How do use concrete window sills on timber frame build...

[Carrerahill]

I have my sills sitting outside ready to go in with the block work, they are 215x75-50: 

http://webtrack.jwgrant.co.uk/ProductDetail.aspx?pg=437&pl1=344&pid=2407

 

My question is just exactly how the concrete sill and the timber frame opening will work together, I have 50x50mm treated that I was going to frame out the window and door openings with all the way around, however if I do this then I will end up with the sill sitting 115mm out over the block work, so really the sill needs to sit back into the cavity. 

 

I was thinking like this:

 

 

So you can see the block wall and the render on it, the sill will sit overhanging into the 50mm cavity by 40mm, I will put a piece of DPC along the back just in case, then I will add my 50x50 treated to the sides and top (not in sketch), the window then fixes to these and it sits on a stub sill with some brackets holding it to the sill trimmer, sitting on the concrete sill. Does that seem right?

 

I was actually just going to bring the blockwork level with the timber frame opening, close the cavity all the way around and use a PVC sill to sit from the cavity closer out over the block but my brick layer said order concrete sills, there is no detail in my BC drawing for this so I am not sure how to proceed - concrete certainly would be more robust and in-keeping with the rest of the house, but I am not sure how it all goes together now!

 

Photos or details of those of you who have done this or seen it done would be great.

 

Thanks

 

 

 

[Mr Punter]

From a thermal efficiency viewpoint, you should set the glass of the window somewhere in the insulation layer, so the window needs to move inside a fair bit.  The window is often attached to the timber frame and you will need to make allowance- perhaps using Compriband tape - for the timber frame to shrink downwards compared to the masonry.

[Declan52]

The cill will sit as you have it as if you are using a cill that suits a traditional build.  You do as you have said and wrap the  cill in dpc.  It will sit in the cavity what ever distance it needs to be to have the raised lip edge in the correct position that you want  the window to be. 

But you can get concrete cills that are for timber frame that are shorter so don't sit in the cavity. 

You can see examples here 

http://www.doranconcrete.ie/bp_cills.htm

[Carrerahill]

35 minutes ago, Declan52 said:

The cill will sit as you have it as if you are using a cill that suits a traditional build.  You do as you have said and wrap the  cill in dpc.  It will sit in the cavity what ever distance it needs to be to have the raised lip edge in the correct position that you want  the window to be. 

But you can get concrete cills that are for timber frame that are shorter so don't sit in the cavity. 

You can see examples here 

http://www.doranconcrete.ie/bp_cills.htm

Thanks for that. I did do a deal with the merchants that they will take the sills back for a small fee - I could be tempted to find some like this...

 

Hmmm... 

[Carrerahill]

42 minutes ago, Mr Punter said:

From a thermal efficiency viewpoint, you should set the glass of the window somewhere in the insulation layer, so the window needs to move inside a fair bit.  The window is often attached to the timber frame and you will need to make allowance- perhaps using Compriband tape - for the timber frame to shrink downwards compared to the masonry.

Yes well my plan had always been to do it something like this, frame out the opening with 50x50 then install it flush to the front of the cavity which would put the inner glass pane into the insulation area, just and the block would come up to the face of the PVC.

 

I should just go to the same height with the wall and use a the PVC sill.

 

 

 

 

 

 

Edited April 1, 2019 by Carrerahill

[dpmiller]

The cill isn't really connected to the window. Our build has a firestop batten to the sides and top of the opening, with the window fitted flush with the front of this. The UPVC frame is then sitting on the TF opening by  10mm or so. We set the cills in on DPC as per @Declan52 and then filled the gap with Compriband.

The blockwork is built over the firestop and in due course the render will extend over the windowframe as a rebate.

[Carrerahill]

25 minutes ago, dpmiller said:

The cill isn't really connected to the window. Our build has a firestop batten to the sides and top of the opening, with the window fitted flush with the front of this. The UPVC frame is then sitting on the TF opening by  10mm or so. We set the cills in on DPC as per @Declan52 and then filled the gap with Compriband.

The blockwork is built over the firestop and in due course the render will extend over the windowframe as a rebate.

 

Yup, I am hearing you. I know for settlement I want the concrete sill slightly lower than the window and it's own stub sill or whatever I opt for so as the frame shrinks it won't end up lying on the concrete sill. 

 

I think what I will plan for, is exactly what you have just mentioned - my block will go up before my windows go in annoyingly or else I could just fit my windows as they need to go and my brick layer would just bring his blocks up to suit the windows. I have old windows, I am going to take the bottoms off them and sit them in position as a guide for the sill position, he is a good brick layer and knows his onions bricks so he will leave sufficient settlement gap.