DPC detail for stone cills

[ruggers]

I'm about to have my cast stone stooled cills fitted into the face brick exterior wall and I need to know if it's best to use 100mm DPC underneath, or a larger stepped DPC.

 

What are the benefits if any of the stepped DPC as per my sketch?

 

My partial fill cavity wall is

100 face brick, 50mm air gap, 100mm PIR, 100mm block. The cills are 65mm in height x 175mm deep so that i can have them set back into the cavity by 25mm, which then allows the windows to do the same. Rear of stone cill to front outer face of PIR=25mm.

 

I planned to install a trimmed cavity closer under the cill, so a stepped angled DPC would interfere with this unless folded at 90 degrees.

[torre]

Ask the manufacturer for an installation detail. Your stepped DPC is definitely approximately the right approach. 

 

Unlike plastic etc cills, stone or cast is porous so you need to prevent moisture getting through and behind it.

 

This is a decent starting point and also emphasises you may need to accommodate expansion/movement between the cill and brickwork, that's another point on which the manufacturer should be able to help

[torre]

Make sure your window supplier knows you're having stone chills if you want the frames to sit directly on them as you need to be sure any frame drainage will work. 

 

Quite often you'll see a stub plastic cill installed on top of a stone cill, which isn't quite such a clean look.

[ETC]

Wrap the cill.

[Oz07]

I just used 100mm below and up sides on a previous house. What's the benefit of bringing up behind the stone we don't do this with facing brick

[torre]

When facing bricks are used to form a cill you'd put a tray behind in a similar way.

 

Compared with vertical brickwork, a cill is catching a lot of weather, both falling directly and run off from the window.

 

Think of it this way, a typical window has a waterproof layer (the plastic cill) directing water away from the cavity. Here the cavity tray is the waterproof layer instead

[ruggers]

On 06/12/2025 at 11:28, torre said:

Ask the manufacturer for an installation detail. Your stepped DPC is definitely approximately the right approach. 

The cast stone company won't provide details, they just produce them only and leave it up to the builders. I discussed two methods of installing cills I seen regularly and they said everyone has their own preference. 

On 06/12/2025 at 11:28, torre said:

 

Unlike plastic etc cills, stone or cast is porous so you need to prevent moisture getting through and behind it.

This is what makes me unsure about the stepped DPC, because any water finding its way in from the outside into the cavity won't have anywhere to go, so it will just remain damp against the plastic tray. Why is a stepped tray needed when below the cill is open cavity, or a window head with cavity tray and weep vents?

On 06/12/2025 at 11:28, torre said:

 

 

 

This is a decent starting point and also emphasises you may need to accommodate expansion/movement between the cill and brickwork.

I have some expansion foam left from the brickwork, looks around 8 to 10mm thick but its knowing how far to set it down between the centre joint with them being in 2 parts and colour matching mortar pointed between them.

 

On 06/12/2025 at 11:32, torre said:

Make sure your window supplier knows you're having stone chills if you want the frames to sit directly on them as you need to be sure any frame drainage will work. 

 

Quite often you'll see a stub plastic cill installed on top of a stone cill, which isn't quite such a clean look.

I didn't know there was another option other than just using 85mm stub cill with a 70mm frame, The stone cill raised section is 40mm to bed the upvc cill onto with the frames fixed using window straps.

[ruggers]

On 06/12/2025 at 15:14, ETC said:

Wrap the cill.

As in 125 underneath the cill, up the back of it and then back over the top where the UPVC frame and cill sit over the cast stone?

 

Any idea what the benefit of an external cill tray as opposed to just having 100mm DPC under the stone cill?
My rear windows won't have stone cills , they'll be UPVC frame and cills direct onto the brick work with DPC or stepped tray underneath.

It just feels like a stepped tray is going to prevent water escaping since theres no weep vents on a cill course where as a window lintel has weep vents every 450 horizontally.

[ruggers]

@Oz07 Thats what I was thinking but I wanted to see what others have done in case I'm missing something. The water cant get into the house because theres an open cavity gap.

[ruggers]

These are the cills, just basic stooled.

 

 

[Oz07]

I always thought the only reason for the dpc was so the brick and stone could move differently without cracking the stone. Wet cast stone doesn't need apparently much stronger the dry cast is weaker. You only have to look at them funny and they chip

[ruggers]

Yeah its creating a slip plane they say.

 

So many methods people choose from, full mortar bed underneath, others spot bed them.

 

Full length 100mm dpc vs full length stepped dpc.

 

Small cut of dpc over the stooled top or not.

 

Expansion foam between any joints if the cills are in 2 halfs. I'm not sure if this is a good or bad idea because the foam between isnt goijg to compress if theres mortar above in the joint. Some use a hybrid ploymer to fill the gap not mortar.

 

The correct dpc detail and the centre pointing are the details i need to get right. 

 

These are being installed tomorrow

[Oz07]

Like I say I just used 100mm dpc. Bottom, up sides and onto stooling. Jointed in middle with standard mortar touch weaker if anything. No probs.