I have a block-built outbuilding, 3m x 5.5m, which has a flat roof with
a 5 degree slope running down its length. The current roof is
corrugated asbestos cement, supported on joists running across the
shorter span, at about 1m centres. The joists are embedded in narrow
slots in the inner face of the wall.
I want to replace the roof with a solid roof. Everything I've read so
far details the construction of such a roof based upon the joists
running down the length of the slope, rather than across the slope. Now
for my building, having the joists running that way would mean I need
5.5m lengths of 3"x9". If I ran the joists the other way (as they are
currently) I could use smaller joists (2"x6"), or greater centres,
either of which makes the whole construction cheaper.
The difficulty I see with running the joists across the slope, rather
than down its length, is securing them. I can't reuse the existing
joist slots - they are too narrow, and widely spaced. Instead, I would
have to position the joist on top of the walls (actually quite
desirable, since this gives me more headroom inside the building). If I
attached the joists directly to the top of the wall, then they would be
off vertical by 5 degrees, so I would have to position blocks of timber
to fit into the gaps between the joists to provide sideways support.
Alternatively, I could run a timber bar along the top of the wall, and
then cut vertical notches in that timber to support the joists in an
upright position. I would then have to cut the top off each joist, to
provide a 5 degree slope to which I could attach the plyboard deck.
Is building a roof with the joists running across the slope feasible,
or should I conform to the practice of running the joists down the
length of the slope?