Repointing Window Reveal?

[jungle_jim01]

Hello again. Firstly thanks to everyone's support and advice on my previous posts about lime pointing. I've had a bit of practice on the Garden wall which has worked well and am now moving onto the main house. The cement ribbon pointing in my sandstone house is just falling out which makes life a little easier. However, the window reveals are rendered in newer cement which seems fairly well bonded. Ideally i'd like to chip this out too and have sandstone reveals. I'm worried about damaging the stone and also unsure of how to seal the window frame once i remove the render? Presumably if i can remove the cement, then i can use the hydraulic lime mortar to fillet around the edge of the frame? Any tips on treating/painting the frame before/after how to make a good bond/seal etc would be really appreciated...feeling slightly nervous about chipping out the cement render!

Thanks

Chris

 

[jungle_jim01]

i should probably mention that the from the outside the window is recessed into the middle of the stone work.

 

[biffvernon]

Whether the stone under the render will be damaged won't be known until you try. Do a small area in an obscure corner first before deciding on the whole job.

To seal the gap between timber window frames and stone I use one of two materials.

Hydrated lime and sand in ~1:3 ratio. This is cheap and easy and fills a large gap. Disadvantage is that lime mortar does not always bond very well to the timber and as the wood moves with the seasons you sometimes get a crack forming. This might not matter much or if the fillet of mortar is extended over the timber then any crack is hidden from the weather.

Alternatively use 'burnt sand mastic'. This can be bought in a tub or mixed yourself. Use the finest sand available and add linseed oil, mixing till you get a workable paste. If it crumbles add more oil, if it is sloppy add more sand. If it is just right squeeze it into the crack smoothing off with a knife. It takes months to set but eventually goes hard forming a strong but slightly flexible bond with both timber and stone. Never really found a definite answer to where the 'burnt' in the name comes from but it probably refers to the practice of heating the sand to ensure that it is very dry. This is probably a good idea.

What I don't do is go to a DIY shed and buy a plastic tube full of brown gunk called 'Frame Sealant - guaranteed for 10 years'.

 

[jungle_jim01]

Thanks for the detailed reply....much appreciated. Presumably once the cement is removed i repoint the stone flush up to the window frame? Also do you have any recommendations regarding treating the wood. What should i use (if any) prior to painting? And should i treat before or after sealing?

Cheers again

 

[jungle_jim01]

I've just managed to tap out a corner as suggested. This was surprisingly easy and with no damage to the stone so i think i'll end up pulling the lot out and making a good job of it. However. This is all fairly new so i'm not sure exactly what i should be doing.... On inspection beneath the mortar, the stone work has been cut back at an angle to allow the window frame to fit in behind. There's a 'finger width' gap between the frame and the stone, which was filled slightly with cement, behind which is a deeper void of a few cm. So a few questions:

1. What do you recommend for filling the gap? presumably NHL mortar as i've been using for the pointing?
2. Should i pack the mortar to fill the entire space?
3. Is it best to chip out all the cement around the frame before re-filling (how will the frame be secured?) or should i do one side at a time?
4. If i use the 'burnt sand mastic' i presume that this goes on top of the mortar and not used to fill the gap?

Lots of questions there! I hope someone can help.

Thanks again

 

[jungle_jim01]

should i fill gap behind with expanding foam? ....seems a bit of out of place with traditional renovation but noticed a lot of posts which mention it?

 

[biffvernon]

Yes, repoint flush to window.
Paint, I would only use a real linseed oil paint such as Alback, supplied in UK by Holkham Paints.
No other treatment or sealing.
Gap filling. NHL and sand if that's what you've got. Hydrated will do just fine too.
Yes, pack space with mortar unless it is very large, in which case use something insulative - sheeps wool, newspaper.
Remove as much cement as is easy so to do. Frame is not going to go anywhere.
Burnt sand mastic fills the gap, not on top, but if it's a big gap I'd use lime mortar.
Expanding foam - never.

 

[jungle_jim01]

Cheers Biff you've been really helpful....i've just been looking at burnt sand mastic on: http://www.masonsmortar.co.uk/site/burnt-sand-mastic.htm which talks about filling the gap and putting mastic on top...any thoughts?

also...would you suggest i paint before or after filling?

cheers

 

[biffvernon]

You could fill most of a large gap with lime mortar and then use the mastic as a sealant on top, but leave the mortar rather less than proud so the mastic is wedged into a groove. And the mortar will have to be dry before adding the mastic.

Doesn't much matter when you paint. Maybe first coat first and then top coat after all the pointing has been done, then you won't spoil it.

 

[jungle_jim01]

Excellent! thanks again...that's been really useful.