18mm hardwood ply thickness variation? Do I need to countersink? (Floor job)

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Draughtsman

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I picked up some 18mm ply from B&Q earier this year for a flooring job but need a few sheets more. Is there likely to be much variation in thickness between the 1st lot and the next few sheets? Do they dry out further in the home after coming off the shelf in the sheds?

Ply being fixed on CLS frame.

Tnx

Dain
 
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They don't shrink due to drying out because they are already pretty dry (typically 6 to 9% MC as manufactured). However, they can swell if badly stored in a damp place. Ply can have a millimetre or so variation sheet to sheet in a stack and they are never perfectly flat either, but they are generally near enough that it make no difference in practical terms
 
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I’d cover my backside if I were you, and buy from the same shop.
Obviously B&Q will have several suppliers but it may minimise the risk of some differences.
 
I don't see the relevance of countersinking.

If they are T&G, take an offcut with you to make sure they fit.

If not, use struts or supports under all cut edges, or you will be able to detect the deflection at joints if not wearing shoes, and they may squeak
 
JohnD said:
I don't see the relevance of countersinking.

If they are T&G, take an offcut with you to make sure they fit.

If not, use struts or supports under all cut edges, or you will be able to detect the deflection at joints if not wearing shoes, and they may squeak
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Not t&g.
 
JobAndKnock said:
They don't shrink due to drying out because they are already pretty dry (typically 6 to 9% MC as manufactured). However, they can swell if badly stored in a damp place. Ply can have a millimetre or so variation sheet to sheet in a stack and they are never perfectly flat either, but they are generally near enough that it make no difference in practical term
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1st lot were stored against a wall in the hallway. I'll take the vernier gauge when I pop over to B&Q for more, just in case.
 
Should I put in a minimal expansion gap between boards when laying them, if so, approx how much?

I'm putting click vinyl down.
 
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Draughtsman said:
1st lot were stored against a wall in the hallway. I'll take the vernier gauge when I pop over to B&Q for more, just in case.
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You are more likely to find that there are differences between mills - so if B&Q buy from multiple mills you may well see differences.

What I've found over the years is that what is sold as nominally 18mm plywood can vary from just over 17 up to maybe 19mm. If you think about it is 19mm is 3/4in in the old Imperial world (a lot of plywood comes from Africa and the Pacific rim and is made to Imperial thicknesses). For most construction purposes, though, a millimetre or so variation between sheets is neither here nor there.
 
Draughtsman said:
Should I put in a minimal expansion gap between boards when laying them, if so, approx how much?

I'm putting click vinyl down.
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For a sub floor where you are glueing and screwing the plywood onto the joists you don't want any gaps between the plywood sheets, but you do want to support the joints.

As an aside when we used to replace floors in restaurants and bars with multi-layer plywood the workshop used to run a 1/4in groove round the outsides with a router and supply us with rips of 1/4in ply for use as loose tongues, which we always used to glue in place. Guaranteed to reduce squeaks
 
JobAndKnock said:
For a sub floor where you are glueing and screwing the plywood onto the joists you don't want any gaps between the plywood sheets, but you do want to support the joints.

As an aside when we used to replace floors in restaurants and bars with multi-layer plywood the workshop used to run a 1/4in groove round the outsides with a router and supply us with rips of 1/4in ply for use as loose tongues, which we always used to glue in place. Guaranteed to reduce squeaks
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Just wondering, why use ply as the really "long biscuits" rather than solid timber?
 
opps said:
Just wondering, why use ply as the really "long biscuits" rather than solid timber?
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Speed and cost. Back in the day (before MDF took over as the material of choice to build counter units) we used a heck of a lot of blockboard and plywood to build units - 6mm in particular (actually 1/4in nominal) was used a lot for unit backs and drawer bottoms (with support muntins, obviously), so most places had loads of offcuts, part sheets etc of 1/4in plywood kicking about in the "bits box" which could be given to the junior/improver to rip down quickly to produce the loose tongues. Faster and cheaper (especially asvit was effectively scrap) than ripping and dimensioning expensive seasoned hardwood to do the job.

Now you know why joiners never throw anything away ;-)
 
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No gap. Everything needs to be tightly screwed down and all joints fully supported. Still best to use a bit of something like PU glue on the edges. Why aren't you using glue to secure the plywood to the joists?
 
JobAndKnock said:
No gap. Everything needs to be tightly screwed down and all joints fully supported. Still best to use a bit of something like PU glue on the edges. Why aren't you using glue to secure the plywood to the joists?
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so you glue the plywood down to the joists and screw?

and you put in a loose tongue as well?

Im wondering as I have a garage conversion to do
 
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