Hardwood flooring questions

I'm about to start laying some engineered hardwood flooring and before I start, I'd welcome opinions.
The current floor is patchy quality old floorboards. My plan is to lay plywood down on top of them. From what I read, I should nail or screw the plywood to the existing floor... but also leave an expansion gap.
Surely there is a contradiction here? If I fix the plywood, then it won't be able to expand. So what would be the right thing to do (and why)?
Also, I've enough underlay to do the room twice and I could put a layer of underlay under and over the plywood.
i.e. floorboard, underlay, plywood, underlay, hardwood-floor.
Are there any advantages or disadvantages to this?
Thanks Bob
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I do not think they are talking of an expansion gap between the plywood and existing floor but a gap around the edges against the wall or skirting boa rd usually about 10mm. You need to use plenty of screws to hold the plywood to the existing floor every 150mm if it was being tiled, you may get away with more spacing under wood flooring. I would not put underlay between ply wood and existing floor as it will affect how tight the screws will hold th e ply which needs to be tight to the floor to take out any movement.
Richard
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On 07/06/15 11:09, WeeBob wrote:

No - don't put underlay under the ply.
Screw ply every 12" around its permeter (or to the nearest joist). Expansion gap should not be necessary for the ply - as you observe, it is fixed.
Then underlay, then flooring - that does need a gap around all edges - 10mm is a typical rule of thumb, instructions should specify more clearly. This is to deal with humidity related (seasonal) expansion.
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On 07/06/15 11:48, Tim Watts wrote:

But to add a point, I would leave a 10mm gap around the room edge between the ply and the wall.

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I'd say that unnecessary. Better to repair the existing flooring to an adequate standard - well fixed and reasonably level. Sand the whole lot if needed.
I'd not want to make the new floor even higher than needed.
--
*A conscience is what hurts when all your other parts feel so good *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 07/06/2015 11:09, WeeBob wrote:

FWIW, I've just done 2 floors in 12mm laminate. Very poor condition boards, having been hacked about for wiring and CH over the years. I screwed the boards down very liberally, and replaced a few where the dip was more than a couple of mm. Seems to have worked very well (laid on top of fibre board), and the 'spring' in an attic room has more or less gone.
I'd do the same again - cheap (about £10/m inc fibreboard) and effective. One thing I'd take the time to do is new skirting (or take the old off and replace). Apart from the fact it'd look a lot better, MDF skirting is about the same price as those horrible skinny trim pieces. I did this in the last room and it looks fine.
https://www.flickr.com/photos/47717472@N03/18546832896/
using
http://www.wickes.co.uk/Wickes-Venezia-Oak-Laminate-Flooring/p/131874
--
Cheers, Rob

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On 07/06/2015 11:09, WeeBob wrote:

If it were me .... screw fix plywood down at reasonable spacing (no more than 200mm) through floor boards into joists.
Put a later of closed cell foam underlay and lay the engineered floor floating with glued t&g joints.
Scribe in the first row, then put 10mm packers behind it, so top can cramp up each subsequent row to it. After floor complete - remove packers and some put in 10mm cork expansion pieces .. not really convinced its necessary.
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On 07/06/2015 11:09, WeeBob wrote:

Thanks for all the advice and tips.
Bob
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