Quick question about laying a DPC.

[MonkeyHunter]

Hi,

Im building a new internal block wall, do i lie the DPC directly on the blocks, or do i lay mortar, DPC, mortar, ive read conflicting reports, is there a consensus of opinion? I don't have time at the moment to try and find it in the building regs, if it's mentioned.

For info, in case the weight of the wall is an issue, the wall is 4 blocks long, being built from 7N blocks. And there will be 5 courses of block above DPC across most of the wall, followed by a 5'x2'x1' fish tank. though not all the weight from that will bear on the wall.

Cheers!

 

[hotrod]

Hi,

Im building a new internal block wall, do i lie the DPC directly on the blocks, or do i lay mortar, DPC, mortar, ive read conflicting reports, is there a consensus of opinion?

The dpc needs to be 'sandwiched' between two layers of mortar to create an effective bond between the courses below and above dpc. Lay a (thinish) bed of mortar on the blocks, lay dpc, then lay a second bed of mortar on top of the dpc and lay the next row of blocks.

 

[MonkeyHunter]

Typical, everything i had previously seen said just slap the DPC straight on the blocks, so when I poured the Foundation I used that as the reference.

And then today I happened across a site that said sandwich.

So that means that my nicely measured DPC level is going to be out, oh well, I guess it doesn't really matter.

How thin is Thinish, 5mm?

 

[hotrod]

Monkey,

There is a lot of disagreement about this topic in general. My personal preference is to 'sandwich' with a thin slither of, as you say no more than 5mm. Seen loads done, as you say by just plonking it straight onto the blockwork.

There's a double skin garden wall, round the corner from me, approx 1m high that 'fell' over about 5 years ago. It had only been there a couple of years and you could see it had hailed horizontally right along the dpc - hence my preferred method. Obviously, if the wall is loadbearing the risk of this type of failure is minimal. I looked into this a few years back and there is a Reg / BS standard that states the dpc should be bedded with mortar both sides but I forget what it is. If I find it I'll post back.

 

[^woody^]

The DPC should be laid on a 10mm bed of mortar troweled level and smooth with no indentations, trowel marks or bits of brick etc. Then lay the damp course on it, smooth it with the trowel or a brick, and then lay another 10mm bed for the first course.

But in practice, the brick/block courses are guaged and any adjustment in level or guage are taken up in the joints, so you may not end up with a 20mm DPC joint, or a DPC joint which is deeper at one end than the other

Edited

 

[^woody^]

The standard is probably in BS 5628 Workmanship on building sites - Codes of practice.

The idea is that the DPC is fully supported on a solid bed so that when the wall moves or settles, there is nothing underneath the DPC to cause a split or puncture.

Any wall can slide on the DPC - even quite tall freestanding walls only need a bit of a push at the top to cause the DPC joint to crack and the wall topple. Somethimes t may be better to use an alternative DPC to the roll of felt.

The cheap polyphene DPCs are more prone to letting the wall slide, and IIRC they are not supposed to be used as horizontal DPC's

 

[MonkeyHunter]

Hotrod thanks for the input.

Woody, sorry but im not quite following you: [My emphasis].

The DPC should be laid on a 10mm bed of mortar troweled level and smooth with no indentations, trowel marks or bits of brick etc. Then lay the first course on it,smooth it with the trowel or a brick, and then lay another 10mm bed for the first course.

The above reads to me as Mortar -> DPC -> Block -> Mortar. is that what you mean?

Edited, as your second post beat this and now its crystal

 

[MonkeyHunter]

The standard is probably in BS 8000 Workmanship on building sites - Codes of practice.

The idea is that the DPC is fully supported on a solid bed so that when the wall moves or settles, there is nothing underneath the DPC to cause a split or puncture.

Any wall can slide on the DPC - even quite tall freestanding walls only need a bit of a push at the top to cause the DPC joint to crack and the wall topple. Somethimes t may be better to use an alternative DPC to the roll of felt.

The cheap polyphene DPCs are more prone to letting the wall slide, and IIRC they are not supposed to be used as horizontal DPC's

Personally I don't think the DPC will make the slightest bit of difference. But guess it should have one. The house is on a slope, and the foundation for the wall starts approx 5' below the FFL, and is approx 2'9" deep then there is loads of brick infill (over 2') before some compacted ash and the floor slab. there is not the slightest hint of damp anywhere below the slab, and even the soil i excavated for the foundation was bone dry.

 

[NS215]

Monkey,



There's a double skin garden wall, round the corner from me, approx 1m high that 'fell' over about 5 years ago. It had only been there a couple of years and you could see it had hailed horizontally right along the dpc - hence my preferred method.

Why would you put a dpc in a garden wall ????????????

 

[noseall]

Monkey,



There's a double skin garden wall, round the corner from me, approx 1m high that 'fell' over about 5 years ago. It had only been there a couple of years and you could see it had hailed horizontally right along the dpc - hence my preferred method.

Why would you put a dpc in a garden wall ????????????

 

[hotrod]

Monkey,



There's a double skin garden wall, round the corner from me, approx 1m high that 'fell' over about 5 years ago. It had only been there a couple of years and you could see it had hailed horizontally right along the dpc - hence my preferred method.

Why would you put a dpc in a garden wall ????????????

I don't know. I didn't build it.

Best of luck Monkey, hope it all works out ok mate.

 

[MonkeyHunter]

Best of luck Monkey, hope it all works out ok mate.

Thanks Hotrod, it will be fine, ill go for the mortar, DPC, Mortar, and either have the DPC run down the side of the wall for the depth of the mortar joint or maybe ill adjust the FFL.

On a related note. My house has/had a aprox 10mm b.itchumen DPM, and the previous owners removed a large proportion of it, and then installed electric underfloor heating with tiles on top. Using the tile grout to make up the 10mm difference between floor levels, all of that has now been removed leaving the slab exposed, so I need to do something to reinstate the DPM, and get a floor I can carpet / tile etc... is it possible to get someone in to reinstate the b.itchumen? or am i going to have to lay a membrane, in which case what would be the best way to go about it? as laying the membrane directly on the concrete would i presume risk puncturing it as it is not particularly smooth.

Edited because of the overly sensitive swear filter...

 

[Austtheboss]

Hi monkeyHunter, what did you settle for in the end? I have the same query as I am doing a shoffice (Shed/ Office). I have a concrete floor with DPM hanging over the internal course. Would you or anyone know if I should:

Mortor>DMP>Mortor>DPC>Motor>Block

or

Mortor>DPM>DPC>Motor>Block

or

DPM>DPC>Motor>Block

Cheers

 

[Nige F]

Monkey,

There is a lot of disagreement about this topic in general. My personal preference is to 'sandwich' with a thin slither of, as you say no more than 5mm. Seen loads done, as you say by just plonking it straight onto the blockwork.

There's a double skin garden wall, round the corner from me, approx 1m high that 'fell' over about 5 years ago. It had only been there a couple of years and you could see it had hailed horizontally right along the dpc - hence my preferred method. Obviously, if the wall is loadbearing the risk of this type of failure is minimal. I looked into this a few years back and there is a Reg / BS standard that states the dpc should be bedded with mortar both sides but I forget what it is. If I find it I'll post back.

i saw a 150 year old garden wall 200 yards long blow over in 1996. where I used to work . Bricks apparently re sell for 60p each @ salvage yard 18 inch thickx 15 feet tall