locating roof joists

I need to locate the joists withing a roof coated with rubber membrane. My studfinder doesn't seem to be doing the job. Any other ideas?
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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 center here.
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Can you get to the underside? If so, measure from a vent pipe.
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unfortunately it's all drywalled and the roof surface is smooth as well (no vents)
Homi
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gd226 wrote:

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.
Ken
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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). http://groups.google.com/group/alt.home.repair/browse_frm/thread/4dcd0ee512926f00/ebc99a676d3f3e1b#ebc99a676d3f3e1b
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 http://www.pbase.com/homi/image/48522649 and of the welded post-plate pieces http://www.pbase.com/homi/image/48522654
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 them. 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.
thanks, Homi
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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.
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gd226 wrote:

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.
R
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Get a better studfinder ! If there are nails in there, a good studfinger will find them.
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