Child Item

How to waterproof concrete with cracks?

My shed has been converted to a granny flat. It smelled strongly of damp, so I pulled up the floating floorboards today to find a wet concrete slab with cracks. There was a thin rubber mat between the concrete and floating floor.

I need advice for sealing up and waterproofing the concrete before I put the floorboards back down. Something that won't raise up the height of the floor more than 1-2cm as the roof is already very low.

I was going to paint it with 'timber protector' but I think it needs something else first. Thanks!

Find more posts tagged with

Tools and Materials

Repairing

Renovation

Flooring

Accepted answers

All comments

MitchellMc

Welcome to the Bunnings Workshop community @kiniget. It's brilliant to have you join us, and many thanks for your question about damp concrete.

It’s great that you lifted the flooring and investigated the source of the damp — that kind of moisture issue can lead to major problems over time, especially if left trapped under floorboards.

Before doing anything else, I’d recommend checking any wall framing or skirting boards inside the space, especially if they’re timber. If they’ve been in contact with the damp slab for a long period, there could be some water damage or even signs of mould or rot. If the water seems confined to the centre of the slab, and the edges near the walls are dry, you might have avoided serious structural damage — but it’s worth confirming before moving forward.

To fix the issue and waterproof the slab, you’ll want to start by thoroughly cleaning the surface. Remove all debris, dust, rubber residue, and anything loose. Use a degreaser or concrete cleaner if needed, and allow the slab to dry out completely. A dehumidifier or fan heater can help speed up the process.

For sealing and waterproofing, I’d recommend Sika 1-Komponent. It’s a flexible, brush-on waterproof membrane that’s made specifically for damp areas and cracked concrete. You can apply it directly over cleaned and prepped concrete, and it’ll help bridge those cracks and prevent moisture from rising up again. Once it cures, it forms a durable barrier and won’t add more than a few millimetres to your floor height — so well within your 1–2cm allowance.

Avoid timber protector products or anything not designed for wet substrates — they won’t bond properly or stop rising moisture. Once the waterproofing layer has fully cured, you can install an appropriate underlay and re-lay your floating floor with more confidence.

Please let me know if you have any questions.

Mitchell

kiniget

Thanks Mitchell! I'll be cleaning the concrete today. Thanks for the recommendation of what to do afterwards

Child Item