Wall Base Plinths – Are they a Good Idea?

05/08/2015

–

Joe Malone

–

Are Wall Base Plinths A Good Idea?

Wall base plinths are generally poorly understood and I attribute this to the confusion caused by a damp proofing industry fad for installing cementitious wall base plinths in the 50’s, 60’s and 70’s. It was a perceived treatment for wall base damp long before retrofit chemical injection became heavily marketed. The basic idea for installing brick wall base plinths, as seen in many thousands of old properties, was a good one, but a whole damp remediation industry ran with a sound principle and because they didn’t understand it, they ruined it. Original brick plinths generally preceded the widespread installation of a physical damp proof course, though you will occasionally find both an original brick wall base plinth and a physical damp proof course, usually of slate. Generally speaking, our experience has been that if you see an original brick wall base plinth then there won’t be a physical DPC installed.

OLYMPUS DIGITAL CAMERA — Original brick plinths work well if correctly maintained. This one was built in 1880.

The thicker wall base gave extra protection against rainsplash and penetrating damp and generally acted as a larger buffer for damp. If you read my last blog then you may remember that I talked about buildings being built on the ‘overcoat’ principle. Well this is precisely how original wall base plinths work, they are generally constructed with lime mortar and so long as breathability is maintained then this thicker ‘buffer’ zone at wall base does a great job of protecting against wall base damp. Unfortunately a remedial damp proofing industry latched onto this idea and started to install retrofit cementitious plinths in the mistaken belief that these too would help protect against wall base damp, which couldn’t be further from the truth. Anecdotally, retrofit cementitious plinths are said to help ‘shield’ the wall base from rainsplash but that function is outweighed by virtue of the fact that they shield against moisture evaporation from wall base.

When it comes to protecting against wall base damp, wall base ventilation and subsequent moisture evaporation at wall base are absolutely critical. If a retrofit cementitious wall base plinth is installed then this critical ability is lost, moreover we often see retrofit plinths that have actually bridged an existing physical damp proof course so now you have double trouble, a bridged DPC and zero wall base ventilation.

Screen Shot 2015-08-05 at 00.48.37 — Retrofit cementitious plinth bridging original physical DPC

Occasionally we also see heavily spalled brickwork to original brick plinths, which results in a render coat being applied to form a new cementitious plinth over the top of the original brick plinth, an action that will most likely lead to severe wall base damp, as in the images below.

To round up, if you see a retrofit cementitious plinth installed then you should have it removed. If an original brick plinth is installed then you should respect the importance of this technical detail and maintain its breathability by keeping the brickwork bare and only repoint using breathable lime mortar.

34 responses to “Wall Base Plinths – Are they a Good Idea?”

