Plaster and sand mastic around sash window

[endecotp]

Dear Experts,

This is a very old building, last significantly renovated in about 1980.

I am repainting a window, and also replacing the sand mastic (aka trowel mastic) between the wooden case and the stonework because chunks of it have started falling out.

Having scraped out the remains of the old mastic, I have found a layer of plaster on the inner face of the stonework. The window is vertical, and I think the purpose of the plaster may in part be to fix the not-vertical stonework. Some of this plaster is loose and chunks have come out, in particular above the window. It seems to be plastered on expanded metal lath, which is also used elsewhere in the building.

What should I do? I could attempt to squidge some plaster (or polyfiller) in there, and then finish with a fillet of mastic. But I doubt I would actually fill all the space. I could fill more completely using something with a nozzle. Or I could just mastic over the gap and leave an air space behind. Then there are bits where the plaster is loose but not fallen out - should I pull it out and replace, or fill over it?

As you can see in the photos the stonework has some cracks and will need some more extensive repairs at some point (££££), so I am looking for a short/medium-term fix, not necessarily something that will last centuries.

What would you do?

 

[Wayners]

I've never done this but.
Your going to have to look this up as I became aware watching fred dibnah.
I think linseed oil mortar mix around the window will work.
Masking along wall to keep clean.

 

[endecotp]

I’ve found some useful info here:

Burnt Sand Mastic & Oil Data Sheets, User Guides by Masons Mortar

Access comprehensive data sheets and user guides for our Burnt Sand Mastic & Oil products. Ensure proper usage and achieve great results!

masonsmortar.co.uk

Quote: “Joints that have lost their lime backing and where there is clear space between the frame and the masonry must be packed, either with well haired lime mortar or a rot proof compressible filler board or backing strip (Sash and case windows have weight pockets and when filling the gap, care should be taken to ensure nothing enters the weight pocket and affects the free running of the window – For this reason expanding foam should be avoided)”

I’m not sure what the “compressible filler board or backing strip” is, any ideas?

@Wayners , I’ve not found much about “linseed oil mortar”, do you perhaps mean the burnt sand mastic described above? I have a tub of the Everbuild sand mastic that Masons Mortar describe as a “more user friendly, but actually significantly less durable of the shelf product”. It’s not bad to work with and is what pros have used on neighbouring properties, but I’m not enthusiastic about using loads of it to replace the lost plaster as well as the old mastic.

 

[Wayners]

Yeah burnt sand stuff. That's it.

Some would run masking up wall and window and fill with foam. Mastic over but I don't like that idea for old property

 

[endecotp]

Yeah i don’t want to use foam, but I might consider a cartridge-gun filler. Trouble is they are slow drying for deep fills.

 

[endecotp]

I think the “compressible filler board” probably means bitumen wood fibre board, which is sold as an expansion joint filler. I can’t find anywhere that sells it in small quantities yet. I’m not convinced I could jam that in and then mastic over it. Hmmm….

12mm x 150mm Wood Fibre Fillerboard Strips (pack of 10)

Buy 12mm x 150mm Wood Fibre Fillerboard Strips (pack of 10) · Used as an expansion joint filler for concrete slab and wall construction to reduce the risk of cracking.

insulationgo.co.uk

Edit: I think Jewson sell the strips singly.