I have a detached double garage, with an uninsulated roof decked with
chipboard (which is slowly disintegrating with age and damp) and covered in
ye olde 'tar and felt'.

All being well, it will soon be reroofed with insulation on top, (mostly to
keep it cooler in summer rather than warmer in winter), thus converting it
into a so-called 'hot roof'. Once this is done, it will (I believe) no
longer be necessary for me to leave the roof timbers (about 2" wide x 9"
deep joists) and the underside of the decking, exposed for ventilation
purposes. I could therefore fit some sort of ceiling, not for aesthetic
reasons, but so that it will not accumulate mucky cobwebs and dust, which
rain down on my prized vehicles and tools, and most important of all it will
help make the most of the interior lighting, (ie the ceiling will be white).
I have already painted the walls white and it made a huge difference to the
lighting, converting a dark and dirty space into a much lighter one that is
far more pleasant and practical to work in. My aim is to make it lighter and
nicer still.

My question is, bearing in mind that its purpose is not so much to look nice
like a ceiling inside the house, but to fulfil a practical need, what would
be the most suitable (and cheap) DIY material to use? I don't really want to
be messing about with plasterboard, (unless you can persuade me otherwise),
so I was wondering about white-finished hardboard (to save me having to
paint). In time, I am sure that it will sag and buckle a little, but as long
as it is not to too extreme a degree I could live with that.

In practice, I will probably leave a gap of several inches on either side,
ie at the ends of the joists, rather than continue the ceiling right to the
side walls. This is because (a) there is no positive source of heat in the
garage so perhaps a little ventilation wouldn't go amiss, and (b) there are
quite a few alarm and power cables fixed along at the ends of the joists,
and it would save having to disrupt them.

There is a photo at http://www.sterry.org.uk/temp/garage_ceiling.jpg to give
you an idea of what I am talking about. You can also see some of the
aforementioned cables.

Any thoughts?

Many thanks in advance, as they say.

--
Rick