Raising the ceilings in a Cob cottage

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CobFamilyMan
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Raising the ceilings in a Cob cottage

Post by CobFamilyMan » Tue 4th May, 2021 1:16 pm

Afternoon!

I've tried to find case studies and examples online for this but surprisingly I've come up empty handed. Perhaps someone on here can help!

I recently bought a small detached cob house. At some point the ground floor has been concreted - I'm thinking it'll stay that way. The ceiling height downstairs is annoyingly low. I've removed the lath and plaster from the lounge ceiling. The first floor is basically joists running from the front wall of the house to the back wall, set into the cob, sitting on a surprisingly thin looking timber wall plate. I have this vision of simply extending the "slots" in the cob above each joist, jacking the floor up by 15cm and filling in the gap under the joists with timber or cob blocks cut to fit as tightly as possible. Alternatively, given the fact that some of the joists have had to be reinforced already, it might be a good idea to replace all the joists and wall plate, but then there's the issue of cutting holes right through the cob to enter the new joists in from outside. Not sure how else they would go in!

Eventually I would want to do the same upstairs. The ceiling up there looks like it is hanging from hangers that simply rest on the loft wallplate so again, looks like it should be a simple job to remove and build a slightly vaulted ceiling in it's place

I know things are never as simple as you expect them to be, but can anyone thing of a good reason not to do this? Or any alternative ideas?

Cubist
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Re: Raising the ceilings in a Cob cottage

Post by Cubist » Tue 4th May, 2021 6:38 pm

Hmmm, scratches head and thinks......

....still thinking, but....

I have no idea what the point loading limits may be for a cob wall but I would guess that its not fantastic and thats probably why 'thinnish' beamsl/lintels were installed to spread the weight load over as large an area as possible. Your first idea, to me, sounds the most realistic but it would be wiser, I think, to use timber to provide the additional height needed under the joist ends. Whats foxing me is how best to secure the joists to these blocks to avoid lateral/twisting movement of the joist.

My concern here is that the depth by which each joist fits into the walls may not be sufficient to allow use of a couple of long frame screws to permanently fix the joists to the blocks and perhaps also the lintels.

I'll keep thinking about it and if I come up with anything else.......

CobFamilyMan
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Re: Raising the ceilings in a Cob cottage

Post by CobFamilyMan » Thu 6th May, 2021 7:41 am

Thanks for the reply. I see what you mean. One idea might be to enlarge the resulting gaps beneath each joist to fit a mini - wall plate under each one. But I was hoping to modify the cob as little as possible.

If I decided to replace the joists, How does one create the holes through which to feed the new ones?

In case anyone is wondering, it's not listed.

Cubist
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Re: Raising the ceilings in a Cob cottage

Post by Cubist » Thu 6th May, 2021 7:29 pm

I'm still scratching my head and its getting a little thin up there with all the befuddling challenges my place continues to throw at me.

Ah well, in for a penny in for a....

You've lost me... why would you excavate 'under' the existing joists? Surely if the intention is to raise the height of the joists and floor then you need to make space above each joist - or am I missing something?

Personally, if the existing joist are structurally sound I would be doing my damndest to disturb them as little as possible. But, if you really must, and only to replace a timber that is damaged or rotten, the technique is to create cavities on both sides of the room that are wide and deep enough to give you the 'swing' room needed to manoeuvre the replacement joist into place. On occasion though, when enough room cannot be created in this way, one of the holes needs to be cut completely through one of the walls to allow the new timber to be installed from outside. Easier said than done and I would seek professional advice before doing this with cob walls.

One question I have is, do the joists run wall to wall or are they supported by another beam running parallel to the walls carrying the joists? I'm assuming not but if there is the job would be a lot easier.

