Cracked cement render, early georgian house

[sjpray]

I would appreciate any advice on the repair of cracked cement render.

Early georgian property, grade II* listed, built in early 1700's and then modified (and rendered) in 1880 when two adjacent properties were combined together. The underlying brickwork is london red stock, lime mortar, and the current render is cement based with a lime coloured skim coat and multiple layers of paint. Based on other articles here and elsewhere I understand that Portland cement goes back to 1860, so it is quite likely that the current cement render is 1880. The cracks run mostly vertically between the corners of windows, and dont appear to be deep or structural. The render is well bonded and given that it has probably been there 150 years I do not plan to remove it. The cracks have been repaired and filled, multiple times, and I am hoping for a more permanent solution.

Current ideas:
- use a cement render to patch - but I am concerned this will crack again given the lack of flexibility
- use a lime render to patch the cracks - but I am not sure this will bond well to the cement?
- use a membrane, eg: http://www.sandtextrade.co.uk/en/Pr...ion/Polyester+Membrane/Polyester+Membrane.htm

Any comments, advice and ideas gratefully received.
Rgds, Steve

 

[DaveBrigg]

The cracks in cement render come in two types. One is where the render has blown, and with no direct support from the wall, the render cracks. The only real solution is to remove the loose render and patch or completely replace, depending on how much you are left with attached.
The second crack appears when there is an underlying problem with the brickwork, and the render is firmly attached but cracks as the brickwork beneath it moves. Any repair or patch is likely to be a temporary fix until the cause of the movement is dealt with. I had what sounds like a very similar problem to yours, with a crack in 19thC cement render starting in one corner of the window and leading to the top corner of the wall. The crack was up to a centimetre wide. Removal of the render revealed a crack in the brickwork beneath, 3 to 4 cm in places. The evidence suggests that there was subsidence before the 1880's, and the resulting crack was just rendered over rather that being filled. After this, the subsidence continued, but at 1cm in 130 years, a structural engineer said there was nothing to worry about.
If your cracks run vertically between windows it may mean that the lintels need checking.
There are issues with having cement render, but as you say it is firmly bonded, leaving it in place may cause less damage than trying to remove it. The best bet is probably to remove a small area each side of the crack, and see what lies beneath.

 

[Nemesis]

And before you do anything, contact the conservation officer, and probably (as this is II*) English Heritage too!


Early cement is much softer stuff, by the way, than modern Portland.

 

[DaveBrigg]

Early cement is much softer stuff, by the way, than modern Portland.

I'm not sure of the exact date of our cement, but almost certainly at least 100 years, and in places it's hard enough to require the use of an SDS hammer drill to get it off. There are a few more modern patches which are even worse, but on a scale of hardness the cement outscores the tudor brickwork by several degrees of magnitude.

 

[Nemesis]

Sure - but before standardisation in the twenties, it was very variable, and early stuff softer than it is today.

Tudor bricks, too, varied in hardness. Not exactly a standardised product either.

 

[michael]

I’ve had a few similar problems on my cement rendered (rusticated stucco) property dated 1880. The property has also had several coats of off white paint applied to it during its history as it would seem have most of the stucco frontages in south coast seaside towns and the large London estates such as Belgravia, Regents Park, and Mayfair.

I had a couple of hairline cracks running from the bottom corner of the upper window seals to the top corners of the lower window reveals. Having observed the cracks for a couple of years they never got any worse, they just looked unsightly so after much deliberation and research including PPUK, I decided to fill the cracks with ‘frame sealant’, the stuff you get in a tube and apply with an applicator gun. I applied a bead of the sealant along the crack, pressed it in with a wetted finger, wiped clean, allowed to dry and then repainted over. So far so good but time will tell.

Other inconspicuous areas of the building had small patches of blown render (hollow sounding when tapped). My understanding is that these old render finishes were made up of three coats :- Render coat , floating coat and finish coat. In my case I found that it was only small patches of the finish coat that had blown and that the floating and render coats underneath were perfectly sound.
I decided to patch repair the blown areas of finish coat with a HYDRAULIC lime mix (hl 3.5) or even (hl 2) if in an unexposed area. It’s not as tough and ridged as cement, not as soft and breathable as hydrated lime but a good compromise being more flexible than the original render which tends to stop the new application cracking around the old. Two years on in my case and all still pretty sound.

These are just my unqualified suggestions but if you want to get seriously involved in old render repairs this site might help

http://www3.westminster.gov.uk/spgs/publications/Stucco.pdf

The site also advises on the types of render (stucco) and the periods it was available. Interestingly it says here that Portland cement stucco was not widely available until the 1850’s when stucco was falling out of fashion but it was used extensively by developers for stucco cornices, window surrounds and other stucco decoration. I think this is what I have.
.

 

[sjpray]

Many thanks for the various postings, esp the last one re. Westminster council which was quite helpful. Having stripped a lot more of the paint off the surface, I am pretty sure it is a c.1880 portland cement render applied over the original 1720's lime mortar brickwork, approx 1" thick, and with a stone coloured surface coat that is scored to imitate stone blockwork, and which has subsequently been painted (and repainted multiple times). The render is well attached, only one loose area, and the cracks are somewhat superficial but have been patched and filled many times over the years which makes them look far worse than they are. A number of the cracks follow the block score lines in the render, and are along the classic stress lines between vertically adjacent windows. Given the formation of the cracks and the solid nature of the render, I believe they are due to a combination of minor settlement over the years (but nothing evident internally or elsewhere in the street) and/or general thermal movement that is exacerbated by brittle nature of the render over relatively flexible lime mortar.

I agree with the approach to use a flexible filler, as the reality is that these old houses are moving all the time. The multiple layers of filler and patching are testament. I am planning a trial using a Sandtex trade polyester membrane, which is a 300mm wide polyester mesh that is stippled onto the surface over the crack, then covered with a primer and then paint coat. Hopefully will provide a flexible covering and allow some ongoing movement without cracking and letting the water in. Has anyone ever seen this before/ know if it can work?

 

[Tilly71]

I’m in the process of buying a Georgian grade 2 listed terrace with survey just arrived. Render in patches blown on rear of house, looks like it was rendered in cement previously. Can the patches be repaired using lime based render or does the whole lot need to come off and start again?

 

[ThornhillMike]

I’m in the process of buying a Georgian grade 2 listed terrace with survey just arrived. Render in patches blown on rear of house, looks like it was rendered in cement previously. Can the patches be repaired using lime based render or does the whole lot need to come off and start again?

Interested in replies to this. I've got the same issue, compounded by a previous owner deciding that a few coats of waterproofing paint would help the situation (of course it very much hasn't). I'm Georgian detached so an awful lot of exterior. The difference between patching and all over hacking/re-rendering is pretty substantial...!

 

[a twig]

Wow, a 15 year old thread comes lurching back from the dead!

Generally “all off” is best. If you are desperate to avoid that, the most critical is usually the ground floor, so you could compromise and only hack off and re-render the bottom storey.

Alternatively if you aren’t currently having any damp issues etc and just want the render tidied up then give the patch repairs a go with a NHL3.5 or NHL2 based render and monitor it over the next year or two