DPM DPC arghhhhhhhhhh- advice please

[P1]

hi guys need some advice, currently having a extension built and the bricklayer has finished the brickwork and the drains have been sorted now he has started on the floor construction.

Yes i know i should have had the floor done straight after the bricks up to dpc had been laid but the builder had assured me that this is a still as good as a method as the normal way.
so here is basically what will happen tomorrow,( the hardcore and sand binding layer are down)



what he has done is cut long pieces of dpm and lapped under the dpc while the bricklaying was going on. so i assume he will be doing what is drawn in the picture above interms of the larger pieces of dpm.

My question is that is this as good and effective against the damp compared to one full piece of dpm, also will the expansion of concrete as it dries create a water and damp proof tight joint between the two pieces of dpm as shown in the picture?

Also is it normal practice to tape any dpm joints where the two large pieces have been cut to form the whole floor using double sided butyl tape and overlap tape from either screwfix or toolstation? just incase so i know as we are laying this tomorrow.

Should i use double sided tape all the way around to be on the safe side??????

What about around soil pipes how does one create a damp proof joint around them?

Sorry for all the questions, but i am real nervous about the technique he is using and have heard about some nightmare stories about damp and solid floor construction so i just want it done right.

Thanks in advance i hugely appreciate any responses..THANKS!!!!

 

[noseall]

so long as the laps are adequate (600mm) then you should have no problems. after all, not all buildings are small enough as to accommodate a sheet of polythene without joins.

why your builder was reluctant to lay the floor prior to building the walls is beyond me! not only has he made the job more difficult (longer in time also), he has run the risk of damaging the inserted dpm strips.

as for services through the membrane then don't worry too much. just do all you can to ensure the continuity of the membrane and that at no time will the concrete be in contact with anything it shouldn't.

as for taping, then it is good practice to tape the joints. it is only when fitting a GAS membrane that you need to be ultra cautious.

 

[^woody^]

There is no insulation upstand on that drawing (to prevent cold bridge).

Otherwise, the builder is not doing anything unusual.

BTW, concrete does not expand when it dries, it contracts slightly

 

[freddiemercurystwin]

The slab can sometimes move a little bit (downwards) as the earth/hardcore beneath etc compacts a little causing the dpm to rip where it turns up the wall. Doing it this way would prevent that.
Out of interest to comply with the regs in the westcountry we're only allowed to do slabs supported at the perimeter by the inner skin to prevent the barrier/dpm splitting (which is then turned up on the outside of the slab edge and is tucked up under the dpc.

 

[stuart45]

In the 70s it was quite common to cut the DPM off flush with the slab before building the inside skin.

 

[P1]

Thanks for the replies its greatly appreciate as least i am a fair bit more comfortable now....

Quick question would a extra layer of DPM before the insulation, or after the insulation to prevent condensation be advisable? i know its not normal practice but i would feel more confortable knowing that there are two layers of protection, what about a vapour barrier?

Noseall, i did attempt to tape the joints cant believe how difficult it was, maybe beacuse its fairly cold the tape was not easy to use as the adhesive was not at as good as it would have been on a warm day...

woody, i assume you are saying that there should be a strip of insulation between the to dpm on the wall? i will insulate with perimeter insulation when i install the UFH will that be adequate?

freddy i guess you do make a good point about the strain on the joints once the concrete settles.

stuart, yes i do remember they never use to lap into the dpc but i guess thats why a lot of them had moisture and damp problems .

I guess i am trying to do it as good as possible so it will last for a very very long time ?



Thanks again i do appreciate the replies

p.s they have just finished laying the concrete so i hope the dpm works!!

Please let me know what you think about the use of a second dpm or a vapour barrier for added protection ?

 

[P1]

also could i possibly use mastic waterproof adhesive/sealent to join the two vertical parts of the dpm before i put down the final flooring?

 

[noseall]

just use gaffer tape, it'll stick to owt.