How far back should you dig out a damp course?

totally_organic

I've recently bought a c1900 terrace. I've noticed a small amount of damp around the back door, and on further investigation have found my damp course (slate) is about 1inch below ground level. I know that it should be about 6inches above ground level... so my question is how far back do I need to take it when I dig it out? Do I need to dig out the whole back garden to bring it down by 6inches (mostly concrete so wouldn't be fun), or is a couple of inches back from the wall enough? Just concerned that if I don't dig back far enough from the wall all I'll do is create a channel for water to sit in and probably make the problem worse...

Any thoughts much appreciated!

Dave

ormus

a 6x6 inch mini trench should be enough. try and make a spillway all the way to the back street to drain any heavy downpours.

you could use a buried PVC pipe to the back gate/street, too?

David_Aldred

Hi Dave,
You do not say whether this is a cavity wall or solid wall and whether the internal groudn floor is solid or or suspended timber - please could you clarify in order to give an appropriate answer to your query. Kindest regards, David Aldred Independent damp and timber surveyor

totally_organic

Hi David,

Its a solid wall (can't say that for certain, but I think it is), and suspended timber floor. There is a step which has been built onto the back where the door is right up against the wall (above damp course level) which I assume needs to come off?

I could take some photos rather than try and explain if that is likely to help?

Thanks again,

Dave

David_Aldred

Hi again Dave,
As you indicate the Building Regulations suggest 150mm (2 brick in height) be maintained between the level fo the damp proof course (dpc) and the level of surrounding ground though this is not always achievable. The reasons I asked for construction were the outer leaf of a cavity wall can tend to do what it likes (to some degree) so long as the cavity is clear to below the dpc level that is lowest (usually that to the inner leaf under the floor joists).

The situation you describe is not uncommon where paths are raised over their original levels and sometimes worse slope down towards the property instead of away from it. The slope of the paths allows rainwater collecting on surrounding ground (which can be quite an area and hence volume of water) to bear against the house walls and even hydrostatic pressure. The increased height of the paths can result in debris bridging the dpc and wind driven rain bouncing off surrounding paths doing likewise. With solid walls usually where they are minimal in width such as around one brick (9" / 215mm) this will obviously not help internal dampness and the situation i smore critical than with cavity walls discussed above.

Ideally as you indicate where possible paths should be lowered to 150mm below the original dpc and then slope away from the property to surface water drain gully away from the house (th elatter to be provided where not present and please not below groudn drain work may require approval from the Local Authority Building Control Officer).

I appreciate the layout of the rear garden does not always make this easy / cost effective and anything less is something of a compromise with an increased risk of dampness especially with a solid wall.

However what I certainly would not recommend is digging a trench next to the house walls and backfilling with chippings as a cheap alternative. This is often called a french drain and not only do they often block with silt over time but if you think about it, all you are doing when you do this is making a sump next to the house walls that the surface water from surrounding ground will flow into and then be directed uder the dpc of the wall and into the foundations / sub floor voids.

This causes moisture to flow / evaporate through the foundation brickwork below dpc and into th esub floor void along with moisture evaporating up from the earth oversite beneath the suspended timber floor. If sub floor ventilation is marginal the result will be an increase in humidity to th esub floor void / decay of ground floor timbers which can include dry rot, increased risk of wood boring insect attack from those insects which prefer to feed upon damp decaying timber and increased humidity within rooms with resultant increased dampness / moudl growth. In effect you will tend to make damp / decay / mould / risk of wood boring insect attack worse instead of better.

If paths cannot be lowered over the whole garden then lower as great a width from the area of concenr as possible and again slope the new paths away from the house and into drain gullies away from the house be that a single drain or a modular drain trench system such as Aco or similar (if you do not know what I mean by these look on:

www.acousa.com/homedrain/index.htm

they are of a type you see when driving over petrol station forecourts etc.

If even this is not possible then put the Aco or similar modular drain trench system next to the house wall and again run the outlet to the nearest surface water below ground drain but ensure the joints to the house / surrounding ground are sealed with flexible impervious material that will accomodate the inevitable differential movement between the drain / house / surrounding paths as they will move seperate to each other over time. The drain system should be laid to a self cleansing fall. Hope this helps, kindest regards David Aldred Independent dampness and timber surveyor

totally_organic

Wow David - thank you ever so much for such a complete answer, this is incredibly helpful indeed and very kind of you. I have taken some photos so if you get a chance to look and feel there is anything else I should do, I'd really appreciate your input http:// s938.photobucket.com/albums/ad229/totally_organic/ (take the space out after http:// as this site won't let me post proper links at the moment so have had to add it in!)

