Need help with floor level issues

[phil woodley]

So here goes I'm attempting to replace my upstairs floor which has been poorly done in the past. the original part of the house has been done with floorboard since 1860s then the extension to the side which I belive was a 1980s self build has been done with bulk standard chipboard with no tounge and groove and has basically turned into weetabix they have not been staggered to tie into the existing it's just one straight cut through and its due some renovation. I'm looking to replace the lot with tounge and groove chipboard however upon lifting the old and new existing boards it is apparent there is a height diffrence and the joists from the extension are currently 10mm higher than that of the original part of the house, it would be perfect if I could lift the existing 1860s floor to this height however it connects to my stair case and would cause no end of trouble in other areas as there are steps down into extensions at the back ect. so I was wondering is it possible to plain or cut the extension joists down by 10mm or is this going to cause significant problems with the strength and if this is the case does anyone have any ideas on how to overcome this issue the extension is roughly 3m in width and length and the original part is about the same I'm not concerned about it being perfectly level I just want the boards to be sat tightly on the joists and not bounce I just don't know how much flex the 18mm chipboard would have. fingers crossed this makes sense and thanks for the help in advanced

 

[Bonni]

Along the full lengths of the joists, fix roof laths 10mm below the top of the joists on both sides of each joist. These laths will be at the same height as the joists in the old part of the house. Where the new chipboard comes in contact with the joists, use a router to rebate the chipboard by 10mm. You may want to router slightly deeper and make the rebate slightly wider than the joist to give yourself a bit of wriggle room. The chipboard will be supported on the laths.