Sean Quinn
05/08/2015
Good article Joe.
You so often see rendered plinths and I wondered if this was in part caused years ago by the wheels of horse carts. Could it therefore be, that the purpose of the plinth was not only for damp protection but also to buttress the base of the wall to protect the building?
Reply
1. Harry Hunt
  01/01/2021
  Thanks for the read, very informative. I’m taking on a Victorian semi which has been ruined by countless ‘damp specialists’ using their PCA ‘qualification’ to justify their work. I wish the industry was correctly regulated or the public educated on these cowboys ruining beautiful historic architecture…
  Reply
Bruce Maltby
05/08/2015
very good article and so true, this is a subject we very often discuss .
Reply
1. Joe Malone
  07/08/2015
  There could be something in this Sean, though I see some cementitious plinths installed where traffic damage could never have been an issue.
  Reply
Bryan Hindle
07/08/2015
An interesting article Joe,
However, as a long term damp specialist, I don’t see how people’s lack of understanding of plinths can be attributed to the ‘damp proofing industry’. Where is the evidence that the industry promoted cementitious plinths? I’ve never met a PCA member who has installed or recommended one, in all of 38 years trading. That’s not to say the odd case hasn’t been suitable, for specific reasons, but I’d be willing to bet that the vast majority of these plinths were repaired/installed by general builders – not damp proofing specialists.
The image of a concrete repair of a failed spalled plinth is typical of a builders repair. In addition, we’ve all seen builder’s render over a DPC to the ground, thus bridging the DPC. Then along comes a good surveyor like you, or a responsible damp specialist like me and the render is cut back to expose the DPC. Some years later a builder re-renders the exposed brickwork a he thinks the naked lower section is causing ‘base wall damp’ (whatever that is), so he makes that good, starting the whole thing off again.
The image of a retro-fit plinth looks like a misguided attempt to carry out the above… The building trade is awash with crack pot ideas which no self-respecting damp specialist would recommend or try to promote. Yet the papers are full of adverts for magical ‘green’ damp proofing using all sorts of untested and dubious methods. No doubt in a few years these will be photographed and it will be said that an out of control damp proofing industry promoted this. In fact it’s the poor consumer laws and lack of statutory control which allows such rubbish. All you and I can do is promote good practice and with blogs this can be done, for everyone’s benefit.
keep up the good work.
Bryan
Reply
1. Joe Malone
  07/08/2015
  Perhaps you need to read the article again Bryan because at no point do I mention ‘specialists.’ I use the term ‘industry’ as a blanket term for anyone that carried out building works related to the management or treatment of damp. You know that the damp industry has changed significantly since the sixties and seventies. There never used to be a need for ‘specialists’ until the myth was created that rising damp was a common occurrence, at which point a ‘specialist’ industry sprang up devoted to treating a problem that was in fact incredibly rare. Most of this work was done by general builders that formed part of the damp proofing industry at that time. The great irony in all this is that ‘specialists’ have been installing cementitious renders for many years on internal walls and causing many similar problems to those caused by external plinths, this too is a ‘crackpot’ idea in all but rare circumstances. The whole specialist industry was devoted to managing, rather than curing the problem because a management solution means sales of chemical injection and internal waterproof tanking systems. A large portion of my working life is spent unspecifying inappropriate systems specified & installed by ‘specialists’ Bryan and yet every time I write something relating to damp I get this sort of unnecessarily defensive response. I know you’re very good at what you do Bryan but you can’t defend a whole industry, nor should you.
  Reply
  1. Lindsay Chambers
    30/11/2023
    I live in a 1900 converted church with a plinth around the whole building. The plinth has a horizontal slot. However in one corner there is no slot and this section looks retro-fitted and there is damp on the internal wall in this area.
    I would love your opinion on this and can supply photos
    Reply
  2. Geoff
    27/03/2024
    The word rising damp was created after the health laws 1875 for DPC introduction in all builds to protect from sewage and poor drainage NOT rising damp, the by laws don’t even mention the word R/D
    Reply
Bryan Hindle
08/08/2015
Good morning Joe,
I’ll drop you a line or bell you to chat about this. Thanks for you closing statement and I really appreciate that. As I’ve always said and written, good practice should be promoted and I also support blogging as a way of spreading this message. Those who take the time to sit down and write up posts like these should be supported.
I have over 170 posts on my blog. Many people question things I’ve written and I love that – I learn stuff from it and it’s a good thing. If I may say, with respect Joe, I do not defend an entire industry, but you blame an entire industry for almost every woe, where damp is involved. Can you see that this is unfair to those members of it who work very hard to improve standards and drive out bad practice?
