How to level flooring in sunroom?

Question

Hey friends! I’ve got a sunroom (5m L x 3m W) which used to be part of my rear balcony and I believe it was closed off previously. As it used to be a balcony the floor joists and floor slope down away from the house about 5cm over 3m I’m looking to lay hybrid flooring throughout the house and thought I should level this room.

The current joists are hardwood 120mmx50mm at 450mm spacing

My approach was to remove the yellow tongue floor and nail/screw new timber to the existing joists to bring it to level and reinstall the yellow tongue back onto the new timber.

If this approach is feasible what size timber would I use to attach to each existing joist and what fasteners to use.

I’ve attached an example I saw online of what I want to do.

MitchellMc · Answer

Hi @Aaron89,

Your plan is definitely on the right track, but the tricky part will be dealing with that gradual 50mm fall over 3m. Every piece will need to be cut down to the exact height required. That way, when you reinstall the yellow tongue, it will sit flat and be fully supported across its entire surface. If the new timber isn’t cut accurately, the yellow tongue will bridge gaps and flex, which could lead to squeaks, movement, or even damage to your hybrid flooring down the line.

The best way forward is to run a string line or set up a laser level at the height you want the new floor to be, then measure down to each joist at multiple points. This tells you exactly how much material you need to add at each location. You can then rip or plane your new timbers so they taper correctly to match the slope. Once fixed to the existing joists with batten screws or construction screws, you’ll have a solid, level platform ready for the yellow tongue to go back down properly.

Please let me know if you have any questions.

Mitchell