Forcing Plywood down, uneven joist

[NorthWest1]

Hi,
Just wondering if anyone can advise on an issue I'm having trying to fit a bathroom floor. My original intention was to fit 18mm cement boards, direct to the joist and go with Laminate on top but after having spoke to the manufacturer, they have advised that the boards need to be laid on a level surface. This is my problem, the joists are not level or flat. The highest joist drops 16mm over a 2.5m span and the others are dropping similar, actually around 20mm. I suppose I could live with a un-level floor but across the joists, spaced at around 300mm , some are 5mm out with the next joist.

I'm now looking at possibly going with 18mm Pywood instead, will 18mm ply deflect 5mm from one joist to another ? I was concerened that even if it did, it would be prone to creaking.

 

[yorksbuilder]

What spacing are the joists?
Could you pack up the joists so they are all level? Or 'sister' them and make sure the new parts are all level?

 

[NorthWest1]

Hi,
The joists are spaced at around 320 mm some closer, there's only 7 in total. To sister them would be pretty difficult, some are close together and there's cables and lateral ties to back wall. I thought about packing, maybe using plywood blocks ( around 5mm plywood ), obviously the the packing wouldn't be continuous because the gaps are long wedge shapes, i'm worried about the floor squeaking down the line. I suppose packing, maybe 50mm x 100mm would be better suited to the 18mm cement boards.

 

[NorthWest1]

What would yousay is the minimum size wood I could use to sister them ? Does it have to be the same size as the joist about 170mm x 50mm ?

 

[stevie888]

I'd use strips of 18 ply say 100 wide glued and screwed each side. You're not lookng to add strength as in normal sistering merely leave a level top.

Ply wont split when you screw it in to place

 

[yorksbuilder]

I'd use strips of 18 ply say 100 wide glued and screwed each side. You're not lookng to add strength as in normal sistering merely leave a level top.

Yes this exactly

 

[NorthWest1]

That would make things easier. When I come to screw the floor ply to the joist, I take it I just use a longer screw and not worry about trying to fill the gap around the screw ?

 

[cdbe]

Done a few (with 100mm deep strips of 18mm ply and 22mm chipboard). Works well and only needed on one side of the joist. And do the first and the last, then screw a straight length of timber across each end and you can just pull the middle ones up to them.

 

[GhostIntMachine]

Another approach is to level the tops of the joists by adding rips of thinner material as required. Materials such as hardboard (3.2mm), and thin plywood (5.5mm) built up to the required level and pinned in place. This will get you to about the right height and the plywood can then be screwed down on top at something like100 to 150mm centres (18mm hardwood ply isn't going to bend a great deal). At that point if the surface isn't completely flat you can take any hollows out by using a self levelling flooring compound, given that plywood itself is rarely completely flat

 

[foxhole]

Laminate in a bathroom ?

 

[GhostIntMachine]

Ply wont split when you screw it in to place

That very much depends on the quality of the plywood - a lot of Chinese ply is poorly glued and can start to delaminate if edge screwed (which you will do when screwing the sub-floor to the plywood "sisters"). I wouldn't ever depend on screws being driven into the edges of plywood. Far better to use solid timber every time

 

[NorthWest1]

I didn't think you screwed into the edge of the plywood, I thought you just used a longer fixing into the joist and pulled the floor sheet to the plywood edge ?
The one thing I notice about the joists in the pic above.....they look nice and square and the chipboard is fixed snug against them, this could be an issue for me, my 100 year old joists look bowed across their height and the any strips may not be tight against.

The absolute best solution would be to have timber firring wedges bespoke to each joist, glued and pinned. I did actually contact a local company but they said the angles I need were too shallow for their machinery. I did wonder if I could use a long plywood strip as a saw guide and cut them myself on the joist.......You'd need some seriously good sawing skils !

I'm surprised more ppl aren't using these No more ply 18mm boards, no more expensive than quality Marine plywood

Yeah, it's a waterproof laminate for Bathrooms, it looks okay, similar to vinyl planks. It's easy to fit and I could tile down the line if needed.

 

[stevie888]

I didn't think you screwed into the edge of the plywood, I thought you just used a longer fixing into the joist and pulled the floor sheet to the plywood edge ?

That's how I'd do it, but I'd "sister" both sides so the floor pulls down solid rather than hsve tendancy to roll. (I do tend to over engineer on ocassion though)

 

[GhostIntMachine]

I didn't think you screwed into the edge of the plywood, I thought you just used a longer fixing into the joist and pulled the floor sheet to the plywood edge ?

Not normally, but I doubt that many joiners would resort to using strips of plywood fixed to both sides of joists to level them up either - you are just doubling the amount of work needed to end up with less robust bearing surfaces. My own preference is to have a 2in/50mm wide bearing surface directly beneath where the screw is driven - because it works

The one thing I notice about the joists in the pic above.....they look nice and square and the chipboard is fixed snug against them, this could be an issue for me, my 100 year old joists look bowed across their height and the any strips may not be tight against.

If you use softwood screwed to the sides of the joists it is always possible to plane the top faces level-ish if you really need to. Plywood isn't as easy to plane and poorer quality plywood can delaminate

The absolute best solution would be to have timber firring wedges bespoke to each joist, glued and pinned

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

Sawing extremely fine tapers is possible by using a plunge saw and guide rails, cleaning up with a sharp jack plane as required afterwards, but I can tell you it can be extremely fiddly to do

As to using marine plywood (at £100 or more per sheet for true aircraft grade - many people seem to confuse marine ir aircraft grade plywood wirh WBP), are you being serious? Structural WBP hardwood plywood at £40 to £50 a sheet is probably more appropriate for this type of work. Not saying don't do it, just that it'll cost a bomb

 

[NorthWest1]

Thanks for the replies guys.

I've got the joists ready to sister, just need to set position and secure, but here's my dilema, do I set the sisters LEVEL or FLAT ?
If I set them level, there would be a step-up into the room from the landing and every other room on the first floor. With a laser level there's a slope to the whole first floor across the span of the joists, its a consistent slope, you wouldn't notice it if it wasn't pointed out but there is a slope. 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 ?

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 ?