How to repair balcony joist and waterproof?

Manish1

Hi everyone,

I am planning to repair the rotten timber joist of the balcony, cover the exposed timber with some weatherproof board and waterproof the whole balcony.  There is no drain, the water simply flows through the edge of the balcony.

Can someone please advise what products should I use for weatherproof board and waterproofing ? For repairing the joist, I am going to trim the joist as far as the rotten part, apply some fungicide and attach the remaining joist with a similar size timber with bolts and screws. Please shoot me with better ideas if you have, for the repair and waterproofing. Cheers

EricL

Hello @Manish1

This looks like advanced timber rot in what appears to be an exterior fascia or balcony beam. The paint has trapped moisture over time, and the wood underneath has deteriorated badly, especially at the end grain where water tends to soak in. The damage is no longer just surface level, the timber fibers are breaking down and crumbling, which tells me the structural integrity in that section is compromised.

For this type of damage, a simple patch or filler repair would not be appropriate. Minor rot can sometimes be treated with a wood hardener and rebuilt with exterior timber filler, but in this case the decay is deep and extensive. The correct repair would involve cutting out the entire affected section back to sound timber. If the beam is structural, the damaged portion would need to be fully replaced rather than patched. That may mean removing and replacing the whole beam or splicing in a new treated timber section using proper structural joinery and fasteners.

Once the new timber is installed, it should be primed on all sides before fitting, sealed at joints, and finished with a high-quality exterior paint system. It would also be wise to address the moisture source, whether that is poor flashing, inadequate sealing along the top edge, or water pooling from the balcony above. Without correcting the water entry issue, the problem will return.

Given the location under a railing and above an entry, I would strongly recommend having a qualified builder assess it in person, especially if that beam supports any load. Rot at the end of a beam can spread further than what is visible on the surface, so a proper inspection by a qualified person is important.

If you need further assitance, please let us know.