Fox

Many years ago I had an Edwardian cottage in which the survey flagged up possible rising damp. Had a series of DPC companies over who all diagnosed damp in different areas (!) and advised varying degrees of DPC. Last contractor was from Rentokil who had a really thorough look, found damp in the exact same areas as the surveyor and advised
1. Rear extension plastered right down to floor behind skirting bridging the DPC, solution chop back plaster
2. Two closed off chimney breasts clogged with soot bridging DPC, solution unclog and install airbricks/ vents
3. Side extension concrete flooring laid too high bridging DPC, solution small area of new DPC which was cheaper that lifting the floor.

Moral of the story is there may be damp that is rising, but a damp proof course is not the only solution. Just because you cannot see or feel the damp doesn't mean it isn't there, but you do need to look at what changes have been made to the property over the last century that has stopped the house from 'breathing' in the way it should. Any chance the DPC is being bridged on the outside of the walls?

David_Aldred

Hi again chillbill,
Glad to see I am not the only who has trouble getting access to the pc (we have four children here and more than one pc but they still hog them).

We are obviously looking at this blind so there is a limit with regard to comments to be made but a few points may be worth considering from the information you have given:

1. The high readings on the electrical damp meter that you say have been found are confined to party walls if I am understanding you correctly. There are issues within your comments to indicate there may be some transient condensation (see note below) but condensation would not normally be found on party walls where both rooms on either side of it are occupied as the surface temps are not usually low enough to hit the dew point (the temp at which condensation would occur).

2. The lack of vent bricks to the base of the external elevations suggests (but does not confirm) that the floors have always been solid construction though they may well originally not have been concrete but perhaps flag / tile floors that were breathable. This meant there would have usually been a certain amount of evaporation up through the flags / tiles / their joints and when replaced with concrete this evaporation up through the general floor area will have been stopped if and as usual (but again not confirmed) a damp proof membrane (think plastic sheet) was laid under the concrete to stop dampness coming up through it.

3. If a damp proof membrane was laid under the concrete floor then it is unlikely it was sealed into the damp proof course (see note below) in the walls and over time there will also have been movement between the floor and the walls such that a gap often develops or at least is not sealed such that dampness can emerge where the floor abuts the walls. This is irrespective of whether the damp proof course (dpc) is effective or not and this situation may be mirrored on the other side of the party wall too.

4. Sat right on top of this open floor / wall joint is timber skirting / other wood such as base of door frames etc and also the base of the plaster. When evaporation is reduced by replacing the flag / tile floors with concrete then the moisture from this area will tend to concentrate its attempts to evaporate at any place it is able to do so and the only place it can is the open floor / wall joint and any weaknesses to the dpc.

5. If the house is only 100 years old it should have been built with a dpc under public health legislation and Building Regulations applicable at that time, though don't expect to see a modern thin black sheet of dpc material, the dpc may well be a layer of slate or band of stonework / brickwork considered to be relatively relatively impervious. If present this does not disappear and although it may crack with settlement of the house the amount of dampness that would emerge up through such cracks woudldbe minimal and would not correlate with the full wall lengths of high conductivity meter readings that you are being told.

6. Your comment that a few years ago both party walls were metered and only one found to have high conductivity meter readings is interesting because if there was no dpc to the property as a whole then every wall should have rising dampness but clearly this is not the case because at that time only one wall was found to have high conductivity meter readings.

7. Another interesting comment is that when you have drilled into walls at their base the debris that came from the drill was dry powder. It is not rocket science to appreciate that debris from a wet wall would tend to come out like mud steaming from the drill bit. There are obviously degrees of dampness from wet mud to dry powder so there are limits to this observation but nevertheless the signs are encouraging that the wall itself may be relatively dry irrespective of any dampness emerging at an open floor / wall joint.

8. Even if we said rising dampness was present to these walls, injecting a chemical dpc to a party wall from one side only that is 2-3 feet thick as you describe would be highly unlikely to form an effective dpc all the way through the depth and breadth of the wall due to the inherent limitations of the system injecting fluid blindly. In the area of the chimney stacks (and these may be back to back on the party wall) if these protrude from the party wall there must be a thickness of 4-5 feet if the party walls are 2-3 feet thick as you imply.

9. Because a chemical dpc would be unlikely to be effrective under such circumstances the reliance would be upon the re-plastering system to hold back any dampness (if present at all) and salts within the wall. Remember it is not only dampness from the wall itself that conductivity meters will give a high reading to but also if salts are present the wall itself can be dry and yet the readings will be high on the meter when placed upon the salty surface because the salts in that plaster may well be hygroscopic.

10. The dark grey mortar and the fact that you say this is a miners terrace raise issues of black ash mortar being used in the construction. Black ash mortar is where coal slag was used rather than sand in the mortar and consequently is high in sulphates. This has big implications for plasters that are applied to such walls and for instance ordinary portland cement often used in the base coat of some plasters (especially replacement plaster associated with previous chemical dpc work) will prematurely fail / degrade / become damp due to the contaminates from the mortar leaching into the wall as a whole and then into the plaster. This situation will be worse around chimney breast areas where the soot from the original coal fires will also compound the problem.

11. From the above it would be prudent to remove a section of skirting in the area of concern and check the floor / wall joint area including that previously chemically injected. Can a knife balde be inserted between the floor and the wall joint? Is there sealing of the area from the level of the chemical dpc injected above floor level down and over the floor with fleible impervious material ? Is the plaster kept at least 30mm from the floor / wall joint ?

