DPM for Window cill

[trevorbayliss]

Do i need to fit a DPM under a new stone cill on my extension. Doing a standard block and brick wall with 100mm cavity filled with 50mm kingpan.

I will have insulated cavity closer's down the sides and under the cill. Is this enough ?

 

[Blagard]

If your "stone" cill bridges the cavity closer both front and rear, yes (to be safe). Otherwise no.

Although if your "stone" cill is impervious to water then it really is not necessary unless it is bedded on mortar that bridges the closer. The mortar is the problem then.

 

[trevorbayliss]

Reading about reconstituted cills, my local supplier has some instructions which shows you need to use a dpm, and use a sealant and not mortar under the cill.

I am probably going to go with stone though and this will sit on the external brick wall. plus probably sit a little onto the wall insulation as i have a gap about 40mm and then 50mm of insulation. So the cill will not sit on the internal block wall. So the cill will sit on the cavity closure and probably finish about the midpoint of the closure. So do you think the cavity closure is all i would need ?

If i did add some dpm how do i make a weep hole under a cill? I have some for the lintel, but not sure what you would do under a cill.

 

[Blagard]

So long as the cavity closer is not bridged by the cill then you don't need any further DPM. The cavity closer acts as a DPM between outer and inner skins. You don't need weep holes, they are to drain any water that gets into a cavity above and you don't have a cavity above the cill.

To be honest I would have thought the stone cill would be outer leaf only and with the top set at the height of the inner leaf. The cavity closer would then be on top of the stone cill and the window and window board fitted on that.

 

[trevorbayliss]

Yes probably fit the cavity closer on top of the cill. I am building my extension and about to start on the external skin soon. Got so high with the blockwork and insulation. All a first for me, but going ok so far.

 

[^woody^]

The idea is to prevent damp from moving from the outer skin to the inner.

It does not matter if there is a stone or brick cill, you would still need a vertical DPC (not DPM) along the back edge of cill, but not under it. ame principle for up the reveals

The cill should not be crossing the cavity.

But nowadays there is a tendancy to use insulated cavity closers, if so a DPC may be redundant.

 

[trevorbayliss]

I am using the insulated cavity closers though.