Rafters Need Cross-Tied? (calculation help)


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Old 08-20-08, 12:35 AM
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Rafters Need Cross-Tied? (calculation help)

Hi, we have an old garage that I'm putting a ceiling in so that I can have storage space above the cars. i plan to be able to walk around up there... but my question related to the rafters, and how to reinforce their span (if needed).

Span of garage is 20' x 20'
Rise is 7'6" In Center
Roof is pyramid-shaped (comes to a point in the center)
Rafters are old 2x4's 16" on center
Indianapolis, IN (for snow load calculations)

Currently it has some really crummy looking 1x4 cross-ties above where I put the ceiling joists. I would like to remove these cross-ties (they run parallel to the floor) in order to free up the area to use it as storage, but from what calculations I've done the span needs reinforcement. I'm just not sure how best to reinforce the spans (if needed) in order to use the space as storage.
 
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Old 08-20-08, 12:08 PM
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You do not need to re-inforce the rafters to allow storage on the joist you put in as long as the joist sit on the walls. For a 20' span I would not use any less than 2x10. The cross ties you are referring to are called collar ties and are needed to help keep the roof from pushing the walls out, and possibly to give the rafters some strength (since they are only 2x4). You can sister new 2x6 rafters to the old ones to gain the strength your looking for.
 
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Old 08-20-08, 12:20 PM
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Bill, the joists are fine. No issue or question there, and I did use 2x10's.

Do you think it would be easier to sister 2x6's or put in some sort of cross-ties?
 
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Old 08-20-08, 01:01 PM
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It will certainly be easier to raise the collar ties. With the pyramid look you described we are talking about a very small amount of collar ties aren't we?
 
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Old 08-20-08, 01:17 PM
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Bill, there are currently 4 1x4 collar ties in one direction and 3 1x4's in the other. That being said, I'm not sure the previous homeowner knew what he was doing. All I can say is the roof is holding up right now.

I'm not sure if there is a particular number of ties I should use, or if it matters how high I push them up?

Right now they're about 2-3 feet above the new 2x10 joists, so when I put a floor up there they'll be in the way. That being said i don't want to push them up too high and have a span issue.
 
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Old 08-20-08, 06:10 PM
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First, double up a joist directly under the peak and put a 4x4 post from under the peak to it. Use simpson or other type of straps to secure to the hip rafters. Then you may want to eliminate some of the ties and see what happens. You will be able to tell if there is any load on them when you take them out. You can always sister the rafters with 2x6's later if you find sagging is a problem. This is really hard to get a handle on without being able to see it myself.
 
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Old 08-21-08, 05:47 AM
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Collar Ties

Are collar ties necessary if the ceiling joists span the width of the garage? Seems to me the joists will prevent the walls from being pushed out.
 
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Old 08-21-08, 03:38 PM
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You're right Wirepuller, the collar ties are not needed for that purpose. I believe they were there to help with rafter deflection since the rafters are only 2x4 and are about 12 feet long. Since the roof comes to a point I suggested the post as a matter of being cautious.
 
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Old 08-21-08, 05:52 PM
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Originally Posted by Wirepuller38 View Post
Are collar ties necessary if the ceiling joists span the width of the garage? Seems to me the joists will prevent the walls from being pushed out.
But does anyone know which way the joists run on the east and west side of this hip roof, if say at least a share of the joists are running north and south?

The idea of underpinning the peak was a good one, from Bill. Post and beam cathedral-ceiling houses are done this way by using a center carrying beam (nailed to center posts) that carries the rafter-beams, to similarly keep walls from spreading.

And also Bill is correct about collar ties used down as low as they are, to assist in strengthening 2 x 4 rafters, to keep them from bellying in from the roof load + perhaps snow load.

Sounds like roof peak is in the neighborhood of 9:12 (with given info of 7 1/2 :10) If the pitch were in the league of 12:12 or greater, the weight pressing down would greatly reduce, even under snow load. But since it is under that 12:12 mark, more attention must be made to keep walls from spreading and rafters from sagging under roof load. The lower the roof pitch, the more susceptible the walls are to bowing out, and also the more stress is applied to the rafters from the roof.

I had another idea that he could brace the rafters without taking away a lot of useable space, by nailing 1 or 2X boards vertical, between the rafter and joist, in from the outside walls say about 3-4 feet. This structural design would transfer some of the roof load weight to the joist at a point along the joist run where the deflection is not as great, since it would only be in from the outside wall only 3-4 feet, let's say.

This method would resemble how 1 1/2 story Cape Cod houses are framed in the upstairs attic rooms. Then that triangular space formed between joist run, framed wall and roof line, is used to store some stuff in where the big open area is used for bedrooms, because it is tall enough to walk in that more center area without hitting your head. Yours would be lower clearance, but the idea would be the same.

Then to make it stronger yet, you could redo the collar ties, only up much higher.

But please let us know first which direction ALL the joists run.
 
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Old 08-27-08, 12:24 PM
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Great replies everyone! Thanks.

The ceiling joists have two doubled up 2x10 beams running N-S and then 16" on center 2x10 beams Running E-W between them. I hope that's the info you wanted.

I like the idea you have ecman51, I was thinking about something like that for the exact reasons yo noted. I tried to calculate load with some rafter span tables (a bit complicated for a DIY'er) and it looks like the 2x4's should really only span about 8' unsuppported.

I don't love the idea of a center beam, just b/c it may be an issue moving long pieces of wood up there. Probably not a big deal, but I'd prefer to keep it open up there if possible.
 
 

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