Damaged mortar, poor drainage, damp

[GraemeR]

Good afternoon. We have moved into a ~1890s cottage in a Cheshire village and I'm looking for some advice on how best to deal with a damp issue. There is a persistent damp area (black mold, flaking paint) on the living room wall behind the skirting and just above it, running from beneath the window toward the corner (see first image). Note that the floor is hardwood on top of a concrete base and is about level with the lowest extent of the white render.

The mortar between the bricks below the render has degraded in a few areas (see close up) and we discovered (after scraping away the pebbles) that there is a stone/concrete base under the pebbles that goes right up to the wall (with a partially degraded layer of mortar at the join) which is possibly hindering drainage. I should also mention that the previous owners had a chemical DPC injection done on this wall.

My main questions are:
- Which material should be used for repointing the exposed bricks? (I am unsure whether they were previously done with lime mortar and whether there is any benefit in using lime given the use of chemical DPC)
- Can the same material be used to repair the join between the wall and base?
- We are thinking of landscaping the front drive - would it be advisable to remove this concrete base (and e.g. install a French drain) or could this damage the foundations?

Any comments or advice are greatly appreciated. Thanks.

 

[aockenden]

Chemical injected DPC’s are a meaningless scam, they do nothing. Therefore they have no impact on the decision to repoint / render in lime (which you probably should do).

Remove the cement, gypsum and modern paints internally and externally and redo with lime render / plaster / paints. Check the ground levels w/ a laser level and lower outside levels to at least an inch or two below interior finished floor height. Check for failing guttering or leaky windows / window reveals in the area.

If you do all that and 6 months later it’s still an ongoing issue then you could look at installing French drains along that part of the house. If that slab of whatever at the base of the wall really is cement then it will have been added much later after the house was built and is unlikely to have any interaction with or effect on the foundation. But either way, you should be careful and meticulous when removing it so you don’t accidentally cause any damage to the foundation stone / brickwork. If it is cement then I’d be inclined to remove it.

 

[a twig]

Have you found any trace of the original DPC? Peering at the photo it looks like there might be one just above the concrete, in which case the exterior ground levels are probably too high and you could do a lot worse than search on here for a Zebcolm trench and employ that.

Does your concrete floor have a DOM tied into the wall?

Definitely use lime, an NHL2 based mortar will do the job for all the repointing and repairs.

 

[Toby Newell]

It might have a DPC, it was mandatory by 1875 in London but slower to catch on elsewhere, unless you have subsidence that breaks the slate they rarely fail.

Raised ground levels are the biggest cause of rising damp.

NHL 3.5 for normal bricks, NHL 2 for sandstone, soft or spalled brick.

The paint isn't helping. The ground level probably is too high, digging a French drain (search here for zebcolm drain) could help.

You are unlikely to damage the foundations if the raised ground level was added afterwards, this is only an issue with say a 1600s cottage with little footings. You may only need to remove 100 - 150mm.

You don't want a join between the wall and the base with a French drain.

 

[Toby Newell]

Chemical injected DPC’s are a meaningless scam, they do nothing. Therefore they have no impact on the decision to repoint / render in lime (which you probably should do).

Sorry old chap, this is incorrect, it is a fraud promulgated by a prolific fraudster called Peter Ward of Historic House fame, a real flat earther. Yes, it is true the 1970s and 1980s were prime time for fraudsters. Rentokill et al made a killing suggesting spraying entire houses needlessly with pesticide when increased heating and ventillation would of done the trick or selling DPC injections when the culprit was raised ground levels, penetrating damp and impervious coatings. However, two frauds do not make a right. It has been illegal for over 100 years, and almost 150 in London to not fit a DPC. Rising damp is a 'thing', it is real, and it can be solved with a modern chemical DPC, to say it doesn't work is a lie. It's simply that it might not always be the correct solution.

Modern injected DPCs are more likely to be slowly dissolving silicate sticks which you can do yourself. These are very effective and cause minimal damage.

There is nothing wrong with them at all rather the issue is with unscrupulous contractors. A DPC chemical treatment should be last, not first on the list, there is no point drilling into engineering bricks or hard Lancashire reds as the chemical will take years to permeate.

Also the standard schedule requires hacking off 1200mm of internal plaster (used to be 900mm then 1000mm) and replacing with a waterproof render, normally sand and cement and a silicate proofer like Sika 1. This obviously reduces permeability and is really not needed in most cases unless there are a lot of transported salts. This is more of an issue than the modern DPC sticks.

