# Forum Home Renovation Waterproofing  Waterproofing cellar ceiling - Underside of a tiled patio

## m1nstral

I'm after some help on what to use to waterproof a concrete ceiling and prevent mould regrowth.  
As you can see in my picture three's a patio with a doorway underneath it. The top patio is slate tiles and there must be a crack as water seaps in and causes the underside ceiling to have mould   
Here is the doorway and an idea of the space underneath. Also, the mould growth if you can see the white stuff on the ceiling    
I believe I've stopped the leak with Gripset Betta Concrete Filler. However mould is still growing and previous cleaning hasn't gotten rid of the spores. I'd like help / advice on:   Getting rid of mould for goodSomething (a membrane of sorts) I can paint onto the ceiling to further stop leaks coming through and act as a barrier for any mould coming through, and future mould growing on top of the membrane.Given the smoothness of the concrete ceiling I imagine some kind of primer may be required and would I be able to repeat the process for the floor. Less about any water coming through but to prevent any mould underfoot.  
Thanks!

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## Moondog55

Actually if you have sealed the leak I'd tackle better ventilation first priority. Maybe by removing a few high level bricks and replacing them with vents and adding a low level intake somewhere in the door as low as practical. Use mechanical ventilation if required

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## m1nstral

Hi Moondog, thanks for the tip. I believe I might've tried that by leaving the screen door locked, but the wooden door open most of Winter. Mould still grew but I didn't tackle the leak before that. I'll try that. 
I still have mould growth now however despite tackling the leak so was hoping I could apply a material on the ceiling that resists mould growth, but also has waterproofing properties.

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## phild01

Ventilation and more as mentioned by Moondog.
An absolute solution is the patio having a waterproofing membrane under the tiles as well.

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## woodbe

> I still have mould growth now however despite tackling the leak so was hoping I could apply a material on the ceiling that resists mould growth, but also has waterproofing properties.

  Fix the leak before you do anything else. If you seal the ceiling the water will be pushed elsewhere (well, until the waterproofing fails) The only way to fix it properly is to pull up the tiles, add a waterproofing membrane as suggested by Phil01 and relay tiles. Stop the water entering the structure rather than trying to stop it leaking out.

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## cyclic

Also, the mould growth if you can see the white stuff on the ceiling [Q 
Your mould looks more like calcium.

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## Marc

I agree.  Water leaking through a concrete slab looks like that. Stop the water and you will stop the white stuff.

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## commodorenut

I have similar looking white stuff on a retaining wall in the garage, that has groundwater run up behind it - it's efflorescence - minerals leeched out of the blocks & concrete by the water.  Ventilation won't cure it.  Stopping the water getting through the porous slab is the only way - which will require a membrane over the top surface of the slab (ie, under the tiles) as mentioned above.

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## phild01

Zooming the pics, agree that ventilation is not an issue, it's like what stalactites are in caves and is not mould.

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## Marc

Not good for the steel in the concrete!

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## m1nstral

> Zooming the pics, agree that ventilation is not an issue, it's like what stalactites are in caves and is not mould.

  Stalactites, never thought of it that way... That white stuff on the ceiling will disperse and fall to the ground with a blow of air and if left to its own devices will just drop to the floor. It's never hardened so assumed it was a mould. Twice over the course of this year I've cleaned it up back to bare concrete and it's grown back, without any visible ceiling leaks. 
Obviously waterproofing the patio is the ideal solution but it's a massive expense I can't commit to at the moment. Plus, I assume the white stuff will grow back again even if I did this.  
All tips welcome!

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## m1nstral

Has anyone tried Zinsser product? https://www.bunnings.com.au/zinsser-...paint_p1560190

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## cyclic

> Has anyone tried Zinsser product? https://www.bunnings.com.au/zinsser-...paint_p1560190

  May work, but are you aware you have to paint it on the top of patio, not underside, and also waterproof up behind the brick walls and up under doors as well.

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## m1nstral

I know I'll have to get onto the top of the patio. But it'll be a big job to dig up the slate tiles and waterproof and re tile. Looking for a less costly interim approach that may / may not work. Painting straight on top of the tiles is likely to be an eyesore. I can't locate the entry point for the leak on top either as there are hundreds of minute cracks on the surface.  
Is anyone aware of a clear membrane I can apply to the tile surface? To prevent pooling where I've sealed the underside and withstand the elements and foot traffic? 
Painting the ceiling is to prevent further growth as the leaks been plugged and the growth is on the ceiling. Behind the brick wall is empty space as the house is on stumps and it's completely open. Will do under the doors as you suggest though, good idea.

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