Hearth design - expansion gap between concrete/vermicrete and outer brick walls?

[Jek]

Hi everyone,

First post from me. I've recently started a cob oven build in southwest Scotland.

I've built a 1x1x1m open topped brick base and have filled it 3/4 full with rubble and sand.

My plan is to pour a 50mm thick concrete base on top of the rubble fill, then above this a 100mm thick vermicrete insulating layer, before laying the fire bricks on top. The firebricks will lie approximately flush with the top row of bricks on my base.

My question is...do I need to leave an expansion gap around the edge of the concrete base or the vermicrete insulation layer? I am concerned about thermal expansion of these causing my brickwork base to crack.

Photo attached of what I have so far.

Cheers,
Jek

[UtahBeehiver]

Welcome,

Will be interesting to watch, not many cob ovens have been build on this forum so it will be a learning experience for me. Based on the base footprint, the oven will smaller. So I suggest you look at David S of Australia threads since his ovens are smaller diameters as well as cast so he may have some insight of forming the dome, etc.

If I get this right, Your bricks will sit on top of a 100mm vcrete base. Just lay the floor bricks, without mortar so they free float on the vcrete. You can use a 50/50 sand/fire clay mix to help level the bricks if needed. A 50mm (2") concrete base is fairly thin but the oven base is as well. Being cob, the weight won't be significant compared to fire brick.But you might want to consider a little thicker concrete base. Not much material cost difference between 50mm and 75 or 100mm.

[myke]

Think you have a good plan so far the concrete should be insulated by the vermicrete so shouldent expand or crack. the vermicrete will "give" a little if needed so wont crack your base. Echo the above comment about not using mortar under the firebricks (I bedded them on a fine layer of sand) but did mortar the entrance bricks.
Hope you are enjoying the build and keep us updated too.

Cheers Myke

[david s]

As Myke said the vermicrete has a bit of give so won't worry your bricks around the outside. Your vermicrete slab should be 5:1 ie 5 parts vermiculite to 1 part cement. Dry mix these and add 3 parts water. Because there's so much water added it is a good idea to leave the vermicrete fo at least two weeks to dry out. If you build straight over it you lock in the water and it will take many weeks and firings to dry properly. Cover it if it looks like rain.