12. Ideally you would want to know what the moisture content of the skirting was in the area of concern. Timber at a moisture content of over 20-22% is at risk of decay but bear in mind the skirting may be damp / at risk entirely due to any dampness emerging at the floor / wall joint irrespective of whether the dpc in the wall itself is effective or not.

13. Carbide meter testing of the masonry to the wall of concern would be prudent. This is a test where debris from drillings to the wall is mixed with carbide that reacts with water and pressurises a flask from which the pressure gauge to the flask gives a reading. A contractor would say readings under 5%w/w are considered to have an effective dpc but the situation is more complicated than that but in any case a low reading on a carbide meter would certainly be encouraging and give a better indication of dryness than simply looking at the dry powder drillings you have previously made.

14. A point worth checking is whether the ground floor of your property is at the same level of that of the adjoining properties because if not there may be a tendency for dampness to push through the lower floored wall which is not the same thing at all as rising dampness.

15. We then move onto issues of higher relative humidity within the property than it would perhaps have had originally. The reason such is suggested is the original fireplaces have been sealed apart from small vents. When not used for an open coal fire the air changes from that open fireplace of say 2-5 times per hour within such rooms will have dropped dramatically. In addition the plastic windows are far more air tight than the previous timber windows and as you indicate there are issues of mould growth typically associated with condensation. In addition the heating regime is allowing periods for the property to go cold (overnight / when you are out during the day) and as condensation is temperature related as the property goes cold the moisture it could hold in the air during the periods of warmth it is no longer able to do so and condensation will occur on the coldest surfaces.

16. Although we said condensation would be unlikely to occur on party walls because the temperature of the wall would not be expected to hit the dew point the issue here is if the plaster is contaminated by salts due to the reasons described above, then it will draw moisture to it from the air in the same way leaving salt out in a kitchen will become damp.

I hope I haven't confused you with all of the above but perhaps you can appreciate that somebody coming round for a few minutes with a conductivity meter and declaring you need yet another dpc may be well off the mark and issues of contaminated plaster, gaps at the floor / wall joint where dampness can emerge and periods of high relative humidity can be mistaken for the requirement to insert yet another dpc. The fact that inserting such a dpc may well not be effective or indeed the reason for the high conductivity meter readings should also be of concern.

Where you go with this information is really up to you but even if you do not have full laboroatory analyis of the walls, I would tend to want to check the floor / wall joint, have a carbide meter test of the walls and a test for salts within plaster before spending £2000 on top of the money you have previously spent on the exisiting chemical dpc to some areas.

A point of further note is that a dpc contractor who claims to say you need another new dpc with a conductivity meter and who then undertakes such work is then often in a fix when high values on such meters are obtained at a later date by others. What they tend to do then is a carbide meter test which if it gives a low reading (under 5%w/w) they declare the dpc is effective. You then have to ask yourself why did they not do a carbide meter reading in the first place and also how come high readings on a conductivity meter meant a new dpc one day but are totally disregarded by the contractor another day when a dispute arises. This being the case one perhaps can see the recommendation to undertake sample carbide meter readings / plaster tests prior to spending further money.
Kindest regards, David Aldred Independent damp and timber surveyor.

chillbill

Hi David - Wow you are officially my guardian house angel!:A

Late replying as just got home from saying bye to my best friend who is emigrating to OZ - what with our buyers pulling out as well has been a very emotional week.

I cannot believe the amount of information you have sent me it is brilliant - some a bit confusing to me so I have printed it off and other half is reading as I type and making lots of hmm interesting noises!

I will feel so much more confident now in contacting a surveyor armed with your information (god help him if he is a cowboy - I will tear him to shreds!)

Thankyou Thankyou and Thankyou again you have reduced my stress no end, I will keep you updated with any progress.

By the way we did have some good news today - we have a viewing on Saturday so fingers crossed:T

chillbill

Hey Firefox thanks for the reply

I am very much liking the unclogging of the chimmney option - sounds cheap and easy comapred to the others. Think your right about he changes to the house as well - central heating, extension fire blocked up etc. - I have strangled the house!

OH will be having a good scout around tomorrow armed with yours and Davids suggestions.

Thankyou again

David_Aldred

Hi chillbill,
Sorry for the spelling errors but hope you managed to get the main idea. See how you go with your chaps coming round to give you a quote. Whilst they are there ask them to give you the readings from the skirting at the base of the wall in the area of concern and write them down and before they come ask them to bring a carbide meter to do a sample test to the area of concern starting at the base of the wall and every 300mm up the wall to the point where the conductivity meter readings tail off and then one more 300mm above that and write down all the readings obtained. The drillings should be done slowly, away from external walls and chimney breats and the plaster plus the first bit of stone should be ignored / not be included in the test material. Just for a laugh ask if you can put your fingers over the end of the conductivity meter and when you get a high reading ask if this means your fingers have rising damp and see what they say. Just one more thing check there is no lining paper of tinfoil which will also give a false high reading. Kindest regards, David

chillbill

Hi David- hadn't noticed any spell mistakes - prob because mine is atrocious! Hav enoted down the extra info. OH very impressed and grateful for all the info. Re fingers on carbide metre - could be dangerous - have sweaty palms when nervous - may blow the machine up!
Thankyou again (and thank the kids for giving you PC time!)
Lisa (just noticed after all these posts had been very rude in not putting name!)