I’m merely pointing out that issue, as a friend in the trade. I see and your response as ‘defensive’ too, especial if you then go on to justify the posts blanket statement, with tales of how terrible the industry is in your answer. We all know about that Joe, but many of the existing specialists I deal with have no part in that. I jump on bad ‘specialists’ and through my blog, I have given consumers lots of information to help them weed out these people wherever possible. , though I know some exceptional one’s too. I’m sure the post wasn’t written to be provocative but it may look like that, if a challenge or critique of anything you write is met with such a response. If you damn a whole industry as bad, you cannot then say that a member of it who speaks up for himself and the better elements in it is wrong to do so and is being overly ‘defensive’ That suggests that anyone in an industry which is attacked in this way, should just shut up and take it. That would be wrong on many levels because it stifles debate and is worse for everyone. It misleads consumers too and actually makes life harder for the good ‘specialists’ out there who get tarred with the same brush. It certainly hurts the better specialists more than the cowboy elements, simply because the better specialists are engaged in the media, whereas the cowboys tend to rely on ad-words to get work, as they don’t get any genuine referrals. Let’s damn those who should be damned and applaud those who choose the better path (even though nobody is infallible). I am not trying to stop anyone writing whatever they like – free speech and all that. Blogs are not discussion forums, because if they become that, it actually damaged the blog itself as it quickly becomes and site for the ‘in crowd’ to ague on, rather than something students and consumers can use as a resource. It’s the posts that count and your post is a good one. I am sorry if you thought I was attacking you in any way Joe, I am not 🙂
As i said Joe, keep you the good work and I hope the new blog is a great success, you deserve it.
We must speak soon, enjoy your weekend.
best regards
Bryan.
Reply
Joe Malone
08/08/2015
Morning Bryan,
Perhaps there is an inevitability about this but I will make my position clear on the damp proofing industry before moving the blog back to its intended purpose, which is primarily technical. I am not an advocate or a supporter of the damp proofing ‘industry’ as it currently exists. I don’t personally recognise the unregulated trade body that promotes the industry, nor do I recognise their unregulated trade qualifications; ‘qualifications’ that can be attained after attending a three day course. Does that mean that all members of that particular body are bad? Not at all, there are many like yourself you have attained a level of expertise through pragmatic site based experience and that I do recognise. I do not think that Chartered construction professionals have any business deferring ‘specialist’ survey work to members of a trade body who are invariably significantly less qualified than themselves to diagnose the problem. You say that you ‘jump on bad specialists’ through your blog and perhaps the situation is similar to a parent criticising one of their children but not being happy when a non-family member makes the same criticism. The industry likes to, and indeed is self-regulating, and your comment is illustrative of that fact.
I will continue to critique where I find justifiable cause but you can be sure that the industry trade body will never be directly mentioned on this blog and neither will it ever be used to promote their members or services. The fact is that where I find defective and poorly specified work, sometimes I know that the installer is attached to a trade body and sometimes I don’t. Whether they are or not is hardly a point worth making but they are still part of a poorly regulated damp proofing industry and that point is always worth making.
Kind Regards
Reply
John May
12/03/2017
Hi Joe,
Thanks for writing such an informative blog.
We bought an old Victorian village school, a little over a year ago. We have a lot of work ahead of us as it has been poorly maintained. So far, we have just about managed to rake out all the cement mortar and replace with lime, turning or replacing spalled bricks along the way.
The building has a cementious rendered plinth supposedly protecting the lower courses of bricks below the damp proof course. This is starting to fall away revealing spalled bricks. Short of replacing these bricks (a monumental task), what are the options? Could I remove the old cement and replace the render with a lime mix, perhaps with a gravel filled ditch surrounding the house?
Thanks.
Reply
1. Joe Malone
  12/03/2017
  Hi John,
