DPM and block and beam floor

[Glee]

Hello and help!

Do I need to put a dpm on top of a subfloor/ underneath the block and beam as well as on top of the beams. It will have a 300mm void as well as air vents. 
many thanks Gary 

 

[Big Jimbo]

Sit the beams on a continuous strip of DPM. It's cheap FFS, and don't forget to seal the ends of the beams which is now a Building regs requirement 

[Glee]

Big jimbo

yes I have placed the beams on a strip of dpm but do I put a dpm sheet above the subfloor?

cheers

[Canski]

On 21/03/2025 at 08:46, Glee said:

Big jimbo

yes I have placed the beams on a strip of dpm but do I put a dpm sheet above the subfloor?

cheers

No you don’t need to. DPC below the beams and a DPM on top of the beam and block that laps up and joins your buildings DPC. 

[Tony L]

On 21/03/2025 at 08:33, Big Jimbo said:

...and don't forget to seal the ends of the beams which is now a Building regs requirement 

 

I only had a quick look at the reg.s, but they don’t seem clear.  I think what you say is true for beams with exposed rebar at the ends, but not all beams have this.  What do you think?

 

This mess, below was built by my cowboy builder, before I got rid of him.  He was supposed to build according to my drawings (2^nd picture), but what I’ve ended up with is trench blocks going up above ground level.  The beams rest on the trench blocks.  There’s DPM beneath the beams, but it’s not really doing much, because it only goes (almost) as far as the end of the beams (& doesn’t go right to the inside of the trench blocks either).  Also, the builder has bridged from the trench blocks to the beams with big dollops of mortar.

 

This might not be such a cause for concern on most plots, but I have a very high water table, so water is going to be rising up through these blocks into my beams.

 

I’d decided to build off the top of this mess, but lately I’ve been considering removing all the blocks, jacking the ends of the beams up with a trolley jack, putting some wider DPC in there & capping the beam ends with K-Kaps (picture below).  Is this idea worth further consideration?  It would be good to hear some opinions from those in the know, please.

 

[Tony L]

Ripped DPC in the red circle & DPC doesn't reach the inside edge of the blocks.

 

 

[Tony L]

& these are the K-Kaps.  You can see the rebar at the ends of these beams.  My beams have no visible rebar.

 

I hope you don't mind me jumping in on your thread, @Glee.  I think @Canski has told you everything you need to know, anyway.

 

Should I get my trolley jack out?

 

 

Edited March 22 by Tony L

[Big Jimbo]

Tony. What a mess. That whole thing is so wrong. That whole thing needs to come up, and be started again. You have no cavity ! I have got to go out now, but I will be back tonight and try and give you some answers later. Your builder is not a builder. I'm shocked.

[Andehh]

Is builder fully paid up?

 

I presume cavity will be built around it?

 

Your missing the outer skin bottom layer?.. Can you chisel out the mortar around edges so it doesn't bridge the DPC?

 

[Tony L]

Thank you, @Big Jimbo.  Here's the beam plan, in case that's any help.  There are sleeper walls under the B&B. The two longest of these are indicated by the green arrows. The sleeper wall indicated by the blue arrow wasn't built.  There's a double beam here instead.

 

I asked (paid) a proper builder to come on site to inspect & report on the work.  His opinion was that we could build off what was there, but we'd have to take the airbricks out & re-fit them properly. Of course, if we build off what's there, we're going to have to accept the B&B floor will be a lot more damp than it should be (higher U value) & our only protection from the excessive damp will be the membrane that goes on top.

[Tony L]

7 hours ago, Andehh said:

Is builder fully paid up?

 

 

No - there are unpaid invoices totalling £10,800.  Let's focus on construction advice on this thread, please, & I'll update my other thread when I have more to report on the dispute/litigation.  This one...

 

[Tony L]

7 hours ago, Andehh said:

I presume cavity will be built around it?

 

Your missing the outer skin bottom layer?.. Can you chisel out the mortar around edges so it doesn't bridge the DPC?

 

 

Yes, the cowboy builder started building up the outer leaf.  It's very bad (see thread linked above, if you're interested).  I may end up knocking the outer leaf down & starting again, or I may just take out the air bricks, or I may re-do the air bricks & the whole of the start of the outer leaf across the facade, because the black bricks are already looking terrible, due to efflorescence.  Those black bricks were quite expensive & the mortar is quite strong, so it would be a job to clean them up & re-use them.

 

These pictures show how things look on site right now.

 

[Tony L]

Actually, this doesn't show how it look now.  This is before the last three or four courses were added.

[Mr Punter]

Well @Tony L that does look a right mess.  Also, it looks very wet and the ground level v floor level is worrying, as is the lack of sub floor ventilation.  I would ask building control to visit and see what they have to say.

[Tony L]

Thanks @Mr Punter.  

 

According to the builder, the building inspector has seen everything & says it’s all good.  The builder has written to me to confirm this.  You’ve got me thinking now, I should explain to BCO, it has become clear the builder has been lying to me.  I should ask BCO: could he please confirm everything that’s been built so far is building regulations compliant?

 

I’m going to be overseeing remedial works to improve the ventilation to the void beneath the B&B.  I don’t care what BCO says – I want to build to a good standard, rather than the low standard at which BCO will say things are acceptable to them.

 

Regarding the ground level: the planners were very awkward with regard to the ridge height, so we’re stuck with the floor so close to ground level.  It’s not a compromise I’m happy with, but I didn’t want to spend a ton of money just to end up with a bungalow or a house with horribly low ceilings.  The first trenches (1m) deep were dug neatly & were shuttered.  When BCO came & said he now wanted to see 2m trenches, we ended up with wide V shaped trenches, due to the wet ground & cave in risk.  A lot of backfilling was done after the trench blocks were laid, so the this may account for the high ground level.  Also, there’s 150-200mm of crushed concrete surrounding the building (1.5m wide apron) for the scaffolding to go on, so perhaps this can be scraped back at some future date.

