"Tim W" wrote in message
...
Hi,

If you look at the joist on the far right of:

http://photos.dionic.net/v/public/bu..._0016.jpg.html

you'll see that it is a 4x2" stacked on top of the original 4x2" ceiling
joist before the roof was converted to a dormer.

I have a few like this, mostly in placed where the builder thought it was
too hard to put in 8x2s for some reason...

I'm proposing to plate these both sides with plywood to stiffen the
assembly
for good measure. Plating also allows me to bridge out the horrendous gaps
in the top left by previous plumbers which will be essential prior to
reflooring with boards.

I will go to B&Q or wherever and get them to cut a 8x4' sheet into 6
strips
of 8'x7.5" wide.

Question:

What thickness and type of ply?

I think 18mm structural soft ply (eg spruce) would be more than adequate,
but it's fairly expensive compared to 12mm ply, which might be enough.

My plan it to screw the ply to the sides of the joist with a *lot* of
small
turbogold or similar type screws (1.5" or so long). Possibly in 4 rows of
screws, each row being towards the top and bottom of each 4x2" joist at a
horizontal spacing of 4-6". This is easy enough with an electric driver. I
don't think glue would help as the joists are rough as buggery.

Any thoughts?

I'd like to do it before running wires down these joists, so I've brought
this job forward.


Simply on the basis of comparison of section properties, you need 4 @ 4" x
2" for flexural strength and 8 @ 4" x 2" for stiffness / deflection to be
equal to a single 8" x 2".

To get the re-assurances you need, that what you are proposing to do is
structurally competent, you should employ a structural engineer.