Just FYI.

 

[fallschirmjager]

I also thought these DPC injections were a total waste of time after watching Peter Wards' videos but I can attest that Dryrods definately work if no other solution is really viable. They obviously do not fix the root of the problem though which should always be the first port of call.

I had the same issue as the OP and sorted multiple damp spots by removing all the concrete render from the exterior of my house, lowering ground levels in some areas and installing a French drain. Concrete render removal will probably be a bit of overkill (and costly) in the OP's case.

 

[aockenden]

Sorry old chap, this is incorrect, it is a fraud promulgated by a prolific fraudster called Peter Ward of Historic House fame, a real flat earther. Yes, it is true the 1970s and 1980s were prime time for fraudsters. Rentokill et al made a killing suggesting spraying entire houses needlessly with pesticide when increased heating and ventillation would of done the trick or selling DPC injections when the culprit was raised ground levels, penetrating damp and impervious coatings. However, two frauds do not make a right. It has been illegal for over 100 years, and almost 150 in London to not fit a DPC. Rising damp is a 'thing', it is real, and it can be solved with a modern chemical DPC, to say it doesn't work is a lie. It's simply that it might not always be the correct solution.

Modern injected DPCs are more likely to be slowly dissolving silicate sticks which you can do yourself. These are very effective and cause minimal damage.

There is nothing wrong with them at all rather the issue is with unscrupulous contractors. A DPC chemical treatment should be last, not first on the list, there is no point drilling into engineering bricks or hard Lancashire reds as the chemical will take years to permeate.

Also the standard schedule requires hacking off 1200mm of internal plaster (used to be 900mm then 1000mm) and replacing with a waterproof render, normally sand and cement and a silicate proofer like Sika 1. This obviously reduces permeability and is really not needed in most cases unless there are a lot of transported salts. This is more of an issue than the modern DPC sticks.

Just FYI.

I’m extremely skeptical, as the story on them makes little sense. How are chemicals injected into spaced holes on either side of a 2-3 foot thick, roughly stacked stone wall supposed to magically disperse throughout that stone wall (including through gaps, clay mortars, etc) and form a cohesive impenetrable horizontal layer through the masonry? I call BS. By what mechanism is that supposed to occur? Capillary wicking action? What happens if there’s accidental horizontal capillary breaks / gaps in the mortar? It’s maybe slightly more plausible when it comes to single-layer brick built properties which are more uniform in masonry structure and are obviously thinner depth of wall, but I still think there’s little reason to believe it would effective. Regardless it remains irrelevant to the question of whether to redo in lime or not.

 

[Toby Newell]

Sorry, I forgot. It is also important to realise that you have a certain moisture 'load' in a building structure, on ground level this is borne mostly by the walls and the floor. If, for example, you have a concrete slab with a DPM this can push more water up the walls as the moisture under the slab cannot expire via the large top surface. This is why concrete slabs and DPMs retrofitted to very old properties can cause issues with rising damp.

By the same token, retrofitting a DPC treatment to walls into a very old property will reduce the expiration of water from the walls and drive more to the floor which might increase damp in joist via piers / dwarf walls with no or imperfect or failed DPCs (often slates if any, which can come loose) or to a retrofitted slab.

Retrofitted DPCs are very often not the right answer.

But sometimes they are.

 

[aockenden]

You said 'it is a meaningless scam they do nothing' this is incorrect, they do work and have been on hundreds of thousands of properties.

It isn't magic, it's science. (Takes deep breath)

First of all you are making things up! the OP has a late Victorian brick built cottage which probably has 9 inch cavity walls, which make up the majority of the UK domestic housing stock, why you mentioned your amazing 2 foot thick (now 3 foot) stone walls I have no idea. But yes, you are correct, an injectable DPC would not work well in your application, but the OP doesn't live in your lovely cottage

Interesting you call BS, ie a scam, on something you don't know anything about! That's like saying the earth is flat when you know nothing about physics or astronomy! You cannot say "that's not true, by the way please explain what you are talking about'! Here we go (I hope you are ok with me dispelling myths)

