Cob or brick & cement? First time build in Winchester, UK

[bigdavenorcott]

Hi all - I've been dreaming of building a WFO for years but finally getting around to it now (mostly because I've got a couple of tons of concrete rubble to hide that I'm putting into the base =D ). Was hoping you guys might be able to throw me a little advice?

I started with the intention of building an entirely clay & cob one, following the river cottage model (don't know if you have that show in the US). The one I've seen that is closest to what I have in mind is Capt Jon's excellent oven on this forum, here: http://www.fornobravo.com/forum/43/c...n-19828-4.html - only I was intending to use cob insulation rather than vermiculite/ concrete.

The decision I'm trying to make is whether to use brick instead of cob - mostly because clay is more of a pain for me to get than brick (I live on chalk geology and the nearest clay subsoil is a good way away; hopefully I also have a pretty large free supply of old red solid bricks). Cost is a big factor - I want this to be pretty inexpensive, and don't intend to buy expensive firebricks. I'd like it to last a few years but I'd rather rebuild another later, than blow hundreds of ? on it now. From what I've read old red solids should do the job: Alternative for Fire Bricks are the old red clay solid bricks. Firebrick substitute

OK to get to the point - my alternatives are, for inside of dome/ insulation/ outer skin:
Cob version: Clay & sand mix / clay slip & woodshavings / clay & sand mix (and a roof to protect it as it's obviously going to weather)

Brick version: Brick dome (using FB recipe ref. mortar I guess) / some sort of insulation (?rockwool held with chickenwire) / vermicrete?/ stucco

The base I'm pretty confident with (reinforced concrete slab, insulating layer, bricks set in thin layer of sand).

Comments very welcome and advice gratefully received!

[wotavidone]

Re: Cob or brick & cement? First time build in Winchester, UK

Old solid reds for free sounds good to me. How about do the best insulation you can afford under the floor, then build the dome from the solid reds? Try it out for a while then insulate over the dome as funds permit?
Always do the underfloor insulation - it's the one thing you can't fix later.
If the bricks are free, the cement, lime, sand and clay would be pretty reasonable here in Oz.

[bigdavenorcott]

Re: Cob or brick & cement? First time build in Winchester, UK

Old solid reds for free sounds good to me. How about do the best insulation you can afford under the floor, then build the dome from the solid reds? Try it out for a while then insulate over the dome as funds permit?
Always do the underfloor insulation - it's the one thing you can't fix later.
If the bricks are free, the cement, lime, sand and clay would be pretty reasonable here in Oz.

Thanks for the reply! I was thinking I'd use about 3-4" loose vermiculite for the insulation layer (contained within brick surround over conc. slab), with foil over and then sand/hearth bricks. Reckon that would move around a bit too much?

Cement/ sand/ lime all pretty cheap here too - fire clay not so much.

[cobblerdave]

Re: Cob or brick & cement? First time build in Winchester, UK

G'day
All sounds good.
On the fireclay. The refractory properties of fireclay are not required at the temps that a WFO operate.
However a clay content in the mix is a great thing. Your mortar will stick to the brick better and the mortar will be able to be squished about a bit without losing its bond. All good things to a first time bricklayer !
Use bricklayers sand which has a clay content already added. Or substitute POWDERED clay.
Regards dave

[bigdavenorcott]

Re: Cob or brick & cement? First time build in Winchester, UK

Awesome - thanks a lot.

[bigdavenorcott]

Re: Cob or brick & cement? First time build in Winchester, UK

Excuse the n00b question but I've read so much now my brain is turning to dough.

For the mortar mix; is it 1:3:1:1 portland cement:sharp sand: powdered clay:lime?

Whilst I'm at it, shouldn't I insulate the top bit too before I run out of steam?

[wotavidone]

Re: Cob or brick & cement? First time build in Winchester, UK

I reckon 3-4" loose vermiculite under the floor might be a bit unstable.
Have you looked at this?
Home | UK Wood-Fired Oven Forum

It's reasonably common for UK builders to use thermalite blocks for underfloor insulation.
I'm a bit frustrated. I've worked my fingers to the bone on google, but I can't find out what cement is used in thermalite.
I want to know if they use the same cement as our aerated concrete blocks, which we call Hebel.

