The Zebcolm Trench

[Zebra]

The Zebcolm trench is so named because I made one, and then malcolm made one. If you make one, let us know and we can elongate the name.

The Problem: ground levels higher on the outside than inside, causing penetrating damp through solid wall, with no possibility of lowering external ground levels.



The Solution: a trench with a retaining wall, topped by mesh grid, which allows air to circulate beside the lower portion of the wall.

First you dig - the rule of thumb is that you can dig to the bottom of the last foundation course. If you need to , you can then dig down away from it at a 45 degree angle, but we don't need to do that here. You may not even need to go this deep, it depends where internal ground level is, ideally you want to get a couple of courses below internal ground level. You might or might not have a DPC. If you do, you want to get a couple of courses below that.

Pour a little trench foundation in concrete, it doesn't need to be much, and build your retaining wall with cheap modern bricks, up to ground level. Point it any way you can, it won't be seen, (with recourse to forefinger instead of pointing trowel, probably!) and backfill any extra soil you removed on the higher side of it.

This trench is about 250mm wide at the top. It could be less - anything is better than nothing - but if it's very thin the air won't circulate so well.



You could leave your trench open, but if you want to close it off, there's some grating that's been found which is quite cheap, readily available, and if you squint, could pass for cast iron from a distance - or at least, isn't ugly. I went for "mini-mesh grating" in black from fibregrid.co.uk - they would supply it cut to size a few years ago.

Mini Mesh Grating

Mini Mesh Grating

www.fibregrid.com

For supports, there are different methods. I went for 25mm steel angle front and back. Brackets screwed to the top of the retaining wall every metre or so held it on that side. The house side was more complicated as I didn't want to fix to the fragile old brick wall of the house, so I set long pieces of 10mm threaded rod in tiny concrete pads every metre or two, and bolted the steel angle to them.




But you could do central supports with a spreader, which is what I think malcolm did.

 

[malcolm]

It's a good idea to write this up. Worth some backround:
Zebra's trench: https://www.periodproperty.co.uk/forum/threads/french-drain.12190/
My trench: https://www.periodproperty.co.uk/forum/threads/exterior-ground-level-decision.14273/

I think it's an excellent solution as it allows the walls to evaporate water away. A French drain only allows water to drain away and drainage isn't the problem if you don't find a pool of water needing to be drained away if you dig down 12 inches.

I put some concrete in the bottom of mine because I had dug to the bottom of the foundations and because I had a natural slope down to a soakaway the neighbours were kind enough to dig for me. Without the slope I think an earth bottom would be better.

There do seem to be some missing photos around here. I'm sure they were here before. I'll try to find them.

 

[Feltwell]

The Zebcolm trench, what marvellous terminology! I'm sure it's helping save Chateau Zebra from damp in a most marvellous fashion.

At some point, I may end up doing a similar thing here - when I sort out the knackered, bodged nightmare that is the patio behind my house. Or possibly get someone else to do it - my back and knees aren't what they once were - reaching that stage in life where sometimes I'd rather work and earn the money to get someone else to do it, rather than doing everything myself.

 

[tvrulesme]

I accidently started building a Zebcolm trench without realising it before reading this post. Do I have to pay royalties?

 

[Cubist]

Probably, but if you don't tell Malcolm I won't tell Zebra. Oops, another elephant in the room..

 

[Niall]

Hey so we have just bought a 1820s sandstone house on the South edge of Edinburgh and she's in decent nick in the main building. However, there is an old/original extension with a larder and utility, with a solid concrete or similar floor and solid stone lime pointed walls. We've got damp in both parts of that area with salt lines visible on the outside and some mold on the inside after the fitting of a membrane behind the plaster in the larder. I'm considering a Zebcome trench (possibly creating the McZebcolm) but I'm not 100% sure of the source of the moisture?

Dug a trail pit outside the slowest to dry part of the wall that got treated with the internal membrane. I didn't find any damp proof course and the soil was pretty dry, and I think the water table is a long way down, as we are on a high point and have a depression in the garden that is bone dry. However in the rain it might a very different story.

Before I start digging is there other sources of water I need to cross off? There are various water pipes of a range of ages in the area, but damp seems to affect almost the whole of the 'utility extension' rather than being focused in one region of it. In addition the previous owners demolished some sheds and extended the car parking area up to and around the utility, in the last 2 years. This is made of gravel, on weed mat, on a layer of rough gravel, on soil. Utility room (which is a step lower than the house) is just under gravel hight and the larder is slightly above the top of the gravel. Is this work they've done somehow linked?

