a brick plinth or two will be built this week.

[robgil]

my clay lump walls are getting it this week , i was supposed to do this months ago but never got round to it and all the advise you guys gave me has long since departed.
the one i am dubious about it this one , on one side there is the old kitchen which has a brick floor on earth but the other side is a newer extension fresh from a 1980's build your own diy extension magazine centre fold.
the new extension floor is a foot or more above the floor level on the other side and i have no doubt it has a dpc.
so , do i use a cement foundation with bricks cemented together ontop? do i use lime mortar? and do i use a dpc below the plinth?
any ideas guys?

 

[LadyArowana]

I will leave more experienced minds to answer the construction question, but I am surprised you are asking at all if K9 ( and presumably the Dr et al ) are there to assist :lol: , sorry couldn't resist. I just love the fact that you took the time to include a canine friend in your diagram. It's that sort of attention to detail that will serve well as your project progresses.

 

[vicky whitworth]

I'm worried about the foundations which seem to be some sort of red berry - possibly cranberry but more likely redcurrants, I think!
Good luck with the walls.

 

[Penners]

I can't remember whether the house is listed, Rob. If so, then of course your CO should be consulted on your plans, and would undoubtedly have some advice on the best technique.

But if not, and given the fact that you have (a) a severely deteriorated clay lump wall, and (b) a modern floor structure on the other side of it, I think I would construct the new plinth in engineering bricks on a concrete foundation. I don't think your choice of mortar makes much difference, so I would use OPC. I would also run a DPM up the new extension side of the plinth structure, continuous with the one under the concrete floor and continuous with a DPC in the new plinth.

This is obviously far from a traditional solution, but under your circumstances I believe it would give you the best job.

I'm now off to wash my mouth out with soap.

 

[robgil]

mmm , food for thought there Penners , the other side of the coin being if i used clay porous bricks and lime mortar without a dpc then any moisture would seep out through the plinth? or perhaps i should use a dpc as well as lime?

 

[Lime]

This demonstrates only too well what can happen when modern techniques of building are attached to old.
I forecast nothing but trouble from now on and the "old" structure will suffer and get the blame.
Sorry, I don't think there is a method to attach the two building systems in this way.
The modern build causes the sub soil to saturate... the old attempts to dry the soil by evaporation.
A recipe for severe damp problems, serious in brick, catastrophic in clay lump construction.

 

[robgil]

thats why i wish to try and use a brick plinth as some sort of (breathable?) barrier rather than replace with clay lump blocks. the problem i am faced with is do i try and put in a water tight barrier or do i let the brick plinth breath. if i let the plinth breath would i get an excessively damp plinth because this is the only exit for the moisture under the modern floor? or , if i use a dpc would it channel the moisture down to the old floor?
the plinth will be exposed brick and the clay lump lime rendered (up until this point both sides of the clay lump wall were cement rendered).

edit - or can i create some kind of ventilation system for under the new modern floor to allow moisture trapped under the dpc to escape?

 

[Lime]

thats why i wish to try and use a brick plinth as some sort of (breathable?) barrier rather than replace with clay lump blocks.

I appreciate your problem and it appears that you do too by your comments.

the problem i am faced with is do i try and put in a water tight barrier or do i let the brick plinth breath.

How could you be sure the barrier will be complete i.e. form a perfect seal where it really matters?
I'd concentrate on preventing the damp entering any part of the clay lump as a priority and use your new construction to remove as much damp as possible.
If you can use the surface of the new brick to evaporate damp that should help... but not improve the living space... it will always be a compromise.
From your added comment below, depending how much space you have available, keeping in mind this plinth will support the old house wall, some sort of external ventilation would be a good idea as long as it was effective i.e. worked and didn't involve the atmosphere in the living space.

if i let the plinth breath would i get an excessively damp plinth because this is the only exit for the moisture under the modern floor?

How long is a piece of string?
Better it is very damp than the water seeping into the clay lump or lower floor.

or , if i use a dpc would it channel the moisture down to the old floor?

Almost certainly it would force the water/damp towards the floor but it is the rest of the clay lump I'd be concerned about.
Anyway I think the floor is already taking more than it's fair share of damp.

the plinth will be exposed brick and the clay lump lime rendered (up until this point both sides of the clay lump wall were cement rendered).

The more exposed the brick the better.

edit - or can i create some kind of ventilation system for under the new modern floor to allow moisture trapped under the dpc to escape?

I've made mention of this already.
What ideas have you in mind?

P.S.
What about foundations?

 

[robgil]

foundation? you mean in the new extension? im really not sure if it has any. i am also not sure about how to go ventilating the ground below the new extension , perhaps putting in a suspended floor , new dpc and air bricks? if i had the ti9me and money i would flatten it and build a timber framed extension in its place. the ground level on the outside is the same all round the house so its just the inside ground level on the modern extension that has been raised.

this is a plan of whats what , as you can see both interior walls are badly corroded and i dont know what the exterior walls look like , they look ok from inside but could be rotten underneath the render on the exterior.

 

[Penners]

Lime has given you sound advice, but my concern is that your new brick plinth really won't give sufficient opportunity to evaporate away any sub-floor moisture. And I entirely agree with Lime's point that your absolute priority must be to keep moisture away from the clay lump structure.

So, although there is no perfect solution to your dilemma, my own approach would still be as I outlined above: a continuous run of damp-proofing from the new concrete floor, up the new brick plinth (in engineering bricks to minimise damp absorption) and continuous with a dpc in the new plinth.

