Levelling sloped floor with underlay.

I would not advise using underlay to 45mm. I would prep the floor boards be sure they are fully fixed and gaps and crack filled. You can then use a screed specifically designed to go over floor boards and I’m pretty sure it can go upto 80mm. Hope this helps

With out seeing the floor. I would take up floorboards and try to leave the floor with batton to make a nice leave floor hope this helps thanks Mick

not really is needs a solid ground floor with underlay what will happen is out time it will wear away because of the movement in the floor u can level the floor out with latex flood the area where its out by 45mm and let it find its level then work your way back from the area hope this helps you many thanks STEVE KEVEREN FLOORING

There are two options both of which are quite labour intensive..the first is to have some fir rings cut which is a timber that from end to end starts at say 45mm and ends up at nothing..This is fixed over the existing floor and then ply fixed over them..the downside is that it raises the floor level in the room..the alternative and much more labour intensive is to take up the floorboards, put the firring strips over the joists and replace the floor boards over so levelling the floor that way without increasing the floor height generally..