Flint wall dpc: a question for JM/Woody

[Shytalkz]

Guys, you two seem to be pretty clued up on damp. I am not being facetious, I have a genuine question, honest!

I'm currently in the process of converting a 200yo (ish) barn, for me personally. The walls comprise flint footings with a brick plinth up to about 450 above current ground level. The plinth brickwork is 215 thick flemish bond; the flint above the plinth is 327 thick and the flint continues down behind the plinth brickwork to foundation level (if I could work out how to post a pdf from my CAD drawings on the forum, I'd sling it up here!)

In some areas, I am underpinning the external walls and building a block wall inside (this is mainly due to getting a window detail to work, where the opening cuts across the existing flank construction and a section of new-build to extend the barn length) with a nom 55 cavity; elsewhere, I am retaining the flint as the internal face, doing a trade-off with insulation levels elsewhere to sort the SAP/CI side of things.

Now, how do I (Can I? Need I?) satisfy the LABCO regarding damp coursing to the original flint walls? The site is nice and dry and there is no evidence of damp at low level.

Pearls of wisdom awaited, cos I'm stumped over this...

 

[fastneattrowel]

fancy asking one of your other ids lol

 

[Shytalkz]

Wondered where you'd got to...

 

[JerryM]

Pearls of wisdom awaited

You're not wrong on this one.

My experience is on brick built houses ranging from 1870 to 1937.

Your's sounds like a palace compared to what i've been used to.

I've not come across flint but presume i can think of it as a stone wall.

In this case i would only use tanking. I use a cement based tanking called K11. It goes on to a height of 1.5m and is a bit more expensive than injection - but well worth the extra on tricky jobs which i include yours as. It's performance is top notch.

The only downside i guess is that you would want to leave the flint exposed on some of the walls which would not work with the tanking. at 215 mm thk the walls are ok to inject (limit is 11") and i feel low pressure cream into the (lime) mortar would be the only option as hp liquid into the stone is a "no no" (won't saturate the stone).

i personally don't like injection into a wall > 4" and would try to maximise use of tanking if poss.

This is a job that i would checkout with my tech support people just to make sure. I am sure they would speak to you direct ***********. any probs i am happy to check out on your behalf.

best wishes, jerry

 

[^woody^]

If a physical barrier is not possible, then there should be an argument that the walls are such that moisture ingress will be minimal.

There is a mention in Approved Document C, relating to work on sensitive historic buildings and an acceptance that intrusive work may not be necessary. It may be possible to coat the exterior or do some ground drainage work to minimise the potential of any moisture ingress.

Also the BCO's knowledge of damp treatment may be limited and mean that he will accept a BBA certified injection system - even if people generally doubt the effectiveness of these. Basically if he has a certificate to say something has been done, then thats the bureaucracy sorted

 

[Shytalkz]

Thanks both

Jerry: flint is a sedimentary rock (see http://en.wikipedia.org/wiki/Flint for a highbrow explanation!), totally impervious to water, so the only way that it can get into/up/through the wall is via the lime mortar joints. I can't see that anything would soak into the stone, tbh. The flints are laid in an entirely random fashion (some of them are abso humongous and go right through from one side of the wall to the other), so the joints are variable, both in width and in quantity per m2.

I've got very low cill heights in some areas at ground floor level - to do with matching new external brick qouins to the openings against those which currently exist - down at 665 above finished floor level, so this would mean that the tanking couldn't be taken to 1500 height all round.

I am minded to do absolutely nothing, to be honest, seeing as there isn't a problem as things stand and I won't be doing anything to make it worse, as far as I can see. But, as Woody says, the BCO might only be happy with a BBA certificate, regardless of whether or not it works, just so that at audit time, there's something on his file. I probably need to sound him out as well, before making a decision...!

I've grabbed the number, before the mod bins it, btw

 

[noseall]

i would say stuff the damp and let the building breathe.

sympathetic uses of period materials combined with modern materials would be the ideal answer, but do the twain mix?

no.

flint is pretty much impermeable but the make-up of the mortar may reveal a more permeable medium.

it is a pity, there is not a solution that can combine a dry interior with a breathable exterior, without resorting to rigid, modern methods.

 

[noseall]

Thanks both

Jerry: flint is a sedimentary rock (see http://en.wikipedia.org/wiki/Flint for a highbrow explanation!), totally impervious to water, so the only way that it can get into/up/through the wall is via the lime mortar joints.

bah, you posted while i was typing.

 

[Shytalkz]

i would say stuff the damp and let the building breathe.

Gut instinct tells me to go that way as well: it doesn't have a problem at the moment, so I can't see that what I'm doing to it is going to create one. My initial thoughts way back when were to blackjack the floor slab and paint that on the wall up to finished floor level (165 above sfl) and I'm still leaning in that direction. Of course, one other thing that requires resolution is where the new-build external walls meet up with the original: I'm hoping that I won't have to put a dpc in the new plinth brickwork, as that seems entirely pointless, the more so if the old walls are left alone.