 

[robgil]

you mean to put the dpc through the bricks like this?

 

[Ushy M]

Just a thought from Mr M - why not run in a length of land drain behind the new plinth and under the extension floor? You could incorporate this as you build the plinth - it would allow air flow to relieve pressure of dampness emerging from under the new extension floor and could be included whether or not you adopt Penners' idea of a dpm.

 

[robgil]

Just a thought from Mr M - why not run in a length of land drain behind the new plinth and under the extension floor? You could incorporate this as you build the plinth - it would allow air flow to relieve pressure of dampness emerging from under the new extension floor and could be included whether or not you adopt Penners' idea of a dpm.

you mean like a french drain? i thought perhaps putting a french drain behind the dpc at low level running to an air brick on the exterior wall? or is that just overkill?

 

[Ushy M]

Yes, a French drain arrangement is what I had in mind - we've installed one on our gable end where the ground levels are different and can't be equalised (on account of being in the graveyard). That of course is outside and I don't know whether running one indoors has a significantly different result.

 

[robgil]

ok , guys , i started to rebuild the plinth , i put a door against the wall so that i would not have to take the wall down , this worked a treat (i put some nails through the door into the wall to try and keep it afloat as it started to sag a bit) although half of the wall was pretty much a dry mix anyway and fell apart as i removed it.

the dpc option fell out of favour when i investigated what was behind the wall. basically all my lovely old floor bricks that should have been on my floor have been used to build a second wall behind the clay lump , this was all bone dry and the kitchen floor was , what looks to be very old tan coloured cement.

the construction of this wall now looks like this,

i dont know how deep or how high the floor bricks go but its a good sign that the cement render has not been applied directly to the clay lump.
i have now concluded that the damp came from a water butt that was left to over flow for at least two years , it went under the house and then had no where else to go except into the clay wall.

i wish to eliminate this dampness by way of removing the source rather than fighting it on the front lines. i will lay pamments to sand in the kitchen , remove the floor bricks and cement render to investigate further behind the clay wall , perhaps add an air brick in the void then put in a false wall made from reed board and lime plaster where the cement render is thus creating a cavity between this and the clay wall. with an air brick in there through to the exterior of the building this should do the job , what do you guys think?

 

[Flyfisher]

i have now concluded that the damp came from a water butt that was left to over flow for at least two years , it went under the house and then had no where else to go except into the clay wall.

I'd say that's really good news - it's always good to find a definite fault and be able to fix it so easily.

It's surprising how much water comes off a roof - well, it surprised me anyway. Last year I interconnected three large water butts and connected them to a downpipe from our outbuildings. The first downpour (and it was admittedly very heavy) filled all three butts within 15 minutes! Imagine that amount of water being deposited into the foundations of any building. I immediately went out and bought one of those gadgets that fits in-line with the down pipe and prevents the water butts overflowing - or rather allows them to overflow into the downpipe.

 

[robgil]

i have always thought that eliminating the cause is much better than eliminating the effects.
i have also found some cracked render on the outside of the building , its a vertical line where the cement and block meets the older clay lump that has a hairline crack down it. water was running down the side of the roof and a trickle was running straight down the back of the render. this and the overflowing water butt has really caused some considerable damage.

this is the wall being repaired.
not the best job in the world , the ledge indicates where the original blocks should be but have receded due to water corrosion (although i may have been a tad out with my vertical level )

i used a clay slurry mortar for most of it , lime for the plinth and a small amount of lime in the mortar for the first coarse of block. hopefully once i plaster the wall the blocks 'grinning' wont show through, hopefully.
you can just about see in the last picture under and around the window where the clay has deteriorated badly as well.
i managed to source some clay lump locally but dont have enough to do all the repair work so i will either have to come up with something inventive or try and source some more clay blocks.
isnt it amazing how a tiny crack in render and an overflowing waterbutt can cause so much damage? i suppose the dpc's and latex/cement floors didnt help much either.

 

[FamilyWiggs]

i have also found some cracked render on the outside of the building , its a vertical line where the cement and block meets the older clay lump that has a hairline crack down it. water was running down the side of the roof and a trickle was running straight down the back of the render. this and the overflowing water butt has really caused some considerable damage.

isnt it amazing how a tiny crack in render and an overflowing waterbutt can cause so much damage? i suppose the dpc's and latex/cement floors didnt help much either.

During the recent Great Snow (January - not the Major Snow in December or the Grand Snow (1 and 2) in February), number 2 son's bedroom ceiling collapsed. It revealed sodden roof timbers and a steady trickle of water, source unseen and unknown. After extensive, protracted (and cold!) explorations on the roof, I concluded that the level of snow settling in the roof-valley coupled with driving horizontal wind and snow day after day, had risen above the level of the lead flashing under the tiles and let water in. A month later and fed up of the cycle of emptying buckets under aforementioned trickle, I had yet another close examination of the lead in the bottom of the valley. I found a hairline crack a good distance from the collapsed ceiling that was letting in a steady trickle which was tracking along one of the main joists and dropping down onto the ceiling.

Must've been doing it for a couple of years I reckon, but the snow was sufficent water volume over a long enough period to cause the collapse. Have now put nasty goo on lead which has stopped leak* and will spend this weekend replacing half the ceiling. We're not due to "do" his room yet, so I have the awful dilemma of whether to match in the awful artex or leave it plain for now.

* I'm sure someone will expect me to have mined my own lead and dragged a charcoal brazier on the roof to do a proper repair. But just this once I was happy to go organic. Organic chemistry that is.