dpm issue

[vibetime]

hi all , i have searched about this issue but am still not clear on a true answer !
so, i have built walls for my cony upto dpc level, to match existing house dpc. where the cony meets the existing house wall (now internal) i was going to lap the dpm up the wall to just above dpc, then when concrete is set, and floor is screeded just trim it to level of dpc, that way damp could not bridge over the existing dpc ?
or does this not matter and i could lap dpm further up past the dpc and make floor level higher ? surely this would in theory allow a pathway for damp to rise past it ?
thanks.

 

[hotrod]

or does this not matter and i could lap dpm further up past the dpc and make floor level higher ? surely this would in theory allow a pathway for damp to rise past it ?
thanks.

Only if your DPM is defective.

 

[vibetime]

how do you mean defective ? could damp not track up the edge of the new dpm, which would be above and below (as it runs past vertically) the existing dpc in the house wall ?

 

[r896neo]

I am a bit confused, How is damp going to track up polythene? Damp can track up through porous materials like brick block wood etc but it can not travel in thin air or in polythene? It's not under pressure being forced up through a gap??

 

[vibetime]

i meant could it travel up the edge of the new dpm to the level it was cut off to ? this edge is the one that is adjacent to the existing house wall .
im bit concerned as a friend did the same and he has damp along this edge
cheers

 

[hotrod]

how do you mean defective ? could damp not track up the edge of the new dpm, which would be above and below (as it runs past vertically) the existing dpc in the house wall ?

Unless there is a hole in your DPM, the only path would be for moisture to travel vertically up the DPM (bypassing the existing DPC) and then transfer into the existing masonry. For this to occur you would need numerous separate factors to happen, including a near constant supply of moisture under pressure and a badly detailed and installed DPM where it meets the existing wall / DPC. Don't forget, moisture will always follow the path of least resistance.

 

[vibetime]

ok thanks for your help, its cleared the issue up.

 

[wavetrain]

You are supposed to fold the dpm down onto the dpc and lap&seal it by say 50mm to avoid the situation you fear. How many people bother to do this I do not know. You could always lap the dpm up the wall above the dpc by say 150mm having first painted 150mm height of brickwork that is above dpc with bitumen compound for piece of mind.

Noseall is familiar with the 1st lapping technique I have mentioned

 

[vibetime]

i cant lap it over the dpc as it would mean grinding it out etc wich would damage the existing dpc.
my floor level was going to be at the same level as the existing dpc, is this not a good idea ?
could i have floor level higher than existing dpc if the dpm finishes at a higher level ?

 

[wavetrain]

I did mean for old and new floor levels to be the same!

I have just re-read your post. I beleive you allow the dpm to rise up the house wall above the existing DPC by 25mm and cover it with skirting board. Before placing your dpm you could apply bitimen paint to the house brickwork below house DPC and upto and touching existing house DPC.

I am sure others will provide better advice for you

 

[vibetime]

cheers for your help that sounds a plan
Anymore advice is welcome tho