Lime Plaster over cob oven

[stephenson1]

So I've been tinkering with a rocket pizza oven for quite a while now. There are some issues I'm working on and at the moment it's how to weatherproof.

Last summer and through this winter I've had a blue tarp over it when not in use. I bind it around the base with a rope.

Two weeks ago I pulled the tarp off and discovered that moisture had gotten trapped under the tarp. This resulted in the thin (3/8" or so) lime plaster I'd applied last spring flaking off under the tarp and a lot of moisture getting into the cob which is now somewhat degraded.

I've read that lime plaster will last well on a stable surface, that although it breathes it should also shed water. Also that lime plaster works well over cob walls because they are very stable; that is they don't change dimension with the weather and crack the plaster coat.

My oven definitely enlarges when hot. It has several cracks that now act as relief joints as it heats up. Does anyone know of a material that is used on a roughly vertical surface that could act as both an expansion space between the cob oven and the a lime plaster shell, and also be rough enough so that I can properly apply the plaster?

Cheers,
Mark

[Donkey]

Tarps are crap for keeping stuff dry. If you MUST use a tarp, build a tent out of it above the thing you are protecting and never let it touch that thing.
The best rain protection of cob is a roof with good eves.

As to the material you're looking for, I'm not thinking of anything off-hand... That your oven expands and contracts like that is an indication that there wasn't enough sand in the mix. It won't effect the use of your oven, but it will be just about impossible to make it stop cracking.

The way I see it, you've got 3 choices:
First, tear it down and do over. This is the crappiest of all choices and the one that I would NOT do if I could help it.
Second, bury the thing in a TON of cob. Make a big sculpture or something over the whole thing and use a layer of cardboard (or other insulating layer) as expansion joint. It's a lot of work but you can get really artsy here and make it very cool in the process.
Third, you can live with the cracks, maybe incorporate them in a decorative plaster design or something.

Whatever you do, build a roof over it and loose the tarp.

[stephenson1]

Thanks for the comments Donkey.

Unfortunately I don't like any of your solutions. I don't really disagree, they just don't suit my situation for a variety of reasons.

Cheers,
Mark

[Donkey]

The only thing that is totally important in what I said above is to fix the tarp(s).. Do NOT let a tarp sit on the oven for long.