Solutions: Dome Cracking & Waterproofing

[Joeyg]

Hi all,

Location: Birmingham, UK
Build time: Summer 2023
Method: david s home brew castable recommendation


I'm looking to try and find solutions to my pizza oven that has experienced some cracking both on the render, but also through to the inner dome. I also need to waterproof the dome as Winter nears here in the UK. Below is a full overview of the build. A few images of the build are included, along with an image of the cracking. Cracks in the largest places are 1.5-2mm. The main crack is also on the inner dome at around 1.5-2mm thick.

Construction method from bottom to top:
Stand
British oak mortise and tenon base with draw pegs

Base slab
Standard concrete mix, re-enforced

Insulation slab
5:1 perlite-crete - 10cm thick

Harth
50mm 42% alumina fire brick

Dome
Home brew castable, (3:1:1:1 Sand: Portland Cement: Hydrated Lime: Fireclay) with polyprop fibres.

Dome insualtion
8:1 perlite-crete, 10cm

Render
0.5/1cm standard British render with waterproofer agent. 6:1:1 sand, cement, lime.



Possible solutions

Cracking
Fill cracks with intumescent cement. It will still expand when heated, but won't allow quite the amount heat loss or water ingress. Intumescent cement can also have up to 25% flexibility when heated so will keep the cracks closed.

Waterproofing
Has anyone ever applied liquid DPM (damp proof membrane) to the exterior of the dome? The exterior rises to about 70 degrees c when up-to temp. I don't want to build a structure over the oven. Liquid DMP does still have a degree of elasticity to it.


Any recommendations would be greatly welcome.


Thanks

[david s]

Water has entered the insulation layer vis cracks in the outer shell by the sounds of it. Wet vermicrete swells when the water it contains begins to turn to steam. Fortunately if insulating blanket is used between the inner dome casting and the vermicrete, it will act as an expansion joint. Without it an 8:1 vermicrete which is denser than the recommended 10:1 mix does not have much elasticity. You must remove all the moisture from the vermicrete before filling any cracks as they will help to allow the moisture to escape. This may take several long but low steady fires. Try to buy a cheap garden moisture meter that you can push into the cracks on the outside to test how wet or dry the vermicrete is.
The damp proof membrane could work ok, although as they are designed to go under tiled bathrooms they’re unlikely to contain any UV protection. I use a product that is both flexible and high build with a lot of UV protection. It is designed to coat tiled ridge caps and called flexible pointing.

[Joeyg]

If going down the route of applying flexible pointing, would it be a case of pulling off the existing render and applying the flexible pointing, or could it go straight on-top? Do you use the flexible pointing as the render at 1cm thick?

[david s]

No, I do a cement render fortified with reinforcing fibres, sealed up for a week to enhance strength, then after several cooking fires to ensure it’s perfectly dry ,I apply the flexible pointing.
If moisture gets trapped between the outer shell and the flexible pointing, it can cause blistering.
It is designed to be troweled on, but I prefer to thin it down 20% with water so it can be applied by brush. 3 coats then results in a layer of 1-1.5mm thick.

[Joeyg]

Thanks Dave, i'm going to give the liquid DMP a go first, then move onto the flexible pointing if that doesn't work!

[RichV]

You gentlemen seem to have more knowledge than the people at forno bravo. My oven has developed some cracks. Looking for some solution on how to go about repairing. Any direction would be appreciated.

Was told issue was from temperature changes. I live in US in North East.

Sorry to hijack this thread.

Thanks,