1. These treatments are primarily meant for normal single and double skinned brick walls and some thinner, regularly spaced stone walls (like the OPs), NOT 2 foot thick irregular stone walls (like yours)
2. The older polyoxo aluminium stearates (POAS) were injected under pressure into the brick, these creams did not penetrate well and have mostly been superseded by better technologies, these are possibly the poorly performing products that were widely over prescribed by unscrupulous contractors that you are railing against, but yes their method of action was capilliary. The problem was that this product didn't work properly in hard stone or engineering bricks.
3. The modern system uses various forms of siloxane, which are basically silicone hybrids, these are not injected at high pressure and not into the brick, rather they are pumped in a normal skeleton gun or via rods inserted into the mortar course laterally. The siloxanes dissolve and move via capilliary action through the single mortar course forming a breathable but waterproof barrier. They can take several months to disperse through NHL 5 and hard cement mortars.
4. Breaks in the mortar can reduce the effectiveness yes (the siloxanes are designed to diffuse up to 80mm in all directions bridging any small gaps). But anyway, any dpc with a few cm of breaks is better than no dpc if in fact a dpc would be helpful.

5. I never mentioned lime! My rebuttal was purely your incorrect dismissal of dpc treatments despite admitting you had no idea how they worked. Lime pointing may or may not help. The main issue for the OP is probably raised ground levels and the external render reducing permeability.

6. Modern DPC treatments work very well when installed correctly and can be done by a confident DIYer, they are not based on magic but years of research by scientists!

7. However. I repeat. Having worked in the building and conservation industry I can tell you that well over 50% of builders are either a. Ignorant, b. Duplicitous or c. Both at least some of the time and that this percentage is much greater in the conservation and renovation sector and in specialised remedial sectors like the damp proofing industry. Often a 'surveyor' is really a salesman, he doesn't get paid to tell you that a French drain might be required, or that your airbricks are blocked, or that you don't have rising damp, you have penetrating damp caused by poor guttering. They are incentivised to sell you a DPC treatment that may have little to no benefit. They also may advise hacking off your internal plaster and apply waterproof rendering underneath citing 'salts' when a better course of action would be to wait 6 months and see if salts are actually an issue. This is because a large proportion of the industry is disingenuous and / or ignorant. The profit margin on free advice is much lower.
8. Do bad actors and inappropriately prescribed DPC treatments affect the efficacy of the treatments themselves? Of course not. Where appropriate they can be very helpful.

9. I would say that a modern (non POAS) injected / rod type dpc treatment should be last, not first on the list of remedies but if done correctly can work and be very helpful in specific situations. Let's be clear, if you already have a DPC, and no obvious heave / subsidence / cracks then a second DPC is unlikely to help, DPCs rarely fail randomly on their own without substantial movement. Rather than fail via breakdown, often a DPC is bridged in cavity walls by falling snotting from sloppy bricklayers or by inappropriately installed cavity insulation. Occasionally inserting a dpc above the bridge can help (after investigation via boroscope) with wet cavity insulation often removal is the only remedy.

Zealots on both sides do not help. Lime is a fantasic material but it isn't magic, neither are dpc treatments, rather a measured, intelligent, hollistic approach is required.

I use traditional methods, tools and materials, I have 100 year old chisels, use hand scrapers and hand sanding, wax beaumontage fillers, shellacs and beeswaxes and hand made swan quill brushes. At the same time I also use the latest modern tools and machinery, carbide scrapers, polyester and epoxy fillers and two part curing finishes. One isn't better than the other, I embrace both, you just need a modicum of expertise and experience to know what to use and when.

I think there are a couple valid points here and I appear to have touched a nerve, so I’ll clarify my position:

IF…

1 - you have a cavity wall
2 - the two brick skins are made from regularly spaced bricks that are contiguously mortared together
3 - you apply the stuff to the internal skin when trying to stop internal rising damp and below the internal floor level
4 - the injected chemicals ACTUALLY behave within the capillary networks as the manufacturers claim (would love to see independent studies on that)

…then it at least has the barebones possibility of being useful. In all other cases it seems indefensible and harmful.

 

[Toby Newell]

I think there are a couple valid points here and I appear to have touched a nerve, so I’ll clarify my position:

IF…

1 - you have a cavity wall
2 - the two brick skins are made from regularly spaced bricks that are contiguously mortared together
3 - you apply the stuff to the internal skin when trying to stop internal rising damp
4 - the injected chemicals ACTUALLY behave within the capillary networks as the manufacturers claim (would love to see independent studies on that)

…then it at least has the barebones possibility of being useful. In all other cases it seems indefensible and harmful.