I didn't give the full picture in my original post as hadn't expected someone as competent as yourself to reply, so there are just a couple of extra things to mention. There is a section of wall which must of been added at some point to fill in a doorway. This has its own little bit of damp course which is 1 brick level higher than the houses original damp course. I assume I should still dig out to the level of the original damp course around this?

There has at some point been some injection damp proofing 2 brick levels higher than the damp course on a different section of the wall. Again, I assume I should just ignore this and dig out the original damp course properly?

Looks like my week off work is not going to be as relaxing as I'd hoped...!

David_Aldred

Hi again Dave,
Just had a scout through your pics and obviously cannot see everything so comments are limited to what I can note from the pics and in addition to previous comments.

1. Walls are painted externally which will tend to reduce evaporation outwards. In addition it appears some areas have been patch re-pointed in a cement type mortar though the latter is not confirmed. Wall looks to be around one brick thick.

These types of wall rely heavily upon evaporation outwards. That is to say the wall is cold so it is prone to condensation on its surface and within the depth of the wall itself and also prone to penetrating dampness. Any condensation or penetrating dampness you want to be able to vent off outwards and not be driven inwards. The wall was designed to get wet with wind driven rain and then vent off during more favourable weather through both the brickwork and breathable mortar.

Painting the wall externally is often prone to flaking / only done part way up the wall so it is still prone to some degree of penetrating dampness and application of hard dense cement mortars / suspect paint retards this venting outwards. The moisture resevoir in the wall goes up and may be sustained. If the wall contains water in its pores instead of air it is colder and more prone to condensation such that dampness within the wall may increase. As the moisture goes up freeze / thaw damage blows the front of the brick faces off. The sun hitting the wall (if it does) may drive the moisture inwards. Obviously the base of the wall is where all this is going to collect.

2. I note a garden or some other wall built into the rear house wall. Given the house wall is solid then the garden wall (if that is what it is) will act as a wick feeding dampness into the main house wall. The vertical chemical dpc at the garden wall / main house wall junction (if there is a vertical dpc here) will not usually resist moisture moving under hydrostatic pressure as they are usually a pore lining material not a pore blocking material given that their function is to control rising damp through capillarity and not penetrating damp under hydrostatic pressure. So as the garden wall gets wet with rain it soaks down and into the house wall because there is no barrier unless there is a pitch polymer or other type of impervious material here.

3. The patio doors are built to the outer face of the one brick thick solid wall. There will be no vertical dpc here to prevent moisture wicking around the perimeter frame of the doors such that the internal reveals are only around 2" / 50mm from the outside. These door reveals are therefore prone to be very cold / acute condensate and also prone to acute penetrating damp even though the silicone sealing the frame to the wall may be in good condition. Accordingly there is a high reliance upon the plaster to hold back any penetrating damp / salts of construction.

4. If the tiled floor is a suspended timber floor beneath I cannot see any sub floor ventilation to remove moisture laden air from the sub floor void beneath the floor. Looking at the outside ground which as you say is high there is a tendency for ground water to permeate into the sub floor void and accordingly a heavy reliance upon the sub floor ventilation to remove the moisture laden air before it increases the moisture content in the floor timbers to above the threshold of decay. If there is a damp sub floor void that has a tiled floor over the floor timbers above it will tend to want to evaporate upwards wherever it can and at the perimeter of the wall there is your porous plaster ready and waiting to absorb such moisture.

5. As the moisture evaporates form the wall it brings with it contaminates such as salts which accumulate at the point of evaporation degrading plaster and finishes. Once this plaster is contaminated it will not recover. The salts will atract moisture from the air and from what I can see this is a kitchen area so the relative humidity will be high even if transient and therefore this is only likely to compound the above issues.

I am not going to spell everything out for you as to what to do because I cannot see the whole elevations / sub floor voids / type of plaster / surrounding buildings etc and these type of things all of have a bearing upon what to suggest by way of resolving things. for a specific case

However you might want to have in mind:
A) making the outer face of the wall breathable but able to resist liquid moisture penetration.
adjust levels / drainage as previously discussed.
C) If the intersecting wall is a garden wall / party wall chat with adjoining proeprty owner / agree under Party Wall Act cutting the wall back from the house and either leaving cut back or rebuild but isolating from house with vertical pitch polymer dpc.
D) Ensuring sub floor ventilation is adequate / check sub floor void timbers.
E) Ensuring sealant around door frame is adequate.
F) Options to replace plaster with a type that is able to be both waterproof and hold back any salts within the wall. However this would not make the walls any warmer and condensate issues could still therefore be troublesome especially at door reveals. Also the workmanship has to be very high / plasterer wants paying whilst wall still wet / plasterer will blame anyone but themselves if it does not resolve.

