Assuming (and sometimes it's a big assumption depending on the
building!) that the joists were installed properly, measure 24" on
center from a known joist. To find a reference joist, you may need to
peel up a corner of the membrane and find a row of nail / screw holes
and work from there.
Other areas may have different building codes, but roofs are 24" on
Your studfinder should be able to find the rafters on the drywalled
underside. Why do you need to locate them on the roof side, do you
need to drill for a vent or something? If that's the case, then one
possibility is to locate on the underside, then drill up from the
underside all the way through the roof to the outside. If you use,
say, a 1/2" spade bit with an extension, then there will be just a
small hole in the ceiling to patch.
thanks for the replies. a little backround on the project ...
so this question is related to an earlier thread regarding reworking of
a roofdeck in which the old rail posts were 4x6s toenailed into the
rubber roof (very wobbly).
the only workable solution I've come up with (no laughing, head-shaking
is ok) is welding a 4x4 square metal tube to a 24' x 16' 1/2 inch
metal plate. In essence, welding a big flange to a metal post. Here's
some pics of the general idea
and of the welded post-plate pieces
My plan is to bolt these post-plate pieces onto the rubber roof and
caulk the edges for waterproofing. The plates would go under the 2x4
sleepers and deck planks.
So, optimally I'd bolt each plate in four corners and there's 6 of
I thought about locating the joists from below, but that would create
too many (24) holes in the living room ceiling.
another question: It seems optimal to have these bolted into the
joists, but is this essential? the weight of the sleepers and decking
*should* hold down the flanges pretty well.
Don't waste your time with a 1/2" spade bit, use a 1/4" or smaller extra
long drill bit, the hole will take a little time to find (leave the bit in
the roof by un-chucking the drill) but it is an easy fix on the drywall, and
even the roof if you don't like the location.
Poke a coat hanger or an awl through the drywall. You should be able
to get the approximate location by thumping. Then poke a coat hanger
or an awl through the drywall. With such a small hole a little dab of
spackling compound or even toothpaste (old renter's trick) will cover
it nicely - you'll be the only one able to see it.
You can also remove a light fixture and poke a bent coat hanger between
the edge of the board and the box. Twist it around until it hits the
side of a rafter.
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