DPCs and Beam and Block floor

[Stu84]

Hi all

 

Working alongside a designer to submit full plans to building control.  Really stuck on where DPCs should go. 

 

Designer has specified a DPC (blue line) immediately above the beams which continues up the wall a little before turning into the bed joint where it meets a DPM (red line).

 

I would have thought it better to place the beams directly on a DPC?  Which leads to my next question.... If you place the beams directly on a DPC rolled out over the supporting inner block work then why would you need a second DPC further up the inner skin as so many seem to suggest?  Doesn't the DPC which the beams rest on stop any rising damp entirely?  Is there a risk of the DPC tearing and so another is put further up just in case? 

 

Thank you for thoughts

 

Stuart

[Mr Punter]

Dpc is very cheap. Damp issues are very expensive and difficult to fix. Having more than one dpc is normal.

[nod]

you need to put 600 mil dpc around the perimeter and sleeper walls prior to the block and beam going down Once the blocks are all in lift the dpc up and set your first blocks for the internal block work on top 

Effectively wrapping the beams Then your high load dpc goes on the top of the first row that you have just laid 

[Stu84]

43 minutes ago, nod said:

you need to put 600 mil dpc around the perimeter and sleeper walls prior to the block and beam going down Once the blocks are all in lift the dpc up and set your first blocks for the internal block work on top 

Effectively wrapping the beams Then your high load dpc goes on the top of the first row that you have just laid 

Thank you for comment. I can see how how this works for the sides, but how does this work for the butt ends of the beams? 

[Stu84]

If there's an expectation of DPCs failing then fair enough!  Is there any need to consider slip of the structure on multiple DPCs positioned only a block or two apart?  Suppose provided its sits on a thin bed all is fine?

 

Thanks

[nod]

2 hours ago, Stu84 said:

Thank you for comment. I can see how how this works for the sides, but how does this work for the butt ends of the beams? 

It’s works the same all the way round 

Just lift the dpc up 

20 minutes to roll the dpc out Belt and braces 

[Gus Potter]

7 hours ago, Stu84 said:

Really stuck on where DPCs should go. 

Have a quick chat with your SE. It's often missed at the Architectural design stage.. you get hung up on the thermal bridging, water penetration and so on.

 

But masonry cavity walls can be designed as vertically or two way spanning so they resist the wind load and stay stable (not buckle sideways) when you also load up say the inner leaf with floor joists or the roof which often sits on the inner leaf that load the wall off its centroid. If you put in DPC's wily nilly you can break the vertical spanning capability of the wall which in this case it look like you might have done.

 

You have no wall ties at lower level so that looks a bit off so this is related to the above as you have a quasi rocker bearing in the wall.

 

I've not spent long thinking about this but I would first look at, and then test my arguement: 

 

1/ Delete the cavity tray that runs from the bottom of the cassette vent back up into the inner leaf. What is it doing when you have vulnerable insulation below it anyway? So now we have done something good structurally too!

 

2/ The DPC on the inner leaf is shown at top of screed level. I would look to drop that to bottom of screed (think buildability)  level and then turn it down behind the insulation against the inner leaf and stop it 20mm short of the cavity lean mix fill to stop any potential wicking effect. 

 

3/ Get another wall tie in below the lowest one you have shown to improve the vertical spanning capability of the wall.

 

4/ Your cassette vents are too high so the lower insulation and cavity are not getting air. Drop them and the outer DPC so they are just over 150mm above finished ground level.

 

Draw that out, think about the structural side and mull it over. That is what I would do in the day job and then think does what I've drawn make sense! 

 

For the sense check, yes I've talked about the SE side but let's have a look at buildability and cost. Are we making things more expensive or are there any Architectural details that prohibit say the height of the cassette vents?