For context the rest of the house seems fine, but its suspended wooden floors with a deep aired solum. This utility/larder was deliberately built cold and cheap, and sports some meat hooks and water hand pump to fill the leadlined cold water tank.


extension you can see salt on the corner and left of the tap




To give an idea of how low the floor level is, larder is a half step up but still right on the line (dog for scale)


Inside we had a membrane put behind the plaster in the larder on the right but there has been visible moisture coming up the floors in corners of flagstones, stuff takes an age to dry and there has been some mild in the back left corner since the membrane. It's unheated and I assume pretty original.

 

[malcolm]

Generally a waterproof membrane anywhere on the face of a wall is normally a bad idea. If you have damp problems in the walls it means you have more water getting in to the walls than can get out. The logic of trying to fix it by further reducing ways the water can get out might be questionable especially as the increasingly wet wall will attract condensation from inside the house when it gets even more wet. It's been an annoyance having to remove all the really hard modern plasters applied to my house so I can go back to the lime that was there in the first place.

For me the trench fixed the problem. The earth seemed fairly dry but it doesn't need to be wet - it's dampness that slowly gets in over many years. Didn't seem to me there was much water to get rid of and for me the problem was it had nowhere to go so it kept building up and making it's way up the walls.

It is a good idea to have the outside ground level 6 inches lower than inside but the wall exposed by the trench will need to be built with lime mortar to allow the water to evaporate before it gets up as far as floor level. A McTrench would probably be a good strategy but don't go repointing the newly exposed stone with anything containing cement. It will only work if it can evaporate water away.

 

[RBailey]

Somebody (on the other place) just posted this:





Looks familiar

 

[malcolm]

Good to see the idea becoming mainstream. The 'how the air drain works' isn't completely perfect. It's not splashing rain that causes problems it's high exterior ground levels so filling a hole with pea shingle won't work either. High ground levels give you water ingress all the time but you only get heavy rain occasionally. If you can open a lid and clear the air drain every few years it'll be useful. Otherwise it'll become full of soil after a few years.

It's just a way to lower exterior ground level to sensible levels and let the water evaporate away from the wall before it gets above internal ground level. Wants to be maintainable so that can happen again the next time someone gets damp and bothers to clear it out.

 

[joe_thomas]

Definitely need to figure out how to do this on my house... Discussing on another thread but this seems logical.

1850's Victorian Cottage - advice sought for Limecrete/Glasscrete floors, lowering ground levels, common sense

Bit of a long one this maybe, first time poster, long time reader! We moved into our 1850's(ish) cottage a couple years ago now. It's a rubble-stone with some brick construction. Not listed, no covenants or restrictions. As soon as we moved in, we started trying to undo a lot of past wrongs...

www.periodproperty.co.uk

Challenges include: pavement (not council owned apparently), foul drain running under pavement, no idea of foundations (1850’s(maybe?)) cottage.

Plan is to break concrete in "trench", dig down, see what it's like; will there be good deep foundations (probably not), will the earth under the pavement fall into the wall (probably), will the foul drain be leaking through my wall (probably), will the neighbours get annoyed at the cursing (yes).

Pretty sure my ground is highly compacted sand, going by the 80-90cm holes I dug for some posts in the garden.

The Zebcolm drain looks like the best option. Giving airflow to an otherwise stifled lower wall.

Old photos below, we're re-painted and patched any cracks/holes... (apart from last one, repainted with gravel removed my trench)

Any thought welcome, and tell me off if I'm spamming multiple threads....

 

[Zebra]

Yes, this looks do-able. I'm not sure what the liability issues would be given that it's a public access area, and given that any damage to the pavement would have to be put right. But if you now dig down in a smallish area, to see what the house foundations (and pavement sub-base) are like, it won't do any harm. Is that concrete beneath the gravel then? I wonder if you used an angle grinder to cut directly down along the edge of the pavement...but no, bad idea, there might be pipes and other services... better to break it out with a SDS breaker. Just have to try a bit and see. It might be an idea to do the trench in sections rather than the whole thing in one go if there's any risk of the pavement collapsing if its sub-base is at all loose.
The gap between wall and pavement looks wide enough for a brick retaining wall plus gap. Looks do-able.

 

[Gloucestershire House]

Dear @Zebra and @malcolm,

Thank you both so much for pioneering The Zebcolm Trench. My partner and I moved into a gorgeous 1881 Victorian semi at the start of this year, and despite thinking, perhaps naively, it was a 'turnkey', have since discovered a few challenges to overcome, particularly related to damp and the entailing issues.

I came across your stories while struck down with the dread C, and trying to self-diagnose our persistent damp problems after a predictably unfruitful visit from the 'Damp Wally' (as Peter Ward would say) who had injected the house for the previous owners.

