DPC laid with no mortar underneath

[flanagaj]

Not sure if it's acceptable to do this, but our new builders have laid the blocks onto the Radon DPC with no mortar below the DPC and only mortar above the DPC.  I did question this and they said it's fine.  I even raised it with BCO and he said it was ok.  Doesn't sound right to me, but I wonder if anyone has any experience of this approach.  This xxxxxxx Radon membrane is the bane of my life and our neighbours said there isn't Radon here (BGS maps show 3%)

 

The thought of having to demolish another bricklayers work is going to send me over the edge.

[MikeGrahamT21]

Whether right or wrong, I’ve had 4 extensions done on my bungalow and none of them have mortar below the DPC, only above, and haven’t had any problems in the last near on 20 years. I have read people mortaring under the DPC but not sure what it brings really, plastic is never going to adhere to mortar, not strongly anyway, and the weight of what’s above will keep it where it should be

[flanagaj]

29 minutes ago, MikeGrahamT21 said:

Whether right or wrong, I’ve had 4 extensions done on my bungalow and none of them have mortar below the DPC, only above, and haven’t had any problems in the last near on 20 years. I have read people mortaring under the DPC but not sure what it brings really, plastic is never going to adhere to mortar, not strongly anyway, and the weight of what’s above will keep it where it should be

I can see why they don't put mortar underneath.  You have a continuous sheet of DPC which is 500mm wide on the horizontal and then it rises 225mm on the vertical before spanning the outer leaf.  Plus it's all taped to the corners, so how you would physically lift it up to put mortar underneath it is beyond me.

[Mr Punter]

Please shut up about radon. It is low where you are. More chance of being run over by a milk float.

[flanagaj]

11 hours ago, Mr Punter said:

Please shut up about radon. It is low where you are. More chance of being run over by a milk float.

It's a shame, I cannot say that to building control. 

[Oz07]

Yeh don't worry I've never seen it done either. Its one of those textbook things which should be done but never is in practice. 

[Mike]

Yes, DPCs should be laid on a mortar bed and, if the DPC has a BBA certificate, you'll find it mentioned there*. However brickies (and, it seems, BCOs) aren't always taught that, or just take the shortcut. It's always been done on sites I've run, but omitting isn't going to cause a problem most of the time.

 

*If not, it may get a mention in the manufacturer's info.

 

16 hours ago, MikeGrahamT21 said:

I have read people mortaring under the DPC but not sure what it brings really

It provides a smooth surface that protects the DPC from any sharp brick edges or projections that could damage it.

 

Edited 6 hours ago by Mike

[Oz07]

Got me thinking how does a flashing interact with a dpc tray? By grinding a lead flashing into a course are you not sending water behind it if running off a doc tray?

[Mike]

18 minutes ago, Oz07 said:

Got me thinking how does a flashing interact with a dpc tray?

The tray goes on top of the flashing, discharging any water onto it.