No nerves. Just facts. You said injected DPCs did not work and had no mechanism of action despite not knowing how they worked.

They do work. Their method of action is well understood.

The issue is in 99% of cases they are missold and are of little to no benefit and in some cases deleterious.

Simple.

 

[Toby Newell]

In conclusion, over the last 30 something years I have conducted condition surveys at thousands of period properties, mostly looking at floors, almost all of which had some extinct or extant damp damage, notably shrinking, rusted nails, iron staining and worm galleries and occasionally rot. I taught myself to learn about damp in order to assist my customers and along the way have been informed by hundreds of their case studies, both good and bad. I have made thousands of suggestions to hundreds of customers, some corroborrated by subsequent professional surveys.

Never have I suggested fitting a chemical DPC!

Still doesn't mean it can not be very helpful in some cases. Many of my clients have the means to remove 16 large dumper trucks of concrete from their back garden (true story) or spend 9 months (true story) digging their basement and fitting limestone and UFH over limcrete, some people, less so, for them a limited treatment might give them some relief and quiet enjoyment whilst they save up or until a richer owner takes over.

 

[GraemeR]

Have you found any trace of the original DPC? Peering at the photo it looks like there might be one just above the concrete, in which case the exterior ground levels are probably too high and you could do a lot worse than search on here for a Zebcolm trench and employ that.

Does your concrete floor have a DOM tied into the wall?

Definitely use lime, an NHL2 based mortar will do the job for all the repointing and repairs.

Thank you for your reply. I can't see any obvious sign of the original DPC. The exterior ground level is approx. 20cm lower than internal floor, although I suspect both have been raised slightly since the original build. I'm tempted to (carefully) remove some of the concrete to see if I find anything resembling the original DPC. I found the main Zebcolm drain thread on here - I'll definitely give that a closer look.

Excuse my ignorance but I don't know what 'DOM' means.

 

[GraemeR]

It might have a DPC, it was mandatory by 1875 in London but slower to catch on elsewhere, unless you have subsidence that breaks the slate they rarely fail.

Raised ground levels are the biggest cause of rising damp.

NHL 3.5 for normal bricks, NHL 2 for sandstone, soft or spalled brick.

The paint isn't helping. The ground level probably is too high, digging a French drain (search here for zebcolm drain) could help.

You are unlikely to damage the foundations if the raised ground level was added afterwards, this is only an issue with say a 1600s cottage with little footings. You may only need to remove 100 - 150mm.

You don't want a join between the wall and the base with a French drain.

Thanks for this and the insights on chemical DPCs - it's been very helpful indeed.

Moving more pebbles at the side (location of my last pic above) has revealed a set of 2'x4' concrete pavers, so clearly not original. I think the next step would be to take them up and look at adding in a Zebcolm-like drain. It will be interesting to see also if there is any sign of the original DPC.

 

[a twig]

Thank you for your reply. I can't see any obvious sign of the original DPC. The exterior ground level is approx. 20cm lower than internal floor, although I suspect both have been raised slightly since the original build. I'm tempted to (carefully) remove some of the concrete to see if I find anything resembling the original DPC. I found the main Zebcolm drain thread on here - I'll definitely give that a closer look.

Excuse my ignorance but I don't know what 'DOM' means.

DOM is a typo, certainly no ignorance on your part, just fat fingers on mine!

Should say DPM, damp proof membrane. If a concrete floor is installed internally you’d expect to have one tied into the DPC like this:

 

[a twig]

Good luck with your investigations, fingers crossed!

 

[GraemeR]

DOM is a typo, certainly no ignorance on your part, just fat fingers on mine!

Should say DPM, damp proof membrane. If a concrete floor is installed internally you’d expect to have one tied into the DPC like this:
View attachment 12398

Thanks for clarifying. I don't recall seeing any evidence of this, but I am planning to partially take up and re-lay the wood floor to get rid of gaps that have appeared, so will have a closer look. I can see that without the DPM, the concrete may potentially be bridging the DPC and contributing to the damp issue.

 

[Toby Newell]

Working in a house now where they laid a concrete drive without digging down, they left an edge gap, but filled with crushed slate above the dpc at the same level as the airbricks, all floors are damp, salts bubbling paint, front elevation bricks gone green from the rising damp.

It's not that rising damp isn't common, it's that the solution is almost always reducing ground level and increasing drainage.