Surveyor disagrees with damp proof people

bubblybee

I am buying a house and have had a full building survey undertaken. The report has come back and it says that there is damp on the external wall of the lounge.

The house had a damp proof course put in 4 years ago and is covered under a 30 year guarantee. The damp proofers went out today and have advised that they have not detected any damp at all. :huh:

The surveyor and damp proofers have talked but cannot come to agreement. Our solicitor has said that the guarantee is not really worth anything as there are lots of legal loopholes that the damp proofers could use to get out of doing any remediation work.

The surveyor says his meter reading was 18%. Saturation is 28% - so he says.

now of course I have paid for the surveyor and should take his advice but I can't understand why there is so much difference in opinion with the damp proofers.
:wall:
We now think that we might get another surveyor in just to analyse the damp issue and then go from there.

Is this the best solution? Why is there such a difference in the damp results? Are damp proof course guarantees any good?

Any help would be gratefully appreciated.

daveyjp

Damp meters are for testing damp in timber - if he was sticking his on plastered wall he needs to get back to college and read his building survey books again where all this is explained.

He should be looking at obvious signs about why it may be damp and not simply relying on a damp meter. If I boil a kettle in a room then use a dmap meter the wall will be damp!

AGBAGB

If damp isn't coming through to the inside, I can't think it would be a problem.

Note on the damp meter reading. A piece of wood out side, but undercover can easily read 18%.

As daveyjp has said the surveyor comments (with no additional info) seem odd. Did he give any indication where the "Damp" is coming from?

bubblybee

Thanks both. No he hasn't indicated where it's coming from but presumed he was talking about rising damp as he referred to the damp proof course. Are there any particular questions I should be asking him to get to the bottom of it?

iamcornholio

Sorry, but your surveyor is not much of a surveyor if he puts a meter on a wall and cries "Damp!"

If you had a full survey, then in the report he should note the condition, discuss all the possibilities of the cause of the dampness, discount the least likely, say why certain possibilities are unlikley and then and then reach a conclusion.

Salts, condensation, defective mortar joints are all possible along with the usual obvious causes, and a host of other possibilities

The surveyor needs to clarify why he thinks it's a probelm define what is causing it, and recommend a solution

AGBAGB

You might want a quick read of this http://www.askjeff.co.uk/rising_damp.html
just for some background info.

diable

Also do a search for any posts with David Aldred may be of help.

Dry_Rot

The problem here is the damp meter. If a DPC is installed and the plaster has been done correctly then the readings should be quite low (not none existant). If higher readings are obtained then there should be a 'moisture profile' taken to help get an idea of the distribution of the high readings. This can then be compared with the evidence on site which may reveal the reason for the readings. The DPC installer will not be keen to hack off the plaster and re-install a system just based on a high meter reading. Let's face it, if the damp is only detectable with a meter, and there is no staining, salting or any visible effects, then the 'cure' is much worse than the symptoms. The installer should be put on notice and the situation monitored - if things get worse then get him back or pay for an independent surveyor. In the mean time, relax and keep all your guarantee documents safe and sound.

David_Aldred

Hi Bubblybee, please can you let me know if the walls are solid or cavity wall construction, are they brick or stone, is cavity wall insulation fitted, are the walls rendered or painted outside, are the floors suspended timber or solid floors and how old roughly the property is. Kindest regards, David Aldred Independent damp and timber surveyor

bubblybee

Hi Everyone, thanks for your really useful advice.

David - The house is 100+years old. I believe that they are solid wall construction, the front of the house is stone but couldn't tell you what the back is as it's covered in render with paint over it. I think the floors are suspended timber with some air brick ventilation underneath - although the surveyor has said not enough.

David_Aldred

Hi Bubblybee,
Thanks for the response and perhaps the following information may be of some assistance with the matter.

According to your info the house has solid outside walls and is over a hundred years old but we do not know how much older than that it is. The vast majority though not all of the UK housing stock was built with a damp proof course (dpc) whether that be a sheet material or layer of relatively impervious masonry and this does not melt away or disappear and the house survived quite well for over hundred years ago without falling down. Around four years ago a dpc contractor installed a dpc to the house and this is likely but not confirmed to be in addition the one the house was built with and the contractor issued a long term guarantee that the system would be effective in controlling rising dampness.

