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I want to hang a motorized projector screen on my basement ceiling. I'd like to know how much weight I can safely hang from the ceiling (joists? See picture below. Arrow marks the spot I'd like to drill/screw at). There's one such structure in the ceiling every 20 inches.

The screen weights 50 lbs and would be hanged from each end. So, 25 lbs on each end.

How do I determine if it safe to hang that and how do I pick the right screw size and length ?

If it makes any difference this is a Montreal house and there's 2 more stories above the pictured ceiling. The room is 18' long, but I don't know if there's a beam or anything midway to hold this joist.

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Before you drill those trusses, I’d be careful because: 1) they are floor (not roof) trusses and carry a significantly larger load than roof trusses, 2) there’s another floor and roof above, and 3) that load will put the bottom chord in “double bending”, which can cause failure.

1) The load looks like it’s being applied at or near the mid-span, where the greatest deflection (bending) occurs. If you could move it to within 1/3 the span of the trusses, it’d be best.

2) I’d check to see what is “loading” the floor above. Some activities have less loading and some are extremely high. Storage (especially book storage), waterbeds, refrigerators, tubs, etc. are “high load areas”. Bedrooms, hallways, open areas, etc. are low load areas.

I’d check to make sure a wall perpendicular to this truss does not support the roof.

3) Any load on the bottom chord will put “that chord” in bending. Very dangerous...especially because you’re adding it mid-span between two chord fasteners. I’d move it as close to the web joint fastener as possible.

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  • > I’d check to make sure a wall perpendicular to this truss does not support the roof What does 'supporting the roof' means ? This is a 2-story townhouse, mostly open space. As such, every truss will be perpendicular to an exterior wall. Are all exterior walls 'supporting the roof' ?
    – Jeffrey
    Nov 20, 2017 at 16:06
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    Yes but some interior walls may also support the roof. You could lay a 2x4 across the span and not have to drill the bottom at all put your load on the 2x4 even a 100 lbs across 2 would be no big deal.
    – Ed Beal
    Nov 20, 2017 at 23:32
  • I agree with Ed Beal. Also, you could drive screws into the joists but drilling removes material and weakens the joist. It is probably inconsequential but why do more damage than you have to?
    – ArchonOSX
    Nov 25, 2017 at 17:18
  • @EdBeal Yes, 50 or 100 lbs. doesn’t seem like a lot, but we don’t design for the “norm”, we design for the critical. So when the client moves that Sub Zero refrigerator on the truss, I can sleep at night. (However, you’re right that the factor of safety would easily absorb a few extra pounds loading.)
    – Lee Sam
    Nov 25, 2017 at 20:34
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These floor trusses can handle a lot of weight. Nothing to worry about regarding the projector. They could probably handle a couple of hundred pounds each. I would recommend that you drill that bottom cord and use 1/4" bolts. They will last indefinitely. P.

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  • This is what I ended up doing, as it is definitely a low-load floor. The bolt is actually closer to the span end than the picture. (I accepted the other answer as I found it and the discussion helpful identifying important considerations)
    – Jeffrey
    Nov 27, 2017 at 18:32

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