I'm installing beadboard panels on my ceiling to cover popcorn ceilings. The boards are .25" thick which means my ceiling fan box will not be flush with the new ceiling. I am not able to find any .25" box extenders. I was thinking of using nuts or washers to make up the difference so that the fan brace will be flush with the new ceiling. I wanted to know if there would be any issues with this or if there is a better approach? Any input is appreciated. Thanks!

Is it a fan rated box? If so, go to HD or Lowe's,they have adjustable extenders. If not, use a fan rated box and space BEHIND the box.

Yes it's fan rated, I put it in myself a while back. I looked at Home Depot but did not see an adjustable extender, I'll take another look, thanks!

So this was a rehab installation, did you use the box that is mounted on a thick rod that extends between the rafters?

Yes, one of the kits that they sell in Home Depot, Lowe's, etc.

Remove the box, loosen the bar and drop it 1/4" lower.

Thanks for the input, I tried that but the bar is already up against the existing drywall and will not go any lower.

Box extenders don't come in different sizes. You use the screws on the extender to adjust the depth. However, there are U bolts on that type of mount. Drop the box, put washers on the back of the box, hold with a little glue or caulk, reset the box flush.