On balance I think some exploration is needed to determine by how much any given joist may be buried in each of the walls above and the state of the cob immediately above each joist end. Originally my thinking was that you could excavate the cob above each joist to create a cavity deep enough to allow the joists, and maybe the floor above, to be 'jacked' up to the required new height and then permit new suitable wood blocks/bricks say to be introduced under each joist end.

steveb
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Re: Raising the ceilings in a Cob cottage

Post by steveb » Sun 9th May, 2021 9:57 am

I am not familiar with cob but as a general point, if there is a floor above as you imply then the windows will move "down" as much as the floor moves up so you may end up with knee-height windows on the first floor. plus all doors will be too low, services as the wrong height etc.

I would do everything to avoid raising the ceiling, and look at lowering the floor if you really need the extra height, removing the concrete would be a bonus. Best option, of course, is to live with it as it is but at the end of the day it's your house and assuming it's not listed it's yours to do as you please with.

MatthewC
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Re: Raising the ceilings in a Cob cottage

Post by MatthewC » Sun 9th May, 2021 2:40 pm

CobFamilyMan wrote:
Tue 4th May, 2021 1:16 pm
I've tried to find case studies and examples online for this but surprisingly I've come up empty handed.
Perhaps that's because everyone with the same desire came up with the realisation that it just wouldn't work!

As Steve says above, lowering the floor and getting rid of the concrete (and using limecrete) would be the preferred solution of most people here - possibly cheaper and certainly a lot less risky. I have never worked with cob but I see interfering with it as a route to having the ceiling fall on your head; you really need to find someone with serious cob knowledge. Doing your proposed job upstairs sounds like it should be a good deal safer though.

Cubist
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Re: Raising the ceilings in a Cob cottage

Post by Cubist » Mon 10th May, 2021 9:58 am

.... and, I'm still thinking about this one. Odd how some subjects can be so gripping that one ends up coming back to them for reasons inexplicable.

In this case its maybe the charm of a building technique thought to be thousands of years old or, more likely, the echoes of childhood joys 'playing in the mud' that so many of us possibly share - but would perhaps not admit to at this time in our lives.

Whilst the technical methods to increase the height of a room by lifting the floor above are, on the surface at least, reasonably straight-forward they are fraught with painful and costly consequences should one fail to anticipate them. Steve and Matthew make fair points - the thresholds of doors in the rooms above would also need to be raised, and the header also increased, services may need to be re-routed, additional stair risers and treads may be needed....the list goes on and on.

On balance therefore Mathew has the right of it I think and the best solution may be to excavate the concrete floor below to create the needed head-room, or, like many of us other PP owners, you must acquire the 'Low Ceiling Crouch' that tends to come naturally after a few painful encounters with an oak beam - after all, it was there long before you were.

That aside, this thread reminded me that a few years ago a couple of episodes of Grand Designs featured the construction of a Cob Castle in Devon which is still available on YouTube. Link below for anyone that may be interested.

https://www.google.com/url?sa=t&rct=j&q ... ptlMSErBx9

88v8
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Re: Raising the ceilings in a Cob cottage

Post by 88v8 » Mon 10th May, 2021 10:03 am

I wonder how low is low?

Generally, I disapprove of hacking at an old house to make it 'convenient', so I would make the absolute minimum changes to the joists. Whole-sale replacement sounds like the sort of thing Bodge the Builder would do.

That said, some of the things I did in our first house 45 years ago now make me shudder.... my halo is not entirely intact.

I do understand the desire not to duck... we lowered the floor in our dining room by 3", nice not having to duck under the spine beam. I'm only 6ft tall, previous owners have been 6'2", 6'4" and 6'7" that I know of, they just got used to ducking in various places.

Your concrete floors should come out, that may give the option of lowering a little, but there's a risk of undermining the non-foundations, and it's a little bit messy.

Anyway, joists if you need to replace any... presuming they won't be on show... easy.... say you have a 12ft joist, use two lengths of 8ft and sister them together with coach bolts. Overlap of 4ft, plenty for upstairs.**

I really really would not mess with the walls.

