How to support garage roof -- cutting header for new addition

amy6152 · 03-20-2012, 11:44 AM

Picture's worth a thousand words--I've included our current setup and future plans. Putting on a screened porch.

Here's my question. The garage (backs up to new porch) is stick built. You can see the header is currently supported by the beams on the patio, but that's all going to change.

The roof rafters need to be cut back the the existing back wall of the garage, which of course poses a support problem. My father (the big project planner) has suggested sistering vertical 2x4s on either side of each rafter at the wall line, double bolting them, and hence carrying that roof load down to the garage wall instead of the existing header. At that point, the rafters will be cut at the wall, and no longer continue out over the patio, so we can build the new structure there.

FYI, the current garage (approximately 20 x 23) is not too "beefy". Meaning, it has four collar ties made up of 2x6s that run the length (at a ceiling joist height), and that's all she wrote. We're looking at increasing the structural support there while we're at it, looking for advice on what we should be doing. Thinking we'll double the collar ties to eight instead of four... should be do anything else??

Pictures are from Google Sketchup.

Joe Carola · 03-20-2012, 12:17 PM

Quote:

Originally Posted by amy6152

Picture's worth a thousand words--I've included our current setup and future plans. Putting on a screened porch.

Here's my question. The garage (backs up to new porch) is stick built. You can see the header is currently supported by the beams on the patio, but that's all going to change.

The roof rafters need to be cut back the the existing back wall of the garage, which of course poses a support problem. My father (the big project planner) has suggested sistering vertical 2x4s on either side of each rafter at the wall line, double bolting them, and hence carrying that roof load down to the garage wall instead of the existing header. At that point, the rafters will be cut at the wall, and no longer continue out over the patio, so we can build the new structure there.

FYI, the current garage (approximately 20 x 23) is not too "beefy". Meaning, it has four collar ties made up of 2x6s that run the length (at a ceiling joist height), and that's all she wrote. We're looking at increasing the structural support there while we're at it, looking for advice on what we should be doing. Thinking we'll double the collar ties to eight instead of four... should be do anything else??

Pictures are from Google Sketchup.

Yes....higher a professional to do it or higher one to help you do it. This is not an easy Diyer project. Very dangerous!

amy6152 · 03-20-2012, 12:44 PM

Can you tell me what's so dangerous about it? Are you talking about transferring this one load, or are you talking about the whole project? Because I've already gotten feedback from the building inspector and he's comfortable with it. Personally I love a little overkill on anything structural, I and would like to have something besides that row of what essentially amount to hinge points, however. So I am looking for some specific feedback.

jklingel · 03-20-2012, 01:36 PM

How are you tying the foundations together? For your other questions, I think it is very difficult to say from afar. I'd consult an engineer who can see the whole thing.

shazapple · 03-20-2012, 01:37 PM

Since you are modifying the rafters I think you need to talk to whomever is building the trusses for your addition (aka a professional). You are basically turning a triangle shaped roof rafter (two rafters and a collar tie), which is an extremely strong shape, into a 4 sided box, which is an extremely unstable shape. There is nothing to stop the sides of this box (the rafter, collar tie, partial rafter, and bolted 2x4's) from moving and rotating.

Why cut back the rafters at all? Leave them in place and build a tie in from where your new construction ends.

Joe Carola · 03-20-2012, 03:23 PM

Quote:

Originally Posted by shazapple

Since you are modifying the rafters I think you need to talk to whomever is building the trusses for your addition (aka a professional). You are basically turning a triangle shaped roof rafter (two rafters and a collar tie), which is an extremely strong shape, into a 4 sided box, which is an extremely unstable shape. There is nothing to stop the sides of this box (the rafter, collar tie, partial rafter, and bolted 2x4's) from moving and rotating.

Why cut back the rafters at all? Leave them in place and build a tie in from where your new construction ends.

Where does the OP say they're using trusses?

amy6152 · 03-20-2012, 03:29 PM

That's a very good point about going from a triangle to a four-sided shape. Makes sense to me, and points out my concerns. Leaving the rafters as they are is not an option; the roof height on the screened porch would be too low (hard to explain). Plus, we're doing a beam and post construction with a cathedral ceiling on the porch.

I'm posting another pic. In this one, I've drawn white lines to represent the shape and I've drawn in a diagonal support that runs from the wall of the garage to the ridgebeam. I'm thinking that in conjunction with the sistering/bolting, adding these supports on each (or every-other) rafter could help immensely.

Thoughts on adding this?

amy6152 · 03-20-2012, 03:32 PM

Oh, and as for the foundations, I have no idea how they're going together. We've hired a mason for that job.

jklingel · 03-20-2012, 04:09 PM

Quote:

Originally Posted by amy6152

Oh, and as for the foundations, I have no idea how they're going together. We've hired a mason for that job.

Probably a good idea, but I always like to learn enough to ask intelligent questions, at least. If nothing else, it keeps the pro on his/her toes. Google is your friend.

amy6152 · 03-20-2012, 05:10 PM

Okay kids, you scared me enough to call an engineer. He comes tomorrow. I'll post what he has to say in case you're curious. Thanks for the help!

jklingel · 03-20-2012, 05:59 PM

always curious what engineers have to say. sometimes they are even right. be sure to ask about the foundation connection, too; that is critical.

amy6152 · 03-21-2012, 02:36 PM

The engineer was just here. He told us to build an extension onto the back garage wall--the extend the wall all the way up to meet the rafters. Put a top plate on it, then birdsmouth the rafters onto it. Then he wants us to put plywood on both faces of the wall for stability, and put collar ties every-other rafter at the height of the new wall.

It's a plan!