WBP exterior grade plywood - 25mm vs 18mm + 3.6mm (or 6 -9 mm)

[Hl.house]

Hi all, time to replace old 22mm floorboards. Joist centres 37mm apart. I am looking at ply options. My local store has either 18mm or 25mm thickness. No 22mm. Problems:
1) Price for 25 mm > two times price for 18mm.
2) Weight, 25mm will weigh much more than 18mm so difficult to carry upstairs especially where the stairs turn.
If I combine: 18mm + 6mm or 12mm + 12mm sheets, screw them together to the joists will it work the same way(structurally) as getting a single sheet of 22-25mm?
Many thanks, hl

 

[^woody^]

If I combine: 18mm + 6mm or 12mm + 12mm sheets, screw them together to the joists will it work the same way(structurally) as getting a single sheet of 22-25mm?

Not unless you bond the sheets and glue the one on the other. And hope they never ever need to be lifted.

 

[tomfe]

22mm chipboard or 22mm pine floor boards?

 

[r896neo]

I'm presuming its a bathroom to be tiled on. If that I the case wbp is the right choice.

 

[Hl.house]

Thank you for your replies.
woody, I don't understand why it will be a problem if the floor needs to be lifted if they are screwed together, not glued? Also, gluing is one way to bond the two sheets and screwing them together is another. What makes the difference in strength?
tomfe, I have read very bad experiences of chip boards so decided too tay away from it. Floor boards are good but patience/time to fix and money(double the price of ply).
r896neo, no it is not a bathroom floor.

 

[r896neo]

Unless you glue the sheets together they will act as seperate flexing structures to a certain extent so 12mm+12mm will not be as strong or more importantly as rigid as 25mm. If its not being tiled on its not critical. If you don't glue them you may end up with a squeaky floor. Wood moving over wood = creaks and squeaks.

 

[tomfe]

Tomfe, I have read very bad experiences of chip boards so decided too tay away from it. Floor boards are good but patience/time to fix and money(double the price of ply).

Just about all modern houses have chipboard upper floors. There is nothing wrong with it, if you pay a bit more you can get better grades and coatings.

 

[JohnD]

There is nothing wrong with it

yes there is.

 

[John D v2.0]

18mm would be fine for that span, if you're worried about steps on the floor you can cut strips of some thin wood to pack the joists up. Or just use some extra underlay

 

[JohnD]

Joist centres 37mm apart.

surely not.

Anyway, 18mm WBP ply is ideal. If tiling, 25mm. Nog any unsupported edges.

Cut it to size before you try to carry it upstairs. I can't carry an 18mm 8x4.

 

[chappers]

yes there is.

Not if properly fitted.
A properly glued and screwed tongue and grooved, all round chipboard, floor will be superior to just screwing down ply

 

[Hl.house]

Sorry not 37mm it should read 37cm apart. I am not tiling. I get it to cut as two sheets of 60x240cm. I will lay it from one end to other reversing the direction with successive rows. Slightly counter suck screws which I will cover with wood putty. The ply is very smooth so I will just use tung oil to finish it as my final flooring. If I fill like changing it I will look into different varnishes or marmoleum covering.
I like the simplicity of it. It is easy to unscrew if access to pipework/cables is required. Current floorboards are cut at so many places by previous owners.

 

[JohnD]

if the joists are at 16" centres, a 4-ft wide piece will span 3 of them, which I think you will find convenient, and will be firmer than two 600mm pieces.

A 2440x1220 is not far off 8ft x 4ft, so do your final measurements before you decide on the cuts. It is an advantage to have joints near the edges of the room so they won't get walked on much, except the area in front of the door probably gets most traffic. Do all your trimming on the edges so they are hidden under skirting. Arrange your lengths (if you can) so no piece has to go under the skirting on both sides of the room. Consider the location of pipes and cables so you can get future access with reasonable ease, especially in front of radiators where you may one day need to change to a slightly different size.

Have a look at your ply before buying. I got some 18mm hardwood-faced WBP from Wickes, it had quality marks on it, the outside veneers are a reasonable thickness and where I left offcuts outside, they resisted the weather well. They were cheaper and better than my regional timber merchant offered.

I later got some from B&Q because they offered cutting, and I no longer had a van. I've discovered the outer veneers are paper-thin, and unsealed offcuts delaminated very quickly when left outside. I got an offer of 18mm Marine but I can't transport it. Cut to 1440x1440 is easy to carry up stairs and to transport.

I'll post a pic of my dye-and-varnish finish later, I am very pleased with it, although there is a colour mismatch between different boards.