Cement Render

[Hobden]

My copy of the SPAB Old House Handbook (OHH, not be confused with OH ), as recommended by Joce, has just arrived. It looks v. useful! Soon I will be an expert on old buildings. Not.

I've had a quick flick through, especially the chapter on walls. We're thinking about buying an old cob house, but unfortunately this has been rendered with cement. Removing it seems like an attractive option, but the OHH is somewhat scary on the point. It suggests that in some cases the hidden cob may have been washed away/weakened by the moisture that can no longer escape and the render could be all that is holding the roof up! Surely not? I had imagined render to be just a waterproof coating, an inch or so thick. To hold anything up it would need to be many inches thick and nobody is going to do that are. Are they? <worries>

 

[malcolm]

I don't know much about how cob behaves, though I attach a couple of images of how wood behaves behind cement render.

The infill in the photos was mud and straw daub which survived very well while the wood was being eaten by beetles, and was the plaster inside was the only structural bit left after the render had been removed (the wattle wood support inside had disappeared). The mud and straw was not intended to be structural in this case but seemed OK.

It would pay to be careful while removing cement render and check the cob looks good underneath the first bit before you remove the next bit. The cement render will need to be removed and replaced with lime as keeping anything wet for a long time will be a bad idea. I doubt the structural integrity will have been washed away from the cob unless there is a big pile of structural integrity sitting outside the base of the wall in the form of mud washed from the wall.

 

[JoceAndChris]

Ohh dear, sorry to have put further worries in your head with that book. They aren't lying though, they're pretty expertful those authors. I'm afraid render can be extremely thick and concrete like and stuck on - here it was 3 inches thick mostly - and you will have seen how dreadfully damaged it was underneath when they took it off, but lime has covered it all up. I was lucky in that though the render was horrendously thick the walls are massively thick so I never truly worried about the house falling down! Some corners were lost though and had to be built up again with old stone we had in the garden. The interior was entirely unaffected.

It sounds like you need to go and view the house again and see if there's any way to tell how thick the render is. Is it falling off anywhere? Is it cracked? How thick are exterior walls, overall? What has been the current owners experience with what's sandwiched between the interior plaster and exterior render? How successfully have heavy items been secured on those walls? What is the roof? Will it need replacing?

If you can only imagine liking the house if you can remove the cement render then it may not be your dream house after all. Sometimes render is a can of worms best left alone. It's removal is always brave and you simply can't know how it will be until you take it off!

The beauty of hiring a company such as the one I used is that they wouldn't get you into a pickle. They would tell you if it were unwise before doing anything major.

 

[Hobden]

Thanks. Was it the cement render that led to the wood getting wet and thus edible to the beetles?

 

[Hobden]

Thanks. Was it the cement render that led to the wood getting wet and thus edible to the beetles?

Oops, crossing posts there - this reply was to Malcolm's post!

 

[Hobden]

If you can only imagine liking the house if you can remove the cement render then it may not be your dream house after all. Sometimes render is a can of worms best left alone. It's removal is always brave and you simply can't know how it will be until you take it off!

No, we like the house as it is, and had no idea it was cement when we first saw it, and still less idea about what that would mean!

The beauty of hiring a company such as the one I used is that they wouldn't get you into a pickle. They would tell you if it were unwise before doing anything major.

If we buy, and decide on remedial action, I'll certainly be back to you.

 

[JoceAndChris]

I had the same experience as Malcolm with wood behind cement render. Lintels that received the brunt of the weather were entirely rotted out and had to be replaced with concrete lintels as green oak shifts and would have cracked the new lime render. Lintels on the fair weather side, fine.

Cement render traps water in nasty ways.

Where the weather hits - the south west corner here - is always worst.

Oh just read your reply Hobden. I actually think you're worrying too much and the house will be fine. If it wasn't going to be fine, I think it would have fallen down by now 8)

 

[Hobden]

I actually think you're worrying too much and the house will be fine. If it wasn't going to be fine, I think it would have fallen down by now 8)

That's what I'm trying to convince myself of! And more importantly, convince OH :lol:

Is there any downside to lime render? I'm wondering why they don't just use it on everything, old, new and everywhere in between. If they banned cement render then at least no more "attrocities" could be committed ... well, I guess I know the answer, it'll be quicker and cheaper to slap cement on.

 

[JoceAndChris]

Oh god Yes the downside is lengthy and upsetting. In times when labour was cheap) stucco was developed for smart Georgians who wanted a Romanesque villa. It aped smooth Ashlar frequently ( scored to resemble the blocks, called lining out) which was and is supremely more expensive. And an infinitely better building material. Look at Bath. It's ironic that the cheap option they developed became ludicrously expensive and difficult to maintain, much more expensive than stone that actually needs nothing!
Lime render must have 4 coats of limewash and those coats need to be kept up as they form a sacrificial layer. A coat every couple of years is probably a good idea. Eventually detailing ( such as lining out or pargetting) in the render becomes blurred and then I don't know what you do, probably masses of laborious hand sanding. Not a problem if you go for a plain finish.
The limewash is laborious, it can't be rollered and must be slapped on in every direction with an enormous brush. Rain dribbles and drops hitting it in the same spot ( like if you have a gutter problem) cause unsightly streaks and runs. I'll get a link to my limewash problems thread for you to see.

So it's expensive, it's not as long lasting as stone or brick, it's hard to find the tradesmen to do it, and it needs frequent repair and renewal. My scaffolding has seriously damaged my beautiful lime work.

I totally see why they got excited about cement render in the 30s and the modern paints. But they aren't suitable.

