I’ve just picked up some IKEA Ivar drawers for the study but the bottoms have sagged and need replacing. I could just replace with another 3mm sheet of hardboard but I’d prefer something that could take some actual weight in the drawers as this is for the study.

I’m thinking of replacing the hardboard with something like ply which should be strong, and most likely attaching a baton underneath too. The question is has anyone done this, and would 4mm ply be a good improvement over 4mm hardboard, or should I go up to 6mm (which would involve digging out the router and widening the channels)?

4mm ply will be better than the hardboard.
And if you put a batten under that should stiffen it up a bit more.

I would guess that 4mm plywood would be stronger than the rest of the Ikea unit.

Funny because it’s true. I just got rid of the Galant drawer unit because I wanted something less in your face, but the Galant really is indestructable. Ivar is the complete opposite, but at least it’s old school solid pine and not their new laminated cardboard nonsense.

This is just an interim measure until I’m some mysteriously rich enough to afford some Vitsoe 606…

Dug out the router but.

Route the edge of the 6mm draw bottom to fit the existing slot.

What Josh said

Glue. Lots of glue around the hardboard.
I’ve also doubled up the hardboard in an old set of ours – just cut to size and glued on the bottom. Solid as.

And it’s for this reason I refuse to buy any drawer unit with a hardboard bottom *

* Admittedly a hard thing to do these days.

You used to be able to rock up at Ikea and request spare parts.
There is a small right angle bit of plastic with a screw through it that can strengthen the drawer bottoms!