After an exploratory dig down the front and side of the house, the problem became clear. Previous owners had ludicrously raised the external ground height a solid 4 bricks higher than the original slate dpc (that I managed to expose). The major problem was the impracticality of the 'proper' remedy - lowering the external ground around the entire back and side of the house. I needed a solution.

It was a real Eureka moment when I came across your idea, a great solution to creating an artificially lower external ground height. Unfortunately, there was no downpipe or suitable drain to tap into, nor the room or space for a soakaway to lay a proper piped French drain. Also, I must admit I was slightly put off this idea by examples of poorly laid drains that actually exacerbate the problem. Luckily, the ground itself was fairly well draining, and I've opted for just an earth-bottomed trench.

We also went for the fibreglass cover, and are really pleased at how it looks. To overcome the issue of supporting this cover, I inserted screws (point in) into the mortar of a single brick-high trim I laid around it. This supports the outside really well, and for the inside, rather than fixing anything to the brick wall itself, I went for a slightly bodge option - stacking a couple of 'unpresentable' old bricks to the correct height. In the pictures you will see this isn't perfectly level yet, but I need to remove the grille soon anyway to finish removing the (yes, you guessed it...) unbreatheable old paint from the wall. I will correct this when I relay it.

For any other budding trenchers, if I were doing it again, I would leave an extra few mm for the covering to fit a bit more loosely between this trim and the house wall as it's slightly tight in places.

Overall, I really enjoyed this process despite the circa 30 bags of rubble (you would not believe the moon rocks they put down the side of this poor house) now stacked up out front. The best part - within a week or two, the musty smell had dramatically improved, so much so, that now a month down the line, it's completely gone, even in winter.

This experience has, however, left me shocked at the thoughtless, corner-cutting, and slapdash treatment our houses have received. I find it absolutely staggering that we were able to build these beautiful, characterful, and long-lasting homes 150 years ago, all for some ill-educated, and couldn't-care-less cowboy to come along years later, and create problems where there were none. We've got the lot - cement pointing all over, cement render (taken off now, good excuse to get an SDS drill), gypsum plaster (also off, thanks SDS), concrete tanking pushing moisture up walls, sealed space under stairs with no ventilation, rubbishy double glazing with no trickle vents (sorted now), woodworm downstairs as a result of moisture, the list goes on!

Anyway, thank you very much and best of luck with the rest of your projects. Pictures attached.

 

[a twig]

What a fantastic job - brilliant project!

 

[Gloucestershire House]

What a fantastic job - brilliant project!

Thanks very much!

 

[Corvidfan]

Hi All,

New member here, and as this thread is so similar to my questions/problems I thought I would ask on here save starting a new thread.

I've a cottage with an identical wall/ground profile as in the original post (but a brick plate with timber frame nestled on top), at some point in the 1900's the owners dug the mud floor out, and laid a thick concrete pad well below ground level, then put a suspended floor resting on railway sleepers.

As such the suspended floor is unventilated apart from into the house so it smells, and penetrating damp comes through the solid brick (and made the cement based plaster that was over the bricks drop off in layers)

So first step is to stop the penetrating damp via some form of trench. BUT my main concern is that the cottage has stood for 250 years or so, and has moved and spread a little bit in that time, will removing the 30cm of buried bricks relieve some spreading pressure in the house and cause some slippage?

 

[Zebra]

Unless you can see some obvious bowing to the walls, then I would say no, it'd be fine to remove banked soil against those walls, indeed it sounds very necessary.

Edit: and if there is some obvious structural issue, then another solution should be sought. But it is highly unlikely the house will fall down.

Edit2: and if some structural instability should occur, it can be managed with accro props or similar while repairs are carried out.....

I don't mean to scare you, only to reassure that should the worst happen, that too can be overcome. Courage!

 

[SamB]

Hey so we have just bought a 1820s sandstone house on the South edge of Edinburgh and she's in decent nick in the main building. However, there is an old/original extension with a larder and utility, with a solid concrete or similar floor and solid stone lime pointed walls. We've got damp in both parts of that area with salt lines visible on the outside and some mold on the inside after the fitting of a membrane behind the plaster in the larder. I'm considering a Zebcome trench (possibly creating the McZebcolm) but I'm not 100% sure of the source of the moisture?

Dug a trail pit outside the slowest to dry part of the wall that got treated with the internal membrane. I didn't find any damp proof course and the soil was pretty dry, and I think the water table is a long way down, as we are on a high point and have a depression in the garden that is bone dry. However in the rain it might a very different story.

