How to start new wall against cob?

[William]

Hello everybody,

This looks like an interesting forum for period property, so I’ve registered and this is my first post, so hello.

I’ve undertaken a variety of work on previous period homes but, for the first time, I’ve moved to a cob house so I find myself on a new learning curve and I probably have more questions than answers - ‘hope that’s okay! I’d also be interested to meet people, see work or even problems associated with cob property near where I live, in mid Devon.

So my first question is how does one ‘start’ a perpendicular wall, of conventional construction, up to a cob wall? I’ll soon be demolishing a small porch (which is so badly constructed that it actually takes some believing) and I plan to rebuild a similar sized addition using either conventional brick or maybe oak framed construction. I’m not listed.

With conventional modern construction, where one would start a wall from stainless wall starters, bolted into the existing wall, that doesn’t strike me as being a viable option when coming off a cob wall so can anyone shed any light?

I did wonder whether some kind of plates or mesh, alternating horizontal and vertical orientation, maybe 50cm apart, built into the new construction and lime rendered into a reasonable depth slot in the cob might be appropriate. If it went say 30cm into the cob, surely any forces concerned would at least to some extent, be less of a point load and more of a turning force, which I’m guessing might work?

I’ve looked around and haven’t found an answer and surely there must be a well-tried, good practice method – can anyone advise?

 

[a twig]

The traditional way of attaching things into cob was wooden rounds driven into the wall and packed in, providing a surface you could then screw into, but not sure that would work in this case.

I’ve no direct experience but there seem to be products like this designed for the application you require?

 

[William]

The traditional way of attaching things into cob was wooden rounds driven into the wall and packed in, providing a surface you could then screw into, but not sure that would work in this case.

I’ve no direct experience but there seem to be products like this designed for the application you require?

Thanks twig. Being my first post, I was starting to wonder if I had done something wrong, forgotten to change my socks or broken some forum taboo as I thought it was at least a half way sensible question, yet until your useful suggestion, not a dicky bird!.
Great suggestion. I was aware that wood wedges/rounds could be used for certain types of wall fixing but thought that technique might not be up to the task, especially with long term stuff like wood shrinkage/worm/rot and all the other nasties. Thanks for the link and the spiral ties you've suggested seem like a very good idea and probably much better/neater than my idea of using mesh plates rendered into the cob, but I'm slighltly consoled by the fact that I might have been somewhere along the right lines.

 

[a twig]

No problem, there are a lot of experienced members who post on this forum but contributions are usually intermittent depending on what people have going on in their lives, so if you have a slightly more niche question (there aren't many with cob experience on here for example) then it might take a while before the relevant person checks in and sees your post. Usually as darker evenings draw in and people are limited in what works they can carry out, participation rates tend to rise. Certainly no taboos broken on your part though! Best of luck and do please post back and let us know how you get on, as that will help provide valuable resource for future members who may find themselves in a similar situation.

 

[elizabeth]

Welcome William, Twig is spot on when he says people generally respond over time.

We own a cob and stone property with a modern extension. We can certainly vouch we have had our fair share of experiences with cob and used some very traditional building techniques to repair with the help of a very friendly family run firm. We are gradually changing the house to lime render.

Helix bars sound a good option for tying in a brick wall to a cob wall. Are you planning to render it? I will add that we have a fine crack on our cement render between the cob and the block extension, different thermal movement.

Be very happy to share our knowledge, the good and the bad if you would like to visit you would be very welcome . We are about a 30/40 minute drive from you.

 

[William]

Thanks for your reply and earlier I was looking further into the stainless spiral ties and will be following it up with a call to Leviat, the firm who supply the Helifix product suggested by Mr Twig. They have an interesting sounding method for fixing the ties and I did find specific mention of cob on their website so this is starting to be a very promising lead.

You ask if I shall be rendering my new construction. I'm not sure! Render would probably be visually more in keeping with 'rough' harled lime but some years ago I did build a oak framed porch with brick infills, which I know doesn't exactly rate as a serious oak framed structure but I still draw pegged all the joints and really enjoyed working with green oak - most satisfying stuff and a gratifying result. But that was quite a different character house and I wonder whether here, I might run the risk turning a typical mid-Devon cob house into something of an 'architectural pseudo hotch potch', which might be better avoided. Somewhat ironically, modern brick/block construction might at least 'appear' to be more in keeping, once rough rendered. So probably yes, rendered. There is another equally horrendous pile of bricks leaning against the rear of the house, so I have two additions to demolish and rebuild. The whole of the rear elevation needs knocking off and re-rendering too but with Winter rapidly approaching, it's time to admit defeat for this year and perhaps try to make sense out of some of the internal disaster zones! ..or take a break and do some planning.

Hopefully you have little cause for concern with a little fine cracking at the block/cob interface and as you say, just a bit of difference beween how the two materials behave and will probably settle down. Thanks also for your kind offer to share your experience of what sounds like some similar challenges. Haven't yet found my way around this forum but would like to follow up if one can pm other members.

 

[elizabeth]

Hello William, I have sent you a message.