Extra support?

Helping my sister with her renovation. She had a cast iron bath which was in part supported by 2 brick pillars. The pillars had raised the floorboards and made them unusable. She is wanting to replace the bath with a lighter acrylic bath. Now I have laid some yellow tongue particleboard where the cupped hardwood floor planks (under the bath were). Upon standing on the board though there is some flex, can only imagine that it would be a lot worse when there's a bath with water sitting on it.

Question is should I install some 2 by 4 blocking underneath where the bath is going to go? If so how far apart should these blocks be? The bearers and joists are hardwood with the bearers about 1200 mm apart and the joists about 500 mm apart. I can just fit some pine 2 by 4 s in but there is some plumbing and wiring that I'll have to navigate around.

The maximum joist centres for 19mm Yellow Tongue is 450mm.
The maximum joist centres for 22mm Red Tongue is 600mm.
http://www.woodlogic.com.au/Product/...288,98,00.html
500's not that much over though. There'd be 450 in between. :wink:
I would have used HardiPanel Compressed Sheets, or Scyon in a bathroom, but it is OK to use particle board. You must put a membrane over it. It's not compulsory to membrane under a bath, but I would since its particleboard.

Extra noggins would help if the sheet's deflecting. Bed the entire underside of the bath as well, not just a couple of piers.

Thanks, John. I thought it was close but there's too much flex for my liking. By the way what type of membrane are you suggesting and is it readily available from places like Bunnies?

There's heaps of different types and brands. I'm no expert, and I wouldn't know which is the best, but I've used Emerclad, and so have most of the builders I know when they do their own. It's like a really thick paint. You paint a coat on, and bed in fibreglass matting strips over the joints in the sheets, and in all the corners, let it dry, then paint another coat on top. If you're using silicone anywhere near it, under it, or over it (e.g. where you silicone around your tap and spout penetrations), then make sure that it's a neutral cure silicone, and not acetic because it will break down the membrane.
If it's being inspected, then you may have to have a license to waterproof. Every different membrane applicator seems to show up with a different type of membrane. I've seen one that's just clear plastic sheeting that's taped up at all the joints. I don't know how good that one would be. :?

Anywhere that sells you the tiles should be able to give you the correct waterproofing aswell.