Alternatives are variations of insulated dry lining (avoiding hygroscopic dry wall adhesive) that has a vapour barrier all fitted to studded polyethylene memebrane with membrane fitted to wall. Crucial the system has an effective (and I do mean fully effective) vapour barrier at all junctions / edges / penetrations to prevent at all costs warm moist air entering the system or it will condense out and cause damp / mould / degradation worse than before. In addition it is often prudent to allow for venting on the cold side of the system externally sufficient to prevent any interstitial condensation. By the way the tiled skirting will have to be removed during all of this.

Address high humidity issues to kitchen by both background ventilation and installation of humidistat controlled extractor fan min rated 150mm diameter 60 litres / second air exchange set to operate below 65%RH and min 15 minute overrun. Cook / boil etc with kitchen to rest of house door shut.

Not exhaustive list and the above comments are general principles only given in good faith and not to be acted upon for specific cases but hope this helps a little with some background info other than pointing the finger at the poor old dpc the house was built with, which is usually generally satisfactory other minor cracks it may have suffered with settlement. Kindest regards, David Aldred Independent damp and timber surveyor.

totally_organic

David I cannot quite put into words how much I'd like to thank you for taking the time to write back. Your comments are extremely helpful.

Given that I only bought this house around 2 months ago, and had a full structural survey which failed to identify any of the above (including failing to identify that a slate damp course existed at all, assuming it must be blue brick - despite sections of the dpc clearly visible eg. at site of drain further round side of building), would I have grounds for asking for some kind of compensation. Clearly knowing all this would have affected the sale price I was prepared to pay, although I have no idea where to start in terms of my rights to compensation and how much to ask for.

David_Aldred

Hi Dave,
The liability of the surveyor is to act with reasonable care and skill of a person in their profession whether that survey is paid for or the advice is for free. The Chartered surveyors have a complaints / arbitration scheme through the RICS but this is rather in house and akin to getting one solicitor to sue another. The vast majority of claims against surveyors never get to court because their professional indemnity usually swallows the pill before that stage and coughs up or if the surveyor is in a panic about how this will effect their future PI premiums they may fork out for any negligence themselves.

The amount you can claim is usually limited to the difference in the price you paid for the property compared to the value you should have paid had the faults been known to you.

What you need to ask is was it reasonable for the surveyor to have picked up on these issues? Was there a paper trail of evidence that the surveyor should have followed? Did they recommend you get advice from a damp / timber independent surveyor? What exactly did they notify you of?

If I can spot the issues described above simply from a photograph you posted a link to then it reasonable to suggest that your surveyor to whom you have paid a considerable amount of money for a full structural survey should have spotted all these things too and in fact a good deal more considering they were actually on site. They then had a duty of care to make you aware of all of these issues and their extent or if they could not fully investigate them place you on notice of the risk and recommend you budget for a waste case scenario until proved otherwise to prevent you suffering a loss.

The excuse / defence of them recommending a damp / timber surveyor further inspect within their report (if indeed they did) may not suffice for general defects that any competant surveyor acting with reasonable care and skill should have made you aware of, otherwise they might as well stay in their office without ever visiting the property and say to you to obtain electrical / gas / drain / tree / damp / timber / roof surveys all done by others and they just sit back in their suits at their desks and charge you a hefty fee.

Your first complaint to the surveyor will usually be met with a standard denial of liability so don't be put off and take some legal advice but the threat of being put into court is usually sufficient for the surveyors insurers to make you a reasonable offer if your claim of negligence is reasonably justified. If you want to go for a small amount of money the small claims court is very cheap for you to bang a claim in but there are limits on the amount you can claim through this process. All of this is just personal opinion and specific cases may well vary such that the comments made herein should not be acted upon other than a recommendation to review your structural survey and take appropriate legal advice about whether a claim is likley to succeed. Kindest regards, David Aldred Independent dampness and timber surveyor.

ormus

youve got two chances of claiming off a surveyor.
bob hope and no hope.

David_Aldred

Hi - Anyone can make a mistake in their job but where a surveyor has been clearly negligent in their duty of care owed then they are insured to cover such an event so be sure of your facts, stick to them, ensure that your argument is justified and reasonable - that is all your legal advisors and a judge are looking for. There are literally thousands of successful claims against surveyors and that is only the tip of the iceburg considering the majority are settled by the surveyors' insurers before they reach court because it is often far cheaper to do so than argue the case with barrister fees etc even if the surveyor feels they might win.

If your claim is clearly unreasonable, malicous, heresay, you are simply after somebody to blame and you cannot back that claim up with truthful facts, then quite rightly you are not going to succeed. Kindest regards David Aldred Independent damp and timber surveyor