Edited March 23 by Tony L
add a picture

[Tony L]

[Tony L]

On 23/03/2025 at 07:30, Andehh said:

Can you chisel out the mortar around edges so it doesn't bridge the DPC?

 

 

Sorry, @Andehh, I forgot to respond to this.  I've thought about trying to get rid of some of the mortar that's bridging the DPC with an angle grinder, but I'm conscious that if I accidentally cut a small channel into the trench blocks right at the edge of the beams, this will be a point from which a stress fracture could start, & I really don't want that.  Chiselling might also give rise to stress fractures.  Even if care had been taken with the installation of the 100mm wide DPC, & there were no mortar bridges, I would still not be happy that the DPC would provide maybe 1mm separation between the beams & the damp trench blocks.  The k-Kaps look a lot more chunky.

 

Any input on my idea of jacking up the beam ends & putting K-Kaps on them would be most welcome.  Or, if there's a better affordable solution, I'm keen to hear about it, please.

[crispy_wafer]

4 minutes ago, Tony L said:

Any input on my idea of jacking up the beam ends & putting K-Kaps

 

a couple or 3 coats of Black bitumen paint on the ends would be easier, probably cheaper too.

[Tony L]

Thanks, @crispy_wafer.  Yes, that would be a lot easier, but almost all the damp in the beams is going to be coming up from the trench blocks & the mortar, so sealing the ends of the beams will still leave me with around 95% of the problem unsolved.

Edited March 24 by Tony L

[crispy_wafer]

@Tony L I’m prone to the odd irrational thought when things go south so please bear with me….  depending on whether the blocks in the floor are grouted in or not, I’d be tempted to hire a digger for a weekend, whip the end blocks out, sling lift the beams a few inches and rollout a wide dpc one beam at a time.  That would give me peace of mind, whether it’s actually required or not I don’t know.  If the blocks are grouted then this becomes a bit more difficult, and becomes a question of whether you are willing to sacrifice a pallet or two of blocks, unless they can be cleaned and reset.  All becomes a question of time and if you have the will and desire, to make it how you want it.

 

This Coming from me who was being unhappy that my builder didnt backfill to my trench blocks on the inner side, so I lifted all blocks round the perimeter and dumped in tons of mot to fill the V to solum level, whilst I was under there I fished out all the crap tins and sandwich packets they dumped, makes no difference in reality apart from my wallet being lighter, but gave me a peace of mind and thus i could sleep a bit easier.

[Tony L]

Thanks, @crispy_wafer.  I have been thinking along the same lines.  I've removed two blocks from the edges just by levering them out with a spade, so I think almost all the blocks could be re-used, apart from those close to where the 110mm pipes, etc, that penetrate the floor, have been grouted in with mortar.  I'm thinking a 450mm wide DPC could go in then be lapped up over the top of the blocks. 

 

My current plan is: remove all the blocks, lift the ends of the beams, put the new DPC in & weave this below the beams (on top of the cowboy builder's 100mm DPC) over the tops of the mortared in brick slips that go between most of the blocks, back under the next beam, over the tops of the ends of the periscope vents for the air bricks, under the next beam end, & so on.  This way, my floor will be isolated from the damp trench blocks & I won't have to make any cuts for the air brick periscope ends.  Also, I don't have to hack away at the mortar between the beams & risk damaging the trench blocks.  If my description has not given rise to a wonderfully clear picture in your mind, don't think about it too hard - when I have time (tomorrow, if all goes well) I'll post a diagram.

 

I'm thinking a trolley jack will be better than a digger, especially if I want to lift the beam ends from the sleeper walls in the middle, rather than restrict these works to the perimeter.  I have decades of trolley jack experience & zero digger experience.

 

& you're right - removing all the builders' packed lunch leftovers is sure to give added job satisfaction.

[Canski]

On 21/03/2025 at 08:33, Big Jimbo said:

Sit the beams on a continuous strip of DPM. It's cheap FFS, and don't forget to seal the ends of the beams which is now a Building regs requirement 

Not according to our beam and block manufacturer and the NHBC inspector today. I see the sense in it though. 

[Tony L]

Here’s the diagram I promised you, @crispy_wafer.

 

Just to re-cap, my builder built this B&B floor off trench blocks instead of a cavity. There’s DPC along the inner edge of the trench blocks, but it’s not doing much, because it’s been bridged with mortar.

 

I’m thinking I should take the blocks out & lift the beam ends one at a time, to run 450mm wide DPC along the routes shown in green & red.  This will leave sufficient DPC protruding to lap over the top of the edge blocks, to be held in place by the first course of the inner leaf, when this goes on (is this how it’s done?).  I’ll leave the cowboy builder’s DPC in place.

 

In the image, below, the thick red line shows where my new wider DPC will protrude from the structure when it’s all been put back together. The DPC shown in red should be easy to install.

 

The DPC along the two walls where the beam ends are needs to go under the beams, of course.  I’m thinking I can weave this so it goes over the air brick periscope ends, & I think it might be easier to go over the top of the infills under the other blocks too.  This is shown in green.

 

I built a model & used paper in place of DPC, to see how the folds work, & everything looked good.  DPC doesn’t fold as easily as paper, of course, so I may end up having to make some cuts, but whatever I end up with has to be a lot better than what I’ve got at the moment.

 

All comments will be gratefully received – thanks.