External Render and internal water proof render - Which one?

[20sHouse]

Hi All,

I have recently bought a late 1920's/early 1930's house . It is made with the softer period brick, seems to have a cavity and has lime mortar/lime plaster behind the later coats of plaster.

There are 2 main areas I need to tackle, following replacing the flat roof behind this white external facia and having the back of this rendered.

1) External render to the white area you can see in the photo, clearly this has seen better days. I have received a couple of quotes and they both want to go with sand and cement. My thought is this is probably fine for the upper part given there is no internal dwelling there but I am wondering whether it may cause problems in the lower sections given this backs off to photo 2... Is sand and cement suitable, or a silicone render? or....?

2) Photo 2 i.e the internal bricks, this was covered in a plasterboard and had the old lime chipped away (probably when it's got wet historically with the flat roof failing). The suggestion again is to put a waterproof render over this, then a plaster skim. Again the suggestion seems to be that the base 'render' is sand and cement with some water proofing agents. Is this suitable? if not what would you suggest? Also there are a few holes in the lime mortar, would you fill these with lime before render/plaster? Again the plasterers seemed to think it didn't need it and the render coat would make this good.

All thoughts appreciated.

Thanks

 

[Cubist]

You do not say what the existing render is on the external facade; do you know what it is? Also, do you know what mortar was used for the brickwork itself?

Assuming that it's original to the construction of the house it could be either a cement or lime based mortar. In principle I am against using different mortars which may be in contact with each other as a close bond can seldom be achieved. Such weaknesses/fault lines can permit the penetration of rainwater into the underlying materials and its tracking through to the interior. It can also introduce the risk of damage resulting from differential movement which can literally tear a wall apart.

I would strongly suggest that you discover what mortars were used for the brickwork and attempt if at all possible to use the same material for both the external and internal render coats.

 

[20sHouse]

Unfortunately I don't know what the external render is, it wouldn't surprise me if over the years it was replaced with sand/cement then painted over with a textured paint. I have attached a closer up picture.

For the bricks, it is almost certainly lime mortar that has been used.

 

[Cubist]

If you are certain the brickwork is constructed with a lime mortar I feel your best option would be to use this for both the external and internal render coats and then apply a lime plaster internally to finish. Sadly, of course, that means all the external render must be carefully and fully removed to avoid any damage to the underlying brickwork.

 

[Stevers]

Although there is no clear bond pattern, there are far too many headers in that internal view of the wall to assume a cavity unless it's an exceptionally thick wall.

Cement renders have a nasty habit of developing hairline shrinkage cracks over time, are too rigid and can lack adhesion. The outside render needs to be softer than the bricks. That suggests a lime or NHL mortar and it's not the time of year for that.

A bugbear of mine is why render always seems to be painted and not left it's natural colour. Keeping it painted just makes for a lot of work, the contrast between materials becomes too dramatic, and if modern masonry paints are used it's damaging to the fabric once moisture is trapped behind the paint as it will be.

 

[20sHouse]

I think that is the case i.e lime internal render and lime mortar, no idea what they have used on the outside render. A couple more pictures of exposed internal render/plaster in another room, plus the external pointing.

 

[20sHouse]

Although there is no clear bond pattern, there are far too many headers in that internal view of the wall to assume a cavity unless it's an exceptionally thick wall.

Cement renders have a nasty habit of developing hairline shrinkage cracks over time, are too rigid and can lack adhesion. The outside render needs to be softer than the bricks. That suggests a lime or NHL mortar and it's not the time of year for that.

A bugbear of mine is why render always seems to be painted and not left it's natural colour. Keeping it painted just makes for a lot of work, the contrast between materials becomes too dramatic, and if modern masonry paints are used it's damaging to the fabric once moisture is trapped behind the paint as it will be.

Ah yes for that part, as it's the balcony doors, I suspect that bit is just a solid wall. I have just measured it through the aperture in the door and external brick to internal brick facing, its about 22cm which I think points to a solid wall.