The contractor who recommeded the dpc four years ago will have done so on the back of using an electrical damp meter similar to the one your Chartered Surveyor has used. The meter is basically a battery with two wires leading to probes and the easier it is for the electric to run between these two probes when applied to something the higher the reading on the meter. If you stick your hand on the end of such a meter you will get a high reading but clearly your hand does not have rising dampness.

These type of meters are useful for detecting areas of concern requiring further investigation especially areas of concern that may not be seen readily by the eye. Since the only thing that is in wood other than the wood itself is usually some degree of moisture then readings taken on wood are fairly reliable unless there is preservative in there which can affect readings.

Wood over a certain moisture content will germinate decay spores that are naturally all around us in the air. Typically wood at around 28% moisture content is sufficent to germinate decay spores but the decay could be sustained once commenced down to around 20-22% the latter of which is therefore termed the threshold of decay. Generally wood below this level of moisture will not decay or if decay has already occurred that decay will become inactive. In a well built house keeping the moisture content of timbers under the threshold of decay should be achievable. The wood can even be wetter than this for short / transient periods but so long as it dries down again and does not become constantly at a moisture above this level then decay should be held at bay.

The problem really starts when folk put an electrical damp meter on a wall surface. The manufacturer of the meter supplies a scale called a wood moisture equivalent (WME) scale on the meter and what they are saying is if you had wood placed against the wall and you got a WME reading on the wall itself with a value above 20-22% it would be at risk of decay.

That may be fair enough as simply a surveyors aid to identifying areas of concern on the wall surface. However things are not as simple as that because the plaster / masonry is not wood and the readings obtained when you plonk the meter on a wall are far less relaible than when taken on wood.

For instance the plaster may have salts within it which will increase the conductivity of the moisture (that is why they use salty water to moisten the pads on the electric chair !) and hence higher conductivity readings are obtained in salty moisture than pure water. There may be traces of carbon or metal elements in the wall / plaster which would unrelaible give high readings. The greatest hassle is the amount of moisture simply absorbed from the air by the wall / plaster will vary considerably both with type of plaster and amount of relative huimidity in the room - a hard dense plaster will absorb far less moisture than an old permeable lime plaster. If foil backed lining paper has been used high readings are given across the metal foil, the list goes on.

The electrical damp meter does not differentiate between these issues when placed on a wall and neither does the meter itself differentiate moisture from any condensation, penetrating dampness, leakage or rising dampness.

Used sensibly the surveyor and the dpc contractor can take readings from the wall surface and for the same plaster / wall material the pattern of the readings obtained can simply highlight areas of concern that should be further investigated but that is all it is telling you. What the readings by electrical damp meter never do is confirm rising dampness and this is stated in the British Standard for chemical damp proof courses BS6576.

Now going back to the property in question with the above information what can we say from the limited evidence given and without actually seeing the problem.

Well we know the dpc contractor did not actually prove rising rising dampness when they said the property needed a new dpc. The contractor simply took a guess at this simply because that is what they are there to sell. If you pushed for detailed laboratory investigation the results may well be no rising damp is now present but the laugh of this is clearly the question is this because the original dpc remained effective and the new dpc was never required (i.e the damp was due to something else) or is it because the last dpc installed by the contractor is controlling previous rising damp?

If the dpc contractor justified installing their dpc system on the back of high electrical damp meter readings 4 years ago, what is the difference now as to why yet another is not required on the back of the Chartered surveyors similar high electrical damp meter readings? Well of course this all nonsense and the dpc contractor is shifting the goal posts to some degree to suite themselves. So let's have another look.

Since the readings by electrical damp meter the Chartered Surveyor obtained are still of concern to them one might be of the view that the underlying cause of such readings was perhaps misdiagnosed by the dpc contractor and because the real reason was not addressed the high readings remain.

