Forcing Plywood down, uneven joist

[aveatry]

A far easier and faster alternative, which I already suggested, was to build-up the tops of the joists using thinner materials such as thin plywood and hardboard rips (which can be of different lengths to create a rough taper) then fix the sub-floor through that into the tops of the joists. Plywood will happily span any 2 to 3mm differences in the tops of the joists. Far faster and easier than using bought-in featherboards or making your own. I have resorted to doing this a few times over the years when sistering wasn't feasible

Going by the picture already done that gives a perfect way to check if standard thickness of sheet material would work and add that to the top of joists as stated above. The problem with adding is getting the right thickness.

 

[NorthWest1]

I splashed out on a table saw, I can have any thickness I need now , its all about whether I need Level or just flat. The slope is so consistent, , its like it was built that way, or maybe it has moved over the years like all houses do.

 

[GhostIntMachine]

If you were to go around a variety of 100 year old houses and put a spirit level on the joists, how many would actually be level, I don't mean flat with respect to each other, actually level ?

How about none, at least not in my experience. Probably not what you want to hear, but I don't really think there is a right or a wrong way. Saying that. tiny steps can be a trip hazard and are probably best avoided. So I'm afraid it's down to your own judgement; personally I tend more towards flat rather than truly level, even though truly level is easier to achieve with a laser and staff, unless the floor is going to end up with an obvious slope on it (it can be disconcerting for the bath water to pool at one end of the bath, and a bit of a problem to have the shower unable to drain until one end has 3in of water in it because the drain is at the "uphill" end). We often get asked on refurbishments where buildings are being repurposed (e.g. Victorian railway warehouse to hotel) to provide the same level in all rooms and corridors on a given floor. It can be a challenge. Sometimes we have to tell the architect that this can't be achieved in which case there will be a ramp in a corridor somewhere. This should be picked up during the survey, but in the real world high corners, etc are sometimes missed

I can easily achieve flatness, which will give me a solid base to fix sub-floor to but should I make my own level that would be out with rest of house ?

What is the room being used for?

 

[GhostIntMachine]

The problem with adding is getting the right thickness.

If you can get it to between 2 and 3mm of the correct height, a sheet of 18mm plywood will more than likely fly over any minor discrepancies. Even if there are minor dips, it's unlikely that anyone will notice them after the floor has been carpeted. In any case minor discrepancies can always be taken out using SLC - which would be necessary in any case if something like Karndean or the like were to be the selected finished floor

 

[NorthWest1]

The room is a bathroom, well, shower.

Just to give an idea of the slope, the highest point is at corner of room ( back wall/gable end) and the joists run front to back of house. This highest joist drops around 23mm over 3m. If you go diagonal across the room, ( around 3.6m ), then the drop is around 45mm. Like I said, you don't notice it when your in room.
Talking about water, If I layed the shower tray to the slope, it would be around 8mm low, one end, it's not much is it ? And I could lay the tray level anyway.

I was more worried about water pooling in corner of room.