  I wrote the blog because this is such a common problem, not just in wall base plinths, but a general masonry problem. The process leading to spalled brickwork is this… An old building constructed with lime mortar manages moisture perfectly well by losing moisture and salts primarily from the mortar joints and this prevents moisture from building up to unacceptably high levels in the bricks. The lime mortar is sacrificial and so of course occasionally needs repointing. At some point in the buildings history, lime mortar gets substituted for Portland cement, which is relatively impermeable. The moisture and salts can now no longer be lost from the mortar joints and so is driven into the bricks. The bricks then become far more susceptible to freeze/thaw action and consequential spalling. The bricks spall most heavily at wall base and once the damage becomes so bad, they are often rendered over with Portland cement based render to hide the damage. The render in turn prevents further moisture evaporation from the bricks and drives moisture levels even higher in the masonry. My advice is to bite the bullet, find reclaimed bricks to match and have them cut out and replaced, ensuring you use lime mortar in the process. You can render using lime but this is high long term maintenance and will cost circa £90m2 to install and then of course would need circa 5 or 6 coats of lime wash. You’d also have to ensure it was 200mm higher than external finished floor level so as to prevent rainsplash bridging or the wicking up of moisture from the ground. I’m unsure what you would be trying to achieve with a gravel filled ditch around the house? If you think this is about high ground levels then a gravel filled ditch is not the answer. Focus on simply reinstating the original technical details and further consider if anything is causing the local ground to be damp. If there are no internal plumbing leaks and the building technical details are correct but there is still wall base damp, then it is almost always attributable to one of the following:
  1. Ineffective or leaking rainwater goods and/or storm drains
  2. Leaking local foul drains
  3. Leaking incoming water main.
  If the building is technically correct then the investigation for wall base damp should always focus on these three key areas. It’s simply a matter of ensuring that the local ground is as dry as it possibly can be. Think about this logically… Even if the building is defective and completely incapable of resisting ground moisture, if there is no ground moisture then wall base damp won’t be a problem.
  Reply
  1. Gareth
    03/10/2020
    Hi Joe, just wondering how many bricks you can safely cut out and replace at the base of a solid Victorian gable end wall at a time? If the spalling is particularly bad, I could understand the option of a lime based plinth being an attractive one. Surely removal of the cement plinth would bring some weak brick faces with it?
    Also, if part of the original slate DPC is less than 200mm above external ground, due to a sloping path, is that a problem regarding rainsplash if you only applied the lime plinth under the DPC? Not totally sure what you mean by “rainsplash bridging” or “wicking up of moisture from the ground” – wicking through the lime plinth?
    Reply
Alex
07/10/2017
Hello Joe
Thank you for your blog, very useful.
I wondered whether I could ask your advice on our house?
We have a Victorian ground floor flat, The main house walls are actually ok (out of interest they have black painted plinths, which I thought were probably an original feature)
Sometime in the 70’s an extension was added, which is a single brick thickness (4inch). This is rendered, and as far as I can see doesn’t have a dpc, I assume with cement render, and has a plinth also, which reduces in height towards the back of the house. The inside floor level is actually slightly below the outside concrete path level at the back of the house, and slightly above at the front of the house.
We have a little damp along the inside of the extension, and salt residue behind the paint and a touch of mould. But the main issue is cold.
I would very much like to improve the warmth within the house.
My thoughts at the moment are to clad the wall on the outside with an air gap, possibly cedral weatherboards. But I don’t know what to do at the base of the wall/floor? Should I make the existing plinth more repellent of splash back (with paint or something) and have the cedral weatherboard go close to the ground (say 20mm so that air can still circulate up the wall? Or have it finish 150-200mm above ground level? A builder has suggested digging out the concrete path all along the extension wall a put in a french drain, which I can understand, but I worry that currently no water is getting to the footings (because of the concrete path) and so if I were to dig this out it may just cause more of an issue as water could then get to the ground around the footings and cause more of a problem with damp.
Another alternative is renderboards with insulation straight onto the wall, but again what do I do at the base, and I worry that if there is no air gap then I may cause more of an issue with damp.
I go round and round in circles with this and I do understand how specific to the property damp solutions are but any advice would be very much appreciated.
Thank you in advance
Reply
M J F Donaldson
21/07/2018
What a helpful article. We are building an “old style” cottage in the New Forest. Two thirds will be rendered and painted with the remainder being 50mm high bricks.. We are considering incorporating a 500 mm high plinth to this section as we have been advised that such plinths were common when New Forest cottages were built 200 years ago.
What would you recommend?
Reply
Jazz Bhatia
08/12/2019