Ivor

**edit... maybe 5ft under the bathroom if you have a cast-iron bath.

steveb
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Re: Raising the ceilings in a Cob cottage

Post by steveb » Mon 10th May, 2021 2:14 pm

you may also find that any structural engineer or building control, if involved (which they probably should be) would look at 100 year old joists and say "those are not up to the job" - consult the timber tables and tell you that each joist now needs to be 12 inches deep and 3 inches wide to cover the span these days, so you could be going up 6 inches and yet lose 6 inches of headroom

CobFamilyMan
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Re: Raising the ceilings in a Cob cottage

Post by CobFamilyMan » Tue 11th May, 2021 10:28 am

Thanks for all the replies, really usefully thought provoking stuff. I'll try to respond to everything... in no particular order:

Regarding the "lowering" of the upstairs windows... Not ideal I agree, but every time I bang my head I can't help thinking that the pro's would outweigh that one con. It's only a bedroom upstairs so I could live with it.

The bit of house in question is a two up two down with a staircase in the middle. The bathroom isn't in this part of the house, so thankfully it's not an issue. It's just a lounge and a bedroom downstairs and two bedrooms upstairs. It's only really the lounge that I'd like to modify. There's also a brick arch fireplace in there which I'm told was probably built in the victorian times so not original, but will need rebuilding if I want to make a feature of it. There are complications with it but that's a subject for another thread!

I had ruled out the idea of digging up the concrete floor because a) what if it was laid for structural reasons and b) will there be something useful underneath, or just earth? I am tempted to break into the middle of the floor just to have a look, and if it's not viable, refill it and put the carpet back! A similar aged house close by did that very thing recently, discovering an old hard floor underneath, and giving them an extra 10cm of headroom. So it could be an option. Could do with finding out why it was concreted in the first place.

There are no beams supporting the joists unfortunately. The joists run from front wall to back wall (approx 5.5m) and if replaced, would almost definitely need to be larger timbers. That said, several of them have been reinforced with new timber and bolts already, suggesting they are pretty darn old. The reinforcements are ugly but were hidden by lath and plaster before I pulled it down. I'm not so sentimental about the timbers that it would stop me from replacing all the joists with new, but as you mention, building control would definitely want to screw up my headroom plans.

The ceiling height (bottom of the joists) in the lounge varies but averages at about 6ft. I'm 6ft2". The lath and plaster was 2" thick before I removed it. The doors that would be effected are all part of a non-structural wall which spans the house at the mid point, so no issues with modifying the frames.

"I really really would not mess with the walls." - Understood. I'm really stuck on this one. The timbers will need replacing at some point anyway. They flex a lot when someone's upstairs, and they have already been reinforced in places. So when the time comes to replace them, there will no avoiding messing with the walls.

The sistering idea sounds viable. Wonder how building control would view it if I did the entire ceiling like that!

Thanks again for all the input. What I really need is for you all to come and see the house for yourselves and give opinions in person :)

CobFamilyMan
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Re: Raising the ceilings in a Cob cottage

Post by CobFamilyMan » Tue 11th May, 2021 11:40 am

Thanks for all the replies, really usefully thought provoking stuff. I'll try to respond to everything... in no particular order:

Regarding the "lowering" of the upstairs windows... Not ideal I agree, but every time I bang my head I can't help thinking that the pro's would outweigh that one con. It's only a bedroom upstairs so I could live with it.

The bit of house in question is a two up two down with a staircase in the middle. The bathroom isn't in this part of the house, so thankfully it's not an issue. It's just a lounge and a bedroom downstairs and two bedrooms upstairs. It's only really the lounge that I'd like to modify. There's also a brick arch fireplace in there which I'm told was probably built in the victorian times so not original, but will need rebuilding if I want to make a feature of it. There are complications with it but that's a subject for another thread!