Should have knocked it down and built a new one out of the reclaims

 

[Shytalkz]

I've grabbed the number, before the mod bins it, btw

Beat ya, mod

 

[Joe Malone]

Sorry, didn't see this and just realised that JM is probably me.

I've dealt with building control before for similar issues in historic buildings and it's usually quite easy to convince them that the standard building regulations don't apply. In fact, technically you can get round any building regulation to long as you can prove that an alternate solution will work.

I'm guessing that the building is at least grade 2 listed? If so have you applied for listed building consent? This situation would actually help your case.

Read my previous posts and you'll already know my view. Buildings of this age are meant to breathe and any refurbishment or redevelopment should be carried out using sympathetic materials. From what you tell me the building appears to have been performing perfectly well so why change things.
The brick plinth is extra protection installed to prevent lateral penetrating damp and protect against rainsplash, you dont see them much these days cos we like to install physical dpcs and cavities. However don't underestimate the value of this feature they work extremely well.
Make sure external ground levels are 200mm below internal finished floor level and make sure the wall base is well ventilated. NB.. This is nigh on impossible if you install a concrete slab.
These standards were laid down in the Liverpool building act of 1842 and are still relevant. I have lots of information that would support a dpc free approach to dealing with an historic building. You just may have to compile a report to convince the local authority. The Society for the Protection of Ancient Buildings advocate this approach and their website is a good starting point for evidence. If you need any more supporting evidence then I'll be happy to send it.

 

[Shytalkz]

Hi Joe - it was actually you to whom I was aiming, along with the Woodster, although Jerry's pov was also of interest

The building isn't listed (thankfully), although I had already considered possibly getting the conservationistas involved, albeit mainly to have ammo to resist losing all the internal flintwork to satisfy Part L. However, I'm using a private BC company, rather than the LA, as they were quite happy to accept a trade-off and would also let me use Tri Iso Super 10 - so I never got as far as speaking with those guardians of the ancient fabric.

But then, at that time, I hadn't started to develop the details to any degree and certainly hadn't thought about dpc issues with the existing walls.

Existing floor and ground level at the rear are more or less coincident; floor level is about 450 above ground level at the front. Due to height constraints to get two storeys in, the majority of my new ffl will be the same as the original, with the front section being stepped down for various reasons. Foundation level at the back is 300 below floor level and I can just about get a slab, insulation and screed in without going below that depth (65 screed 100 insulation - trade off for walls - 100 slab), as the subgrade is ok to not have to dig it out and import hoggin.

As it happens, I am underpinning the rear wall, as this has wibbled a bit, due both to roof spread and founding in soft soils, aided by an outlet for cows' **** over the years, so I could drop the ground level and - wait for it - put a french drain in . This would be on the basis that, as the wall is underpinned, I'm not going to cause a problem with water being diverted down to foundation bearing soils...! I do have to put a level entry on that side of the building though, for Part M, as my disabled bog is at the higher (rear) floor level, so can't let them in at the front - unless I put a ramp in the central hall area and wouldn't that look nice?!

That's an interesting point that you make re the brick plinth. Do you reckon that I could argue the toss over missing out a dpc in the new plinth then? The construction here would be 102 facings in Flemish with snapped headers, 150 block backing, and both a 55 cavity with 190 block inner leaf and 80 cavity with 100 block inner leaf (there are reasons for these two variations!).

I must admit that I hadn't thought of SPAB as a source of support, so thanks for that pointer, I will go look see and then come back to you.

It's pleasing to note that the overall concensus (3:1) is to leave the existing fabric alone and, as you will see from my earlier posts, that was the way I was leaning.

As an aside, what the hell does OP mean? Other person? Original poster? Organic pumpkin?

 

[Shytalkz]

Update: spent a wodge on publications on the SPAB site, very interesting place.

Spoke with the BCO and he's happy to work along the lines of leaving it alone, if it's a dry site (check) and there are currently no problems (check); may also let me not put a dpc in the new plinth to the gable extension, but is going to think about that. As I pointed out, if he makes me put one in, what happens where new meets old. Teeth sucking and no comment...

 

[Shytalkz]

Hey check out the Provident personal credit advert at the foot of this page: 183.2% APR

 

[JerryM]

fancy having the same initials as Joe.

had we not and i missed your post you'd have got 3 votes to nil and carried on without any doubts.

it's the mortar that concern's me. yes all's fine whilst the building is allowed to breathe (as it was designed to). the trouble is that our way of living has changed and maintaining the ventilation is a heavy trade off against energy costs.

i would at least get some readings/assessment of the damp in the mortar joints just for peace of mind.

in terms of the 1500 tanking ht, there is potential to relax it depending upon the ht/extent of the rising/penetrating damp.