Hi, firstly than you for a very informative blog, I am not in the damp proofing industry but have noticed on many properties the bridging of dpc by plinths
One method that might be reasonable is the use of the plinth with stop and drip beads . That way the look remains consistent but with a gap between the now two plinths
For our benefit, what is your professional and experienced view of this method and what gap would you recommend between the two beads, assuming it’s deemed an acceptable solution
Reply
1. Joe Malone
  13/12/2019
  Hi Jazz,
  I just wouldn’t deem it to be an acceptable solution, though I’m not entirely sure what you are describing. I assume however that you are still talking about applying a retrofit cement plinth, albeit, with a belfast bead installed. Bare bricks are fare better at managing moisture, and even if the bricks are heavily spalled, I’d much rather these are cut out and replaced with reclaimed bricks.
  Reply
RS
28/09/2020
I have quite a few perished looking bricks in the first few courses of my Victorian house (unfortunately the last owner built up soil against the house for a planting bed) It does have an original bitumen DPC, I was going to get some reclaimed use plinth bricks and build a couple of courses up as a way of the covering the perished brick work, however im guessing you would advise against this? and instead just the slower method of removing and replacing the offending bricks one at at a time
Reply
1. Gareth
  08/10/2020
  I was considering the same, but also to protect any weakened bricks from future snow etc. Could use engineering bricks and turn a few lengthwise into the wall to tie it in – perhaps removing some of the blown bricks to create the space to allow it. Other blown bricks could perhaps be stabilised and filled out with lime before being covered by the new courses if too many to replace. One concern I had was the new ‘mini’ plinth not really having any proper foundations and whether tying it into the main wall could even add stress to the old weakened bricks if there was any movement of the new bricks due to this.
  It would be interesting to here a professional opinion on this option. I read on one site to expect to pay ~£17 (per brick) for replacing bricks in a wall – and that wasn’t even old bricks and didn’t account for the use of reclaimed stock and lime, which seems to be a bit specialised for regular builders.
  Reply
Brian Priday
20/11/2020
Hi Joe,
Very informative article, thanks.
I would appreciate your views on my problem. The house is mid terrace 1900 brick/dressed stone, bay front with black ash/lime mortar. Historic and present damp issues. The DPC (brittle bituminous felt) is at outside ground level onto the random stone foundation wall, which is also the wall plate bed for the suspended floor joists. The existing timbers are all rotted, wall finish has deteriorated and both show previous remedial efforts, (failed). Under bay area is portland cement/gypsum plaster, as are various areas throughout that have been ‘repaired’.
My main query is what to do with the front wall to increase its water proof level without markedly reducing its breathability. Is there something that can be done externally to the foundation face to help. There is 10cm fall in 5m of frontage for possible drainage, with about 75cm from bay front to boundary wall.
There is also a 15cm x 25cm hole in the outside skin extending just below ground level about 15cm into the wall left when gas was introduced many decades ago, which needs filling with an appropriate mortar for black ash/lime. Any guidance on that would be great.
Best wishes
Brian
Reply
1. Joe Malone
  23/11/2020
  Hi Brian,
  There is too little information for me to understand what your issues are, but I’ve sent you some general principles for dealing with, and understanding damp in old buildings.
  Regards
  Joe
  Reply
Keith Stafford
19/05/2022
I have an old property built circa 1930 that has a cement plinth (probably fitted after the build), which is falling away in places. should i just get the whole thing removed and get a chemical DPC installed? Under the plinth I can’t see any physical DPC. The wall is red brick and does not have a brick plinth, it’s vertical, some of the brick under the cement plinth is spalling. Regards Keith
Reply
1. Joe Malone
  24/05/2022
  Hi Keith,
  Yes, get the plinth removed but my advice is to forget the DPC injection? I’m not sure why you would even consider that? Do you have evidence of damp internally?
  There are thousands of buildings in the UK that function perfectly well without a physical DPC so long as you have adequate wall base clearance. If you can’t see a physical DPC then it is probably bridged, in which case I treat the building as if one is not installed. This increases the requirement for wall base ventilation, meaning that you want to ensure that your external ground levels are a minimum of 200mm below internal finished floor levels.
  You may also need to consider cutting out and replacing any heavily spalled bricks at wall base. As a general guide, replace any bricks where spalling exceeds a depth of 10mm.
  Regards
  Joe
  Reply
Allistair
20/07/2022
I’d be interested in your take on the photo of my plinth here https://drive.google.com/file/d/1HPxCOJQjSRwrIdK1ZTB3UI2Ep7N6dnKE/view?usp=sharing
The background photo shows how the plinth traverses the bottom of my house. The inset photo is a close up of a part of the plinth broken off showing underneath.