What we know is the outside wall of concern is solid so it is inherntly prone to condensation and penetrating dampness. The design function of that wall is when it is subject to wind driven rain and condensation such moisture should be able to evaporate outwards but this may have been changed by such matters as re-pointing in hard dense cement mortar etc such that this moisture tends to permeate inwards. As it evaporates from the wall the moisture may leave behind on the wall / plaster surface salts from within the wall construction which were previously in solution and these salts may accumulate on the wall surface and as we know salts attract moisture from the air and go damp if you leave it out in a kitchen and in any case salts effect the value electrical damp meter readings.

Your surveyor says the ventilation under the timber floor is inadequate and if this is the case the timber floor is at risk of decay because the moisture vapour level under the floor will be excessive and the timber will absorb this moisture laden air until in balance with it and be at risk of germinating decay. However it should be realised that it is not only the wood which will absorb this moisture laden air under the floor.

The wall itself including above the level of the dpc if the dpc is below floor level will absorb this moisture to be in balance with it whether or not the dpc in the wall itself is effective or not and this moisture laden air will be trying to evaporate upwards into rooms increasing the relative humidity in the rooms. The easiest place for this moisture laden air to evaporate up into rooms is at the floor perimeter where it will readily be absorved by skirting, base of door frames, plaster at the base of the wall etc.

In other words a damp moisture laden air in the sub floor void will result in low level dampness to surrounding walls / plaster / wood at the base of the wall which can be misdiagnosed by a dpc contractor / surveyor as risng dampness when they get a high reading on an electrical damp meter and take a guess as to the reason for those readings especially if they are doing so for profit or they have not thought the situation through.

So we have a wall which is subject to moisture from under the suspended timber floor, the wall is subject to condensation both on its surface and within the depth of the wall itself and prone to penetrating dampness. We definately have one and most likely two damp proof courses and we have according to the Chartered Surveyor high electrical damp meter readings. The dpc contractor says the readings were low when they attended and the Chartered Surveyor said their readings were high - if both are to be believed the moisture of concern is transient. Rising dampness due to failure of a dpc would not be transient.

Last but not least of interest are the values quoted by the Chartered Surveyor of 18% readings being found. If they took these readings on a wall surface we know that the value of the reading is pretty useless as a single reading. It is more the pattern of a collection of readings we are interested in such that we are looking for areas of the same plaster type that simply have a higher reading than surrounding areas of the same plaster irrespective of what the value of the reading actually is.

The other point of note is if the Chartered Surveyor took the reading on wood we know that in any case 18% is under the threshold of decay (20-22%) and the wood will not rot at this level of moisture content. Somebody above posted that the readings will be zero if everything is okay but this is not the case as there is always some degree of moisture present which is absorbed from the moisture vapour in the air (termed hygroscopic moisture content) and nothing in a house is absolutely dry. It is more our perception of what is considered to be acceptably dry or perceived to be dry for a given situation.

I know the above is rather long winded but you did ask for info about getting to the truth of the matter. What would I do from here? Well amongst other things a good start would be to fully investigate the conditions beneath the suspended timber floor, look for issues of penetrating dampness, look at changes to the outside of the property which may have altered the way it was designed to evaporate outwards after wind driven rain penetration, test the plaster in the area of concern for salts which may reveal ground salts, salts associated with pollution, salts associated with unwashed sand used in the plaster etc. A dew point analysis to look at issues of condensation would be helpful but preferably monitoring over a few days would give a better idea of condensation problems. Look at how this property has been altered from when it was first built.

I hope the above helps at least give you some idea. Clearly without blowing my own trumpet one perhaps can see the benefit of having an independent specialist surveyor look at the problem rather than a general building surveyor or a dpc contractor looking for work (and then usually saying there is nothing wrong with their previous work). A list of such Independent specialist surveyors may be found upon the Property Care Association (PCA) website and look in the section 'find a member' and then select Freelance or Consultant (the rest are contractors looking for work) for one in your area to have a chat with. Hope this helps, kindest regards David Aldred Independent damp and timber surveyor.