Attic rafter strength

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  #1  
Old 12-03-05, 07:03 PM
flat_broke
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Attic rafter strength

Originally Posted in Attic forum but I am not getting much traffic there.

I have removed all the old insulation in my attic (ruined during re-roofing job) and installing new/more. I noticed that the ceiling joists have quite a bit of give when I am moving around up there. I realize they are overspanned for thier size. What could I do to strenghen them so that they can support the weight of all the new insulation and an AC air handler at some future point? Should I install more collar ties?

Here are some specs and a picture link (Almost to scale):
1. built: 1890
2. joist/rafter spacing: 24"
3. joist size: actual 2"x4"
4. Rafter size: actual 2"x6"
5. No ridge board installed
6. No floor installed

Attic framing pic.
 
  #2  
Old 12-05-05, 08:13 AM
L
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The only way that you would get the roof to support an HVAC unit would be to replace the rafters with engineered trusses. Use an HVAC unit that is installed at ground level rather than roof mounted.
 
  #3  
Old 12-05-05, 12:12 PM
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flat_broke,

you did not state the span. The easiest way to beef up the ceiling joists is to sister a new ceiling joist to the existing based on the span you going. You will not affect the ciling materials except for a few nail pops you will have to pound back in and patch. Since you are only asking for a load in a specific spot (under the ac) you should be able to dbl up the (new) joists at that that location and take care of all your needs.

I hope this helps.


Brian Garrison
 
  #4  
Old 12-05-05, 02:40 PM
flat_broke
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Thanks for the replies. If I understand what 'span' means, then the span of the current 2x4 joists is about 16', 24" apart. I do not plan to be up there much once the rewire and insulation project is done. The AC unit I mention is the blower only the compressor will be outside.

It appears that to support the load of the blower for my span I could sister 2x10's to the two or three joists that would need to support the load? Should I worry about the missing ridge board or adding more collar ties?
 
  #5  
Old 12-05-05, 02:49 PM
L
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There are a lot more issues involved than anybody has touched on yet.

Is snow load an issue? Wind load? Siesmic Zone? Will your local bldg. dept. allow you to alter the roof structure as you are proposing, or will they require that the alteration be done with trusses? There's only one place to go to get the answers to those questions.
 
  #6  
Old 12-05-05, 11:55 PM
flat_broke
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Thanks for the input. My local building department has no guidelines to speak of. So long as I pay my permit fee, there is no problem. (Small town)

As far as load goes, The house has withstood 115 years with the current roof design. I am just trying to improve the structure.

Some snow but no eartquakes here in southwestern Iowa. Almost all the houses around me are the same design. None have central air so I have no installations to compare against.

I am not an engineer so please be forgive my ignorance. I am trying to learn!
How will sistering on a larger ceilng joist worsen my situation? Is the weight of the new joists causing the rafters to spread what I should be worried about?
If I had a truss design, could the current rafters be transformed into a truss?

The blower install is probably 2 yrs off so I have some time to assess the situation. However, I am installing the insulation and new wiring now and the ceiling joists are accessible. Thanks.
 
  #7  
Old 12-06-05, 08:16 AM
C
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structural engineer

I know nothing about this, but had you considered hiring a structural engineer to come out and tell you what needs to be done? The structural soundness of your roof seems pretty important to me.

If you do make modifications and the roof fails, will your homeowners insurance cover it, if the modification is deemed to be unsafe by their inspectors?
 
 

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