Pebbledash - Advice needed on repair please

[michael]

Some relatives of mine are about to repair and decorate the front facade of their 1930’s Bay fronted semi-detached house in Golders Green London. The 1st floor of the façade is covered with original undecorated pebbledash which forms part of the bay between the top and bottom bay windows. I’m guessing that this pebbledash bay consists of a lime and sand mix applied to a timber structure, probably vertical timbers spanned with horizontal laths. The ground floor on the other hand is plain brickwork (painted).

Unfortunately, this pebbledash bay has a vertical/diagonal crack that needs to be addressed.
And in my amateurish opinion, I have suggested that they get their builder to carefully open the crack a little, inspect the timbers behind and if sound repair with a mortar comprising of a mix of 3 parts sand to one-part hydraulic lime NHL 3.5 (no cement) and then pea shingle thrown on to it.

Because the repair will no doubt be visible right on the front of the house and because the rest of the pebbledash is all a bit grubby, they will be looking to paint it, and so to maintain the breathability of the lime and protect the timber structure behind, whilst at the same time provide durability, I have suggested applying:

One coat of Earthborn Ecopro silicate primer followed by
One coat of Earthborn Ecopro silicate masonary paint diluted with 20% silicate primer
One or two coats of Earthborn Ecopro silicate masonary paint undiluted

https://earthbornpaints.co.uk/product/ecopro-silicate-masonry-system/

Please advise if I’m totally wrong on this and how they should go about making this repair

Many Thanks

 

[Gareth Hughes]

If it's a 1930s house it's very likely that the pebbledash will be cement based, rather than lime - so the choice of paint to go on top has relatively little effect on the performance,
Assuming the windows haven't been replaced with plastic, the cause of the cracking might be failure of the timber framing behind the pebbledash, or of the lathing (which is likely to be a non-rustproof metal mesh).
If the windows have been replaced, it's possible there could be major issues since the timber mullions of the original windows will have been part of the structure of the bay, whereas the replacements will not be intended as such - even if they have been pressed into service inadvertently. It's not unknown for bays to part company from the rest of the house in such cases.

 

[michael]

Thanks Gareth
I’m surprised that it might be concrete. The mortar is quite a sandy colour so presumably a high sand content if cement. Unfortunately, yes, the windows are plastic but the crack in the pebbledash doesn’t look too severe, nevertheless if it is concrete and the water has got in then I’m sure that the already compromised mullions won’t be in the best of condition. Don’t know whether to patch up and leave well alone for now and see if it falls out in the future or disturb it further and make major repairs now. Could trapped damp in the pebbledash bay cause other issues, I’m thinking of the dreaded dry rot. There is certainly no evidence of damp on the inside wall.
Anyway, lots to think about, many thanks for your advice.

 

[paulc]

I’m surprised that it might be concrete. The mortar is quite a sandy colour so presumably a high sand content if cement.

Sounds very similar to my 1920s built semi. That too has a pebble dash cement render on the top half with several cracks & hollow sound spots that need attention. Lime mortar was used throughout for laying the bricks, and lime plaster internally. None of the internal walls are truly flat or square, and in places, you can see where the plasterers stopped for a few beers.

One thing to watch out for if replacing wooden windows with uPVC - There may well be a concrete lintel or even a wooden beam supporting the inner leaf of brickwork, but quite often, it is the window frame supporting the outer leaf. So unless any remedial work was carried out when your windows were replaced, keep an eye out for sagging frames and cracks in the brickwork.