GPG wrote:
I'm really talking about the ones at the top which open outwards
about a horizontal axis (I don't know what they are called, sorry).

Top casement sash or hopper sash!

It seems, to me, to be a common problem that they will not fully shut
- well, none of mine will. So :-

a) If it is as common as I think, why won't they shut? Too many
layers of paint? Wood has expanded? Other?

Do the windows close and then 'rattle' when the casement stay is 'locked'?
If so, slightly adjust the rear stay pin away from the sash (can sometimes
be done by tapping the base of it with a hammer as this is used as a
fulcrum) just enough to stop the window rattling but allow the stay to
'hook' over the front pin.

If its draughty, then the only way to reduce this is to use a foam type
stick-on draught excluder or such-like.

Another reason could be that the sash has 'twisted' or warped. If this is
fairly major, then the only 'cure' is to renew the sash. If it's minor,
then the sash can be 'adjusted' by moving one of the hinges in or out as
needed and then adjust the stay as previously explained - or just adjust the
stay (a little risky as it could put the glass under stress and it will
break).

If the window refuses to shut, then check the following:

1 Excessive paint covering the hinges themselves making them stiff to
operate.
2 Worn or rusty hinges.
3 Loose screws holding the hinges.
4 Too large a screw head holding the hinges and 'catching' on the
opposite hinge when closing making the sash 'hingebound'.
5 Poorly fitted hinges again making the sash hingebound,
6 The sash has swollen and the leading edge of the rebate is catching the
frame preventing the sash from fully closing.

The cure or repair of the above will be obvious once you have identified the
cause(s).

b) What should have been done, or not done, to prevent this problem?

Not a lot that you can do really other than carry out an annual check on it
and keep it painted - I would advise stripping the paint off every couple of
decoration cyles to prevent a thick build-up which could cause problems over
the years - although due to the clearances on a casement sash this is
unlikely to be a major problem (other than on the hinges).

c) What is the most cost- and/or effort- effective way of curing the
fault?

See above

d) What, if anything, can or should I do to stop it happening
in the future?

Just normal maintenance really - but remember, that as the window is made of
a natural product, this will twist, warp, move into add shapes and
ultimately rot at some time in its life.

Hope this is of some help?

Brian G