I have a 5/8 subfloor over 24 in on center joist spacing. Things I’ve read is the subfloor should be at least 3/4thick for 3/4 naildown. This floor already has staple up radiant underneath. It also has some variation I’m shimming the worst joist from underneath the rest of it isn’t terrible. I was thinking of planing any high spots using floor patch or self level on others and then adding another layer of 1/4 over too. I know I haven’t found a self leveler or floor patch that is supposed to be nailed through just wanted to get some opinions. I don’t want to build up with shingles or felt because of the radiant and possible off gassing. Let me know what you guys think thanks for your time.

Wow, that is the wrong joist spacing for 5/8th ply. If you use half inch ply it would be much better than quarter inch. I think that by covering tar paper shims with half inch ply you would not get any fumes from the tarpaper. I would consider 3/4 inch ply with such wide joist spacing.

Ok thanks for the info I think I’m gonna at least move up to adding 3/8 ply and try to find a self leveler or patch that someone’s had reasonable success nailing through maybe use a roofing nailer so I’m not beating on those spots or glue over those spots

I still think half inch ply is the best solution. Ask an engineer. Another thin layer over a weak sub-floor does not equal the strength of the same thickness of sub-floor as when the layers are added together, unless they are incorporated by a structural adhesive. If you use a leveling compound and glue the 3/8th inch ply with a full trowel method, you may be able to get the strength of a one inch sub-floor. You will need to have complete contact between all the clean existing 5/8 ply and the adhered leveling patch. Using a half ply over the existing 5/8 ply with glue assist would be my choice.Using screws where the tarpaper shims are needed will work using the correct fastener at schedule, at a minimum.