A Victorian brick built house that has never had its bricks mucked about with is probably a very wise choice.

 

[JoceAndChris]

This thread from summer illustrates the types of headaches limewash gives the owner.

http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=15567


I will resolve it with a complete repaint next year ( after having Skillingtons back to repair the damage caused by the scaffolders the roofers used).

Roof before render. Why didn't I listen?

 

[Gareth Hughes]

There does seem to have been an increasing number of incident like this in recent years. It's usually possible from the photographs that accompany the reports to see that "heavy rain" is not the only cause of the problems
http://www.bbc.co.uk/news/uk-england-devon-36926950
https://www.middevonnewscentre.info/further-cob-wall-collapse-cullompton/

 

[88v8]

I imagine the original builders would have been amazed at the notion that their cob or half-timber buildings would still be standing a few hundred years later.

One need only look at photos of cottages in the 19th & early 20th century to see that many of them were pretty much being allowed to tumble down around their owners' ears, even back then when the knowledge and skills to maintain them were surely ubiquitous.

Ivor

 

[JoceAndChris]

Hobden, can we have a link to this house so we can all have a look? We promise not to buy it!

I simply can't believe cob houses would have a value if they were so utterly fragile. Surely the collapse is the exception rather than the rule, and may owe a debt to ground heave or subsidence as well as cement render and wet conditions?

 

[Hobden]

So it's expensive, it's not as long lasting as stone or brick, it's hard to find the tradesmen to do it, and it needs frequent repair and renewal. My scaffolding has seriously damaged my beautiful lime work.

Er ... just when you think you're out of the jungle!

 

[Hobden]

This thread from summer illustrates the types of headaches limewash gives the owner.

http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=15567

I will resolve it with a complete repaint next year ( after having Skillingtons back to repair the damage caused by the scaffolders the roofers used).

Roof before render. Why didn't I listen?

All very good hard-won advice. Thanks!

 

[Hobden]

There does seem to have been an increasing number of incident like this in recent years. It's usually possible from the photographs that accompany the reports to see that "heavy rain" is not the only cause of the problems
http://www.bbc.co.uk/news/uk-england-devon-36926950
https://www.middevonnewscentre.info/further-cob-wall-collapse-cullompton/

Thanks for that confidence boost Actually, I had seen it before ... and still not sure if I should be worried or not.

 

[Hobden]

Hobden, can we have a link to this house so we can all have a look? We promise not to buy it!

I'll hold off that for now if I may, simply because I've been discussing something that isn't yet mine to discuss - don't want to drop the existing owner in it!

I simply can't believe cob houses would have a value if they were so utterly fragile. Surely the collapse is the exception rather than the rule, and may owe a debt to ground heave or subsidence as well as cement render and wet conditions?

See, this is the confidence boost I need, thanks

Oh, decisions, decisions ... well I guess nothing will happen before Christmas so we can relax for a while. And I can read on ...

 

[elizabeth]

Hello Hobden,
we have a cob home here in Devon and we have been gradually reversing the damage done by cement render. There are a lot of cob cottages here and though collapses do happen, it is thankfully not common but dramatic when it does. Admittedly it is quite nerve racking hearing all the horror stories but having learnt a lot about cob, helped us understand what we were dealing with and reduce the fear of it. We have some pretty excellent lime/ restoration company's here who are very helpful.

 

[Hobden]

Hello Hobden,
we have a cob home here in Devon and we have been gradually reversing the damage done by cement render. There are a lot of cob cottages here and though collapses do happen, it is thankfully not common but dramatic when it does. Admittedly it is quite nerve racking hearing all the horror stories but having learnt a lot about cob, helped us understand what we were dealing with and reduce the fear of it. We have some pretty excellent lime/ restoration company's here who are very helpful.

Have you been removing the cement render a bit at a time? Or did you go for it all off at one go? Any horror stories underneath? And in particular did the removal brings chunks of cob off with it? How thick was the render? Apologies for all these nosy questions, but it seems like you've been there and done that and I'd love to benefit from your experience ...

We've been researching lime restoration companies, but are getting quotes in the order of £150+VAT for removal of render and lime plastering over. And that's without any repairing of the cob if it's in bad shape or gets damaged during the removal! It seems quite a lot to me, but of course I've no real idea of the effort or skill involved.

I wonder if many of the collapses are due to cement render preventing the cob from drying out properly? It would certainly be useful to know that. Are there any "official" investigations with results duly released for public consumption?

 

[elizabeth]

Hi Hobden,
happy to help with the questions,so don't worry about asking. We did our cottage in a piecemeal fashion. This allowed the cottage to adjust gradually to the changes and also suited our budget as we were quoted for the work and then had to allow a contingency for any hidden extras like replacing rotten lintels.
We were told that cob expands and contracts throughout the year and being covered in hard inflexible,cement render, it was inevitable we would experience cracks which trapped water. When we removed the render the stone plinth on the South (weather facing) side was glistening wet, despite being the middle of summer and having had no rain for about 3 weeks.
Since having the lime work done, the moisture in the end of our main beam in the cottage has reduced significantly. As to the condition of the cob,when they took the render off,some places 3 inches thick, the cob was in a poor state, the straw used in the initial building of the cottage had rotted and was no longer binding, causing the cob to turn back to dusty soil. So some areas needed rebuilding with cob blocks. Thankfully this was in patches and not a whole wall.
We have so far tackled 3/4 of the cottage with one more wall to go.
We would be happy to invite you in to see our cottage if you happen to be nearby. I'm not sure how to organise sending you a private message but I can work that out later if you would find a chance to see up hand the work we have done so far useful.
Regards
Elizabeth