laying dpc should this be on mortar or not?

[simonjay]

Right, now i am confused.
I like the idea of only lapping the DPM by 50mm. It has concerned me that the only thing holding the wall in place is its own weight as the mortar surely can not get a good key on a smooth peice of plastic.

To add something else, I was told once upon a time to lay an UNEVEN bed under the DPC so that gravity will lock the wall above in place and it cannot slide. It certainly makes sense. If you do that, you have to put mortar below and above obviously.
The DPC is textured plastic for a key, whilst the DPM is smooth so it makes sense to only lap the DPM 50mm.
Which way round to put DPM and DPC - I haven't seen a conclusive argument one way or the other.
Simon.

 

[^woody^]

Uneven beds are definitely bad practice. The bed needs to be even, level, and smooth to avoid punctures and stress

Buildings wont slide all over the place, so that it not an issue. But an added bonus with DPC/DPM material is that is allows the inevitable thermal movement of the different bricks and blocks above the DPC

 

[theoldun]

Posted: Fri Jun 15, 2012 7:15 pm 

^woody^ wrote:
The NHBC show an illustrative diagram with no supporting information to back it up


Wrong again woody, supporting information is in black and white.
Why not try reading supporting information?
Should a knowledgeable person require supporting information to understand a simple illustrative diagram or drawing?
Regards oldun

Did you not read the supporting information?

Posted: Fri Jun 15, 2012 8:05 pm 
^woody^ wrote
Err no, it does not detail the chances of the dpm tearing on the sharp arris, or the route for moisture. It's a poor detail.

Well, why not write and inform NHBC that it is a poor detail?
We would expect a first year apprentice to fully understand the diagram even without supporting information


Posted: Sat Jun 16, 2012 11:12 am
^woody^wrote

I've had a look at quite a few tech documents about this simple thing, and there is no definitive answer


Then you have not looked far enough woody.
Marley, Anderson, Rubberoid and Plysolene do not give any information..
Visqueen do give detailed instructions and a drawing in their BBA Certificate , however their method is no where near common day practice in as much that the sealed joint between dpm and dpc is below the thickness of screed and oversite concrete, whereby dpm does not turn up and over brickwork and lapped by the dpc.
As it would appear that NHBCs approach is an acceptable alternative, and is every day practice then the way to go is for dpm and dpc on top, all as our original post.
Could we now put this post to bed?
Regards oldun

.

 

[^woody^]

Yes, when you can quote me a definitive BS or EN stating that the DPC should be installed in a particular way then that will do me

Referring to a single diagram in a single guide is not really conclusive. Especially when there is no supporting justification to detail why that method is the "only method" or should be used in preference to any other.

And I can't understand how you can quote four manufacturers which don't mention any installation details at all, and a further one who mentions a completely different installation method to support a proposition that your method is correct.

The thing is, if this was a definitive method, then it would be published and researched more widely. If you refer to BS 8215:1991 Code of practice for design and installation of damp-proof courses in masonry construction, the BRE, to Zurich guides, to the Scottish or NI guides then you wont find any mention of it other than it needing to be lapped.

In fact, if you want to rely on a single image, then look at the picture in the NI technical guides, and it looks like the DPM is above the DPC to me.

 

[browfish]

Crikey, i thought i had asked a simple questions that probably had a simple answer. Thanks for all the input.
I have started the work and lapped the DPM over the blockwork, then laid a thin mortar with DPC and then laid blocks on 10mm mortar as normal.