# Forum Home Renovation Rendering  Sealing a leaking tank from the outside

## Wombat2

Not sure if this comes under Rendering, Plumbing or Bricklaying  :Cry:   
We have 140,000 liters of water storage under the garage made from concrete filled concrete blocks measureing 7 x 9 x 2.4 meters. It is built into the slope so the front is at ground level and most of the back is out of the ground - the external wall has ground slope as a diagonal front to back while most of the under house is 3/4 out of the ground (not back filled) Where it was expected to be buried there is black (tar?) painted on the wall. The external (not under the house) has been rendered and painted.  
Problem is after 15 years the mortor lines are cracking and water is leaking - causing paint and render to lift and more important the loss of about 2000 litres a day.   
There are various products that claim to waterproof from the outside - even when water is flowing - I  am wondering how good are they?, do they work? - are any better than others and what would people recommend?

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## cherub65

Used silasec before worked well. You will have to remove paint first though.  SILASEC

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## liamd

Dear Wombat2. You have a common case wityh core filled concrete blocks. The products used for waterproofing are based on rapid set cement. You need to strip the paint. the blocks should be slightly rough. If the blocks are just damp then a product such as Davco K11 Slurry will work. If there is visible water seeping then you need to rub in a flash set cement to block the pores and fine cracks. The Davco Powder X is rubbed into the surface as a dry powder and sets in a couple of minutes. The excess is brushed off with a stiff brush and then overcoated with the K11 Slurry. if your mortar joints sre suspect, you will need to repoint them with an underwater setting mortar. These are used by plumber to fix leaking pipes. If you suspect that there is joint movement, the cement waterproofer is not very flexible. In this case use it as a base coat and finish with a flexible waterproofing topcoat. I would recommend a turps based bitumen as the bitumen gets into the pores. But if you use a bitumen you can only ever recoat with more bitumen. If you are getting seepage through broken joints, you should consider fixing reinforcement mesh across the joints to hold the waterproofing mortar. Hope this helps
LiamD

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