Of course you should insulate over the top as soon as you can, it is just that if expense is a problem it can be done later, not like the floor which is difficult to change after the oven is built.

[bigdavenorcott]

Re: Cob or brick & cement? First time build in Winchester, UK

I reckon 3-4" loose vermiculite under the floor might be a bit unstable.
Have you looked at this?
Home | UK Wood-Fired Oven Forum

It's reasonably common for UK builders to use thermalite blocks for underfloor insulation.
I'm a bit frustrated. I've worked my fingers to the bone on google, but I can't find out what cement is used in thermalite.
I want to know if they use the same cement as our aerated concrete blocks, which we call Hebel.

Of course you should insulate over the top as soon as you can, it is just that if expense is a problem it can be done later, not like the floor which is difficult to change after the oven is built.

I've come to the same conclusion about the vermiculite! Am now planning to pour 3-4" of vermicrete over my slab layer, then (if I can find some affordably) 2" of that solid insulation sheet stuff underneath the cooking floor itself. Still trying to get quotes from a few places.

I'd be surprised if your aerated blocks aren't essentially the same thing. After a bit of googling, they're both aerated autoclaved concrete - think their U-values are pretty identical. Are they cheaper than pouring thermacrete?

Cheers

Dave

[david s]

Re: Cob or brick & cement? First time build in Winchester, UK

It is worth getting dense firebricks for the oven floor. Some solid red bricks can't take it and will spall, there's probably no way for you to find out if they are suitable. They will probably be ok for the dome though.

[bigdavenorcott]

Re: Cob or brick & cement? First time build in Winchester, UK

It is worth getting dense firebricks for the oven floor. Some solid red bricks can't take it and will spall, there's probably no way for you to find out if they are suitable. They will probably be ok for the dome though.

Thanks - I'm keeping my eye out for some reasonably priced ones. Storage heater bricks look like they'll work well and are sometimes really cheap

[wotavidone]

Re: Cob or brick & cement? First time build in Winchester, UK

I've come to the same conclusion about the vermiculite! Am now planning to pour 3-4" of vermicrete over my slab layer, then (if I can find some affordably) 2" of that solid insulation sheet stuff underneath the cooking floor itself. Still trying to get quotes from a few places.

I'd be surprised if your aerated blocks aren't essentially the same thing. After a bit of googling, they're both aerated autoclaved concrete - think their U-values are pretty identical. Are they cheaper than pouring thermacrete?

Cheers

Dave

Probably more expensive but I think, if they don't degrade, that they are an excellent insulator. I helped my mate build an oven in January, and he bought a pallet of 24'x 8" x 6" blocks, for about 2 dollars each. His oven sits on a layer of them, and the enclosure is made from them as well. Certainly holds heat well.

[bigdavenorcott]

Re: Cob or brick & cement? First time build in Winchester, UK

I'm reading a lot of people have used straight vermicrete as an insulation later under the hearth. Do you guys have a view on whether that's enough? I could easily do 4-5" of that and put the firebricks straight on top.

EDIT: Nah I need something else don't I.... Now thinking lay 4" vermicrete over my normal conc slab, then for the sub-firebrick area a sheet of calcium silicate board. That should do it

[Archena]

Re: Cob or brick & cement? First time build in Winchester, UK

Don't mind me - just lurking...

[bigdavenorcott]

Re: Cob or brick & cement? First time build in Winchester, UK

For want of any further progress so far (turns out I need to go to work rather than build my wfo), here's a couple of shots of the base in progress.

Bottom filled with closely stacked large concrete demo rubble, then topped off with rammed chalk subsoil.

I was going to pour a concrete slab over this then a layer of vermicrete, but I think instead I'm going to add a layer of precast concrete slabs I've got lying around then pour a thicker layer of vermicrete straight on top of that. Think the concrete slab is overkill - that base ain't going anywhere I don't think; happy to be corrected though!

[Archena]

Re: Cob or brick & cement? First time build in Winchester, UK

Are you building an oven or a house? :wink: No, that thing isn't going anywhere... EVER!