On 07/06/15 11:48, Tim Watts wrote:
On 07/06/15 11:09, WeeBob wrote:

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?

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.

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

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.