Before I start digging is there other sources of water I need to cross off? There are various water pipes of a range of ages in the area, but damp seems to affect almost the whole of the 'utility extension' rather than being focused in one region of it. In addition the previous owners demolished some sheds and extended the car parking area up to and around the utility, in the last 2 years. This is made of gravel, on weed mat, on a layer of rough gravel, on soil. Utility room (which is a step lower than the house) is just under gravel hight and the larder is slightly above the top of the gravel. Is this work they've done somehow linked?

For context the rest of the house seems fine, but its suspended wooden floors with a deep aired solum. This utility/larder was deliberately built cold and cheap, and sports some meat hooks and water hand pump to fill the leadlined cold water tank.

View attachment 10905
extension you can see salt on the corner and left of the tap


View attachment 10906

To give an idea of how low the floor level is, larder is a half step up but still right on the line (dog for scale)

View attachment 10907
Inside we had a membrane put behind the plaster in the larder on the right but there has been visible moisture coming up the floors in corners of flagstones, stuff takes an age to dry and there has been some mild in the back left corner since the membrane. It's unheated and I assume pretty original.

What is the ventilation in those rooms? If there is no damp outside but you are getting visible damp inside, especially on a waterproofed wall then there's a good chance it's condensation and you just need to install humidistat type extractor fans, particularly in any room laundry is done in.
In any situation with mysterious water appearance you should certainly track any pipes, active or obsolete to make sure there isn't a slow leak.
On the other hand having the external ground level flush with the internal floors, however dry the external soil, is asking for trouble and personally I would trench all the way round, but probably just backfill with aggregate to allow some breathing. Obviously if once the trench is dug (which will also allow you to spot any old pipework coming in) you will see if there is any water build up and can french drain if needed.

 

[Corvidfan]

Unless you can see some obvious bowing to the walls, then I would say no, it'd be fine to remove banked soil against those walls, indeed it sounds very necessary.

Edit: and if there is some obvious structural issue, then another solution should be sought. But it is highly unlikely the house will fall down.

Edit2: and if some structural instability should occur, it can be managed with accro props or similar while repairs are carried out.....

I don't mean to scare you, only to reassure that should the worst happen, that too can be overcome. Courage!

Really appreciate the response thanks, I thought I would get notified but didn't so forgot I had posted for a while!

I can't see anything structural, the brick seems well in place (but the timber frame plate above has opened up at the corner) and I am more concerned with the damp and smell at the moment!

Hopefully the below drawing loads up, this is what I am currently facing, slab thickness is unknown (but at least 100mm)

My plan was to trench down to the concrete level, and put in drainage/soakaway pipes for the front and two sides as the rear of the house will have a modern extension dropped down a 500mm so plenty of scope to run to proper drains there. back fill the trench with gravel, but only up to a level that is below the floor level (which may change)

the dpm would be cut back to concrete level.

Then aside to that, apply some mechanical ventilation under the floor by a pair of fans configured as supply and extract.

 

[MBozz]

Hi all,

This all makes for very interesting reading, so thanks for sharing. I was wondering if you might enlighten me as to possibilities in the stone farmhouse we are currently in the process of purchasing. I should put in a disclaimer - while I'm from the UK, I live in Asturias in northern Spain, and that's where the farmhosue is. The climate out here is pretty similar to somewhere like Cornwall (where I'm from), so plenty of rain.

We've fallen in love with the house but it has A LOT of work to do, first and foremost sorting out the damp that is penetrating through the walls. Issue is, the house is built into a slope, and buried almost up to roof level in the rear, and also significantly at the sides. It also seems to be built on/into bedrock. The building is divided into two (LHS stone built home, RHS stone/brick built barn), and water will literally pour like a tap through the wall of the barn following heavy rains. Id say tanking is definitely not an option, as that water will just build up and the problem get worse. But is it possible to dig out such a mammoth amount? And is it structurally viable to do so? And if not, what would the alternatives be? Some sort of land drain system in the slope above the house to try and divert water away from it?

Any help would be hugely appreciated.

P.S. I'm not shy of whatever work is needed, and happy to move huge amounts of earth if desirable. Just don't want to compromise the structure. In terms of experience, Ive worked on a stone house in Bristol that had damp issues, and found lowering external ground levels, replacing concrete render with lime pointing, and massively improving ventilation worked a dream. But this is another beast...

 

[a twig]

Wow! Looks stunning!
For me I’d say your best bet is to try and install a geotech membrane on the outside of the wall - something like this https://www.permagard.co.uk/permaseal-8-geodrain-external-waterproof-membrane with a drain at the base to take away the water that is “caught” by the membrane - fair bit of digging to do it but should block and divert pretty much all the water coming down the hill before it can enter your wall, then usual lime etc on the inside to let any moisture that sneaks through out again