I had ruled out the idea of digging up the concrete floor because a) what if it was laid for structural reasons and b) will there be something useful underneath, or just earth? I am tempted to break into the middle of the floor just to have a look, and if it's not viable, refill it and put the carpet back! A similar aged house close by did that very thing recently, discovering an old hard floor underneath, and giving them an extra 10cm of headroom. So it could be an option. Could do with finding out why it was concreted in the first place.

There are no beams supporting the joists unfortunately. The joists run from front wall to back wall (approx 5.5m) and if replaced, would almost definitely need to be larger timbers. That said, several of them have been reinforced with new timber and bolts already, suggesting they are pretty darn old. The reinforcements are ugly but were hidden by lath and plaster before I pulled it down. I'm not so sentimental about the timbers that it would stop me from replacing all the joists with new, but as you mention, building control would definitely want to screw up my headroom plans.

The ceiling height (bottom of the joists) in the lounge varies but averages at about 6ft. I'm 6ft2". The lath and plaster was 2" thick before I removed it. The doors that would be effected are all part of a non-structural wall which spans the house at the mid point, so no issues with modifying the frames.

"I really really would not mess with the walls." - Understood. I'm really stuck on this one. The timbers will need replacing at some point anyway. They flex a lot when someone's upstairs, and they have already been reinforced in places. So when the time comes to replace them, there will no avoiding messing with the walls.

The sistering idea sounds viable. Wonder how building control would view it if I did the entire ceiling like that!

Thanks again for all the input. What I really need is for you all to come and see the house for yourselves and give opinions in person :)

a twig
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Re: Raising the ceilings in a Cob cottage

Post by a twig » Tue 11th May, 2021 8:27 pm

Almost definitely right there steveb - although my joists are oak and have done a perfectly good job - technically they are 2" too thin for modern regs.

philpjuk100
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Re: Raising the ceilings in a Cob cottage

Post by philpjuk100 » Fri 14th May, 2021 9:23 am

"Ducking" under beams can be learned, I have been in this house for 13 years now and used to bang my head leaving the bedroom, over a few months I learned to duck automatically when leaving the bed room until last week when I opened the door and noticed another landing door open, forgot to duck and found myself on my knees on the floor wondering where the Elastoplast was, (no fun being bald ). I do not have the option of modifying the house as it is one of the main beams of a hall house. Would I raise the ceiling if I had the chance? I don't think so.

Gothichome
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Re: Raising the ceilings in a Cob cottage

Post by Gothichome » Fri 14th May, 2021 11:03 am

My thoughts on this subject. Run several steel jacking beams through the walls perpendicular to the joists, cut the perimeter of the outer wall at the existing sill beam and jack the whole second floor up. Install a cripple to take up the space and start treading mud in the back yard to fill in the cripple. I think this kind of job would be best left to the house jackers. Treading on mud though I think will be up to you.

MatthewC
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Re: Raising the ceilings in a Cob cottage

Post by MatthewC » Sat 15th May, 2021 12:20 pm

CobFamilyMan wrote:
Tue 11th May, 2021 10:28 am
I had ruled out the idea of digging up the concrete floor because a) what if it was laid for structural reasons and b) will there be something useful underneath, or just earth? I am tempted to break into the middle of the floor just to have a look, and if it's not viable, refill it and put the carpet back! A similar aged house close by did that very thing recently, discovering an old hard floor underneath, and giving them an extra 10cm of headroom. So it could be an option. Could do with finding out why it was concreted in the first place.
Finding an old hard floor underneath is actually a very real possibility, given the modern tendencies of owners and builders. Are there any signs that the floor level has been raised - like doors not being the right proportions due to having a lump cut off the bottom?

I would doubt that the floor was installed for any structural reason. If there is a hard floor underneath it's likely to be thinner (perhaps only 3") than if there isn't. If there's no hard floor there then the concrete is likely to be 4" - 6" thick with some rubble as well.

Matthew

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