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Discussion Starter · #1 ·
I have a NEC 9pg+ that weighs about 120 lbs. My home theater space is in a finished attic space. The ceiling is plaster on lathe suspended by 2X4's that are attached to the roof struts (see diagram). I would be attaching 3 pieces of unistrut to three of the 2X4's using 3 or 4 lag bolts per piece. I want opinions as to the structural integrity of mounting the projector using unistrut to these 2X4's since they are not actual joists, but just support for the ceiling. If it is safe how big/long should the lag bolts be to be sure that I don't split the 2X4's.


Thanks
 

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It depends greatly on the span of the 2X4's. I cannot find a joist span table that even lists 2X4's. For 2X6's at 20-inch on center they can span no more than about seven feet. So for 2X4's I'd say more than say five feet and you likely need to brace the center of the 2X4's as they will not support typical loads. If you have access to the 2X4's from above you could always scab larger joists to their sides and/or provide center bracing. Now, it sounds from your description that your weight will be about 40lbs per 2X4 . That is not huge load and the nice thing about wood is that it does not fail instantly and catastrophically like steel. It fails slowly over time and it is easy to see it happening and stop it.


Brian
 

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Discussion Starter · #3 ·
Is that a table to determine correct strengh for floor joists? if so since there will be no additional weight above the joists (empty space under the roof peak) I should be okay, right? They span about 7 feet and with the unistrut the load will probably be about 50-60 lbs per joist, but other than the weight of the ceiling they don't have to support anything else. I have no access short of ripping out the whole plaster ceiling and i think I'd rather go digital or at least keep it floor mounted than try to replace the whole ceiling. If anyone thinks I'm nuts to try this please speak up. I don't want to be the guy whose projector fell on him.
 

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Jack1000,

I don't know if the gallery is working but I have pics of my unistrut install there.


One thing you might consider.

Why put big lag bolts up through the unistrut into the 2x4s of the ceiling?

Wouldn't the holes of the lags diminish the strength of these 2x4s?


I suggest you place the unistrut on top spanning a few of the 2x4s then the weight rests on top rather than with lag screws from the bottom. All you would need are some drywall screws and washers to keep the unistrut from sliding around on top of the 2x4s. You could then use threaded rod to drop down from the unistrut to another set of unistruts attached to the projector.(perhaps where the leveling feet were, unless you have attachment points available on the projector)


I would be glad to elaborate more if it helps.


rule of thumb:

Be sure to overbuild since the most likely projector position is usually just above the seating area.
 

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Discussion Starter · #5 ·
The ceiling is wet plaster on lathe, with some loose fill insultation above it. While I would like to span something across the 2X4's wouldn't I have to open up a big chunk of the ceiling to do it?. If you know a way I can do it by opening small portions I'm all ears because I like the idea of not having to use fat lag bolts in the 2x4's. I really don't have the skills to repair a plaster ceiling with a 3 foot hole in it and I think cutting through all that lathe would stress the integrity of the rest of the ceiling. I love my old (1918) house and real plaster is awesome, until you have to open a wall up or even hang a picture.
 

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At this point you should either consult with:


1. Structural references on capacity of 2X4 lumber. I am an engineer but its been years since I did lumber design. There are reference books for this stuff. The span table is for 40lbs per square foot live loads on the floor deck above. Dead loads are greater.


2. An actual structural engineer. A simple call to one might yield your answer without any fees at all.


Brian
 

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The other thing to realize is that the plaster will be cracking WAY before the 2 x 4's deflect excessively. Its cracked plaster that I would be worrying about. OTOH, 120# does not sound like all that much weight to support.
 

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Quote:
Originally posted by Jack1000
The ceiling is wet plaster on lathe, with some loose fill insultation above it. While I would like to span something across the 2X4's wouldn't I have to open up a big chunk of the ceiling to do it?. If you know a way I can do it by opening small portions I'm all ears because I like the idea of not having to use fat lag bolts in the 2x4's. I really don't have the skills to repair a plaster ceiling with a 3 foot hole in it and I think cutting through all that lathe would stress the integrity of the rest of the ceiling. I love my old (1918) house and real plaster is awesome, until you have to open a wall up or even hang a picture.
Imagine cutting a 6x6 round hole right in the middle where the PJ will mount (as you might do for a can light say). Slide the lengths of Uni-strut through that hole into the dead space and do as previous poster [to yours] suggested.


I have my G70 (180+#) mounted 2 4" lag screws to each of 3 joists above and have no concerns about the load on such pieces of lumber. Problem with 2x4"s is that you can't get a 4" lag screw into them any which way you try. Maybe you could on the diagonal, but I wouldn't recommend it. :)


If you can figure a way to do this by laying the unistrut on top of the 2x4"s, I think you'll feel perfectly secure about the mounting structure. Plus, the hole that you'll need to repair will be over the PJ, thus hidden, so you don't need to be an expert plasterer to figure it out...


Alternately, find a spot where you want a 6" can light and put one in... :)


---Jim
 

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jack1000,

Do you have an access hatch to the attic anywhere else in the house?

Perhaps in a closet?


My thought was to put in the unistrut then 4 threaded rods would come down through the ceiling.

All you would have is 4 small holes, not much bigger than the rods.
 

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Discussion Starter · #10 ·
I have no access to the space other than what I am willing to cut. With the 6X6 hole suggestion how would you fed the threaded rods through the ceiling. Could I cut a slightly larger hole push the unistrut through and lay it on top of three joists and then attach four rods through a 1 ft square area and then attach another layer of unistrut to that or would it concentrate to much of the load in to small of an area?
 

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jack1000,


Can you use PM to send me your email address and I can forward some pics that might help.


Bob
 
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