Can you cob straight onto existing concrete foundations

Hi All,

Looking to build my first cob house this year on a existing building plot which basically already has a solid concrete foundation.

My question is, is a rubble trench necessary if existing foundations already exist.

So, Can I build a stem wall straight on top of a concrete foundations and then the cob onto that? I have been told the stem wall is required to let the cob walls 'breathe'

Thanks in advance for any help on this as thius would make my project alot easier as there is ALOT of concrete to get rid of if not.

Best Regards
James

Hi James,

You can build off of concrete, but I would have to know more about the configuration of the concrete work, and what type of stem or "sill" wall you are considering to build, to give any clear guidance. Could you possibly post pictures.

Regards,

jay

Hello Jay,

Thank you for your reply.

Here are a few photos as requested. Basically the back and right side wall of the existing house will be kept and I was hoping to cob further out and extend the front all the way along the exiting concrete. The stem would will hopefully be made from existing rock from another building directly on top of the concrete.

I hope you can make sense of these.

Best Regards
James

Hello James,

I understand more now, but that has only generated more questions.

1. Is this in a desert environment?

2. Do you mind sharing the exact location?

3. How long have you been doing Cobb/Adobe work?

4. What is the circa date on the original building, it appears to be of vintage age?

5. The concrete appears to be crumbling in some locations, and unless you have extensive experience with concrete, I would recommend speaking with at least three concrete contractors of good reputation to look at the foundational elements. This structure appears vintage to me, that being said, the foundation may not me concrete at all, but stone with a thick concrete parging and in situ slab poured on top. If my assertion is correct you would be strongly advised to seek advice from a P.E. and/or a restoration specialist. If you have the skill sets, you can encapsulate the existing foundational elements in new dry laid stone work (properly done) and then continue to build as you see fit. I have reservation about destroying and/or modify vintage architecture without regard to the original craftspeople, and respect to their original work when ever possible; which includes doing new work in traditional systems as they would have.

Regards,

jay

Hi Jay,

Sorry for the late reply, I have been away for the past week.

Here are the answers to your questions.

1/2. It is not in a desert Environment, it is in the mountains in Bulgaria, in the Rose Valley.

3, I am quite new to cobbing but have done a couple of courses and feel confident to start building.

4, Not exactly sure of the date of the original building but only the back wall which is made of stone will be staying as the current side walls are made from adobe bricks which need renewing (cob walls will be used instead)

5, I will try to find out about the concrete foundations as I have a architect friend who helped me to purchase this and knows her stuff. I will let you know.

My main concern was whether you can actually build a stem wall directly onto the concrete as I have heard some people say the stem/cob walls all need to breathe and with concrete I may have problems with rising damp entering the stem wall as it will not have the usual rubble trench. Although after cobbing for the last 4 days with a group, the instructor did say I shouldn't have a problem doing it this way. What are your thoughts on this please?

Again, thanks for helping me on this Jay.

Best Regards
James

Hi James,

Thanks for keeping me in the loop. I have enjoyed following along and will love to see your progress. I believe you can come directly off the concrete as well, as long as the concrete is of good construction and quality. Your architect should be able to aid in this. As for the capillary effect of concrete to wick water into the cobb, that is very possible, but can be subjugated by several means. Something a simple as a membrane between the cobb and the concrete could possibly work.

Have you learned about adding lime to the first course of of the sill cobb going into the stem wall?

Have you learned about lime plasters yet?

Are you going to have a stone course for the stem wall plaster in cobb?

How thick are the walls you plan on building? (I would recommend at least 600 mm for the sill/stem wall, tapering to 400 mm minimum for your region and climate.)

Are you doing any CAD or blueprints? I would strongly recommend doing so. It could aid folks like me assisting in the design process.

Will these structure you are building have any "bones" (i.e. structural elements) besides cobb?

How will you build the roof?

Regards,

jay

Hi Jay,

That's great.

I'm not sure exactly what materials I will be using for the stem wall (will have to see what is available), probably brick for the walls with lime (NHL5) with Leca as the insulation.

I did some lime plastering at the weekend, seems quite easy actually.

I am not sure what you mean Jay by "Are you going to have a stone course for the stem wall plaster in cobb?"

I was planning on 600mm walls with no tapering. As I underdstand the only reason for tapering is to save on cob and would prefer the extra thermal mass.

I am planning on a green shed roof, to keep it nice and simple.

Just started to design it in google sketch which I will be more than happy to share.

Here is my initial design, seems very small now

Best Regards
James Ellison

Sorry meant to say 650mm walls.

Hi James,

Send me the actual file that you are working on to:

tosatomo@gmail.com

I want to look and study your concep more.

Regards,

jay

Hello James,

Hope the project is working out for you. We would love and update.

Regards,

j

Any updates?

James, I'm also wondering how the project went, as I have a similar old house to renovate in the north of Bulgaria.

I would be using those adobe bricks again, with a mud mortar between them and a lime plaster on them when bricked up.

Guessing at what isn't shown in the photos, I suspect some of the mud bricks in the right hand wall may have been water damaged due to tiles having fallen from the eaves. The other issue with some earth-built village houses are that they've been cement rendered in an attempt to modernise, and once water gets in, the earth walls can hollow out behind the render. That could be why James wants to rebuild that wall.

I'm likely to have the same issue with my Bulgarian cottage. I still haven't made it there to see it (September, I hope!), but in photos it's clear the roof is sketchy at best in a few places, especially along the eaves on one side. The walls look solid in pics, but actually testing them out may show something different. I'm wondering what surprises that  white-painted render is hiding!

Once the roofing's repaired, would cobbing to fill in any damaged sections rather than rebuilding a mostly-sound wall work, provided nails  were carefully knocked in or screws screwed in for the cob to stick to? I'm thinking a bit like the way cob corbels are built, using plenty of long straw, except used to fill holes and furrows made by water running down the wall rather than building out from the wall. I'd much rather repair than rebuild, it it's a workable option.

I'm hoping I can find people who understand the old way of building in my village. Unfortunately due to the mass exodus of younger people to the cities and abroad, much of the old knowledge may have been lost. I hope not, as the vernacular building styles there, so different in each region to adapt to the climate, are beautiful!

Jane if you get some more photos of the problem areas we can give more definitive answers.
I am keen to help if I can.