Laying a plywood floor

[cgadmin23]

Hi All,

I am in the process of tiling my bathroom floor but need to relay the floor first.

Most of the boards are in very poor condition so I am going to remove them and replace them with Plywood.

I am also installing a wetroom and have a tray former that is 22mm deep.

Can someone please advise what type and depth Plywood I need to use. I have seen18mm in most DIY shops - Is this enough depth, and is exterior plywood adequate?

Do I have to leave any gap between the Ply boards when I lay them?

Do I need to use special screws?

If the tray former is deeper than the plywood (In my case it may be 4mm deeper) do I just need to cut into the joists until it is flush with the floor?

Are there any other tips I should be aware of?

Many thanks

Chris

 

[mikethetile]

hi chris

18 mm ply is thick enough to replace the floor, i assume your ripping the bathroom out before starting,

no gaps ............butt boards up to each other and fix @300 centres max

i fix @ 150mm centres

use spax srews

use noggins between joists to support ends of boards

leave a small gap around edge, about 3-5 mm, ply is stable and doesnt really expand but the gap ensures no squeeking

exterior ply is adequate and shouldnt delaminate in these cicumstances

you can take the top off the josts to accomodate the tray but take care to remove the minimum required..........5mm will be ok

its difficult to explain the stress on joists without drawing a diagram

but the joist is in compression at the centre of the span so when you get to the edge the bottom of the joist is pushing in to the wall and the top of the joist is pulling away

so care is needed not to cut too much away from the top

check the joist to make sure that there hasnt been holes drilled through for pipes etc

if so your cut out needs to be a min of 200 mm from the nearest hole

the good news is that they over specify anyway but to keep the floors all level a smaller room such as a bathroom has a lot deeper joist than required so there no danger ,just leave the max depth you can

if your not sure about reducing the depth of the joists then dont

just overboard your 18 mm ply with either 4mm or 6mm wbp ply, this will give you a step up into your bathroom but you can get over this with a threshold , you would need a threshold anyway as your tiling will raise the height of your bathroom floor

if your having a flush tray it will pay you to tank the floor and up the walls to protect the susbtrate

good luck with the job and hope it all goes well for you mate

 

[Gazzatile]

Mike just about said it all I would just say I would also tank both walls and floors

 

[cgadmin23]

Mike,

Thanks very much for your reply. The information is clear and in depth, so I very much apprciate it.

I have already taken out the old bathroom and am preparing the room before tiling. I will tank both walls and the floor before laying the tiles to ensure it is water tight.

I just wanted to ask a couple of questions before I start relaying the floor:

1) Whats the best way to fit a noggin? Brackets or screwing straight through the joist?
2) Should I position the plywood in such a way that if I ever had issues with the pipes it would be easier access them?
3) When you say 300 centres do you mean screw it down every 300 mm? and is this just along the joist or perpendicular to the joists as well?

Many thanks again

Chris

 

[mikethetile]

hi chris

where possible srew through from the joist to the noggin, you can do this by slightly offsetting the centres on the noggins

if not screw through at an angle through the noggin and into the joist

screw @ 300 centres along the joists and along the noggins

put traps in where you think you may need them but its going to be difficult tanking and tiling the floor without having visable access covers , so its personal choice wether you do that or not

ive gone through the ceiling below before now to repair underfloor plumbing as it was a cheaper option than lifting and relaying a tiled bathroom floor

 

[cgadmin23]

Thanks for the info Mike!

You've put me in a much better position than I was. Much appreciated!

Regards

Chris

 

[vosnack]

I would suggest screwing the noggings with screws on an angle with two screws on each side of a noggin. I think it's less stress on screws hence better joint.

 

[vosnack]

The arrows are the screws. Never mind the Blue Hills, I had to ruin a sample picture from my computer to upload the idea

 

[aph257]

i've used 4" nails in the same manner as above, i thought it was OK?

 

[Wood's better]

The reason screws and nails are angled (toshed/toe nailed call it what you like) is because as it is not parallel with the end grain it is stronger. It has greater resistance to withdrawal this way.

However, with modern screws or ring shanked nails (improved nails) the strength is pretty good.

Exterior ply, if of a good quality veneer should be fine. True Marine ply uses the same glue as exterior grade, except the venneers are of better quality and have less gaps between the joints in the veneer (less voids for salt water to get into)

Persoanally I would use 25mm ply if possible. The rule of thumb is 18mm for 400 joist centres or 22mm for 600 centres. However, when tiling you are increasing the weight of the overall floor,so I would try to stick to 25mm. Then overboard with 6mm Hardi if height allows.

Good luck

Richard

 

[vosnack]

I personally never use nails. Screw is set in a wood properly. Maybe I'm wrong and a lot of people will disagree.

 

[Mr Tiler]

i just have a quick question guys... if you have to screw ply at 30mm centres does that mean you need a noggin every 35mm along the joists? thanks

 

[Gazzer]

30mm centres......


A bit OTT ......lol

 

[SJPurdy]

If joists are at greater than 300mm c/c then noggins are required at 300mm c/c is my understanding of the BS recommendations

 

[Chalker]

i just have a quick question guys... if you have to screw ply at 30mm centres does that mean you need a noggin every 35mm along the joists? thanks

35mm!!!

going to take some screws ! More screws than ply!