The house walls were reconstructed in the early 90s with breeze block, cavity wall and external brick skin, but I have no idea how to tell whether I have engineering bricks, or a DPC, or whether this plinth is functioning as some form of damp control and whether it’s needed.
Anything I can look out for? We are having the driveway re-done and there is an angle to the plinth > 2%. We want to angle away from the plinth for a portion of the driveway but it will require having a paver come up the plinth by 4cm to angle back to a linear driveway drain.
Reply
Elaine
28/02/2023
Hello, I wonder if you can advise. We have a 1930’s house, had very wet joists and floorboards in the front bay, no damp on walls, had concrete render/ plinth removed, can see black DPC Now we have very damaged chipped bricks exposed at the bottom of the building where the plinth was and builders are going to cover with slate. Is that a good idea? Are there any other options to cover the bricks? Grateful for your advice. Many thanks.
Reply
1. Joe Malone
  02/03/2023
  Hi Elaine,
  No, this is a very bad idea. Badly spalled bricks will need replacing with matching reclaimed bricks. The general rule of thumb is that if spalling reaches a depth of 10mm or more, then cut out and replace the bricks. If just lightly spalled then I’d live with them. The slate will prevent the masonry from losing moisture by evaporation and drive up wall base moisture content.
  Make sure that lime mortar is used if the house was originally constructed with this.
  Regards
  Joe Malone
  Reply
Sinead
14/02/2024
Hi Joe, I wonder if you can help, I have a plinth on both sides of my house, one is quite high and is causing a lot of damp inside my hallway, not sure what to do, I have previously just had the wall inside replastered but after reading your article, ma be I should get the plinth removed? My house is Victorian 1800s I’ve lived in it for 27 years and have always had damp on that wall
Regard sinead
Reply
1. Joe Malone
  14/02/2024
  Hi Sinead,
  I’m assuming you mean a retrofit cement plinth? If so, then the answer is most definitely yes. Get it removed and be prepared to see spalled or damaged brickwork. Usually, as a bare minimum, you’ll find that some repointing is required.
  Regards
  Joe Malone
  Reply
  1. Sinead
    14/02/2024
    Thank you for your advice Joe, yes it is a cement plinth ,comes up quite high too
    Reply
Rachel Thomasson
01/08/2024
Joe – This thread and the lead article are so helpful. I live in an 1870’s house, soft yellow brick built in Flemish bond. Dreadful cement pointing over original lime (will be ground out and re-limed next spring), lovely stone base plinth discovered under cement – now exposed. The bricks below the plinth are slathered in cement render and the faces are tightly bonded to it. problem 1 – some of the brick faces high above the render are covered in thin (3mm) cement (presumably to hide spalling) and it has been impossible to source replacements from reclaim yards. Could the pointer turn them?
Problem 2 – as we can’t find anything close re:brick replacement, any advice for the base of the house once the cement render is removed? I suspect the bricks will be a mess so we are considering line render. Thoughts would be very welcome
Reply
1. Joe Malone
  09/08/2024
  Hi Rachel, I’m not sure I understand your first question, “Could the pointer turn them.”?
  Regarding question 2… I would only looking to cut out and replace these bricks if spalling reaches a depth of 20mm or more. I would not advise advise installing the lime render as you will be installing a long-term maintenance nightmare. I would just be patient and keep looking for matching reclaimed bricks, I’m sure you’ll find something sooner or later.
  Regards
  Joe Malone
  Reply
Tom
05/09/2024
Hi there. I’ve recently had to lower the ground levels around our 1903 property. The ground is now more than 20cm below floor level (around 3cm before). From the general info I gleaned I removed the retrofitted plinth which was a cement/slate sandwich. Whilst hacking it off, you could actually see moisture trickling out in places and worms had even managed to cast inside! I’m now getting concerned this may have been preventing splashback from rain after the downpour we had today. The bricks are in a pretty poor state after removal though well above your 10mm recommended threshold, albeit saturated about four courses high. The area is completely shaded. I wanted to ask your opinion whether a ‘false plinth’ could work, ie some kind of breathable panel affixed to studs with an air gap clad with matching brick slips (if that makes sense). Or would a angled lime plinth be the way forward? Thank you!
Reply
1. Joe Malone
  06/09/2024
  No Tom, neither of the options you describe is a good idea. You want bare masonry and should simply concern yourself with ensuring the ground levels are at the recommended height and the masonry and pointing are in good repair.
  Regards
  Joe Malone
  Reply
  1. Tom
    10/09/2024
    Thanks Joe, the pointing is completely shot. There seems to be some black powder residue one course above the foundation course (an original DPC? if so, what could it be?) So pointing it with a lime putty and replacing bricks is the consensus. What about swapping out the bricks for something more impervious (Staffordshire blue? ). Thanks again
    Reply