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Old 11-28-2011, 07:04 AM   #1
nukefission
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Default Converting roof trusses to rafters

I have a great blank slate to work with and need some help. This garage is 24x30 and has 11ft ceilings with engineered fink trusses on 2ft centers. I would like to install an assymmetric, clear floor, two-post lift in here, but for that it would appear I need at least 13ft. Would it be possible, and not prohibitively expensive, to convert these roof trusses to rafters, thereby changing the ceiling from flat to vaulted?

What I mean by "converting" is to attach one (or more) stronger (2x6 or 2x8) rafter members adjacent to each truss along the slope, make the appropriate butt cuts for them to sit on the top plates, then cut everything out below the rafters.

I will be getting professional opinions as well, but I wanted to see if anyone on here has experience with this sort of thing.

Also if you have any other ideas on what I should do with this space, I'm all ears. There is so much potential that at times I'm at a loss on what to do. At the moment the plan is:
-Acid etch then coat the floor with Rustoleum Epoxyshield 2-part solvent
-Tear out existing cabinets
-Add lots more wiring everywhere for outlets and lights, and maybe in-wall speakers
-Insulate/drywall or OSB
-Build little exterior attachment to house behemoth compressor (to free up space and so I don't have to listen to it)
-Baseboard heating around perimeter

Thanks for the space!
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Old 11-28-2011, 07:52 AM   #2
Ironcrow
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Default Re: Converting roof trusses to rafters

The problem is the tension in the bottom member of the existing trusses is supporting the spread load from the weight of the roof. With rafters, you have nothing to hold this force - the roof sags and the walls bow out. The solution is to install and support a ridge beam. I think it is worth doing, but it won't be completely easy and cheap.
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Old 11-28-2011, 08:29 AM   #3
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Default Re: Converting roof trusses to rafters

The short answer is that you can do almost anything. You need professional help to aid you in determining the most efficient and least expensive method for achieving your goals. If you have a specific location for the lift then I would just modify the existing trusses in that area to accommodate it. One possible solution might be to install scissor trusses at midpoints of the existing trusses. Another might be to modify the existing trusses. Engineered lumber might be useful here. You might also want to explore installing a girder truss on one side of the area you want to modify and then running a ridge beam from there to the exterior gable wall. This will allow rafters to take a lesser load.
In any case, your engineer will determine the loads required by code and offer you options with relative costs and ease of implementation advice.
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Last edited by bczygan; 11-28-2011 at 08:34 AM.
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Old 11-28-2011, 09:09 AM   #4
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Default Re: Converting roof trusses to rafters

Or you can install 2 parallel beams as purlins. Either way on the beams, just sneak them up against the existing and use metal ties.
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Old 11-28-2011, 09:38 AM   #5
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Default Re: Converting roof trusses to rafters

Trusses resolve to vertical loads, rafters have a combination of vertical and horizontal loads. In your situation, the horizontal loads on the walls will need to be addressed. This can be done within the truss system and is essentially like designing a large skylight. You have to box the perimeter, where the lift will conflict, and convert the loads to vertical. You don't want horizontal loads above the garage door if it can be avoided.
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Old 11-28-2011, 09:41 AM   #6
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Default Re: Converting roof trusses to rafters

put on an addition to house the lift and then add on your compressor room to that. you can always use more space.
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Old 11-28-2011, 09:45 AM   #7
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Default Re: Converting roof trusses to rafters

No, you cant do it
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Old 11-28-2011, 09:59 AM   #8
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Default Re: Converting roof trusses to rafters

Quote:
Originally Posted by nukefission View Post
I have a great blank slate to work with and need some help. This garage is 24x30 and has 11ft ceilings with engineered fink trusses on 2ft centers. I would like to install an assymmetric, clear floor, two-post lift in here, but for that it would appear I need at least 13ft...
Check out this thread. If you get a floorplate lift you'd be okay and not incur additional building costs if money is a major concern. I know that's not what you want but it's an option (better than no lift at all).
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Old 11-28-2011, 11:07 AM   #9
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Default Re: Converting roof trusses to rafters

Just get Mohawk lift and be done with it done in 2 or 3 days no permits,engineering fees, less labor on your part
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Last edited by robertlynk; 11-29-2011 at 02:53 AM. Reason: wrong name Tomahawk to Mohawk
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Old 11-28-2011, 11:40 AM   #10
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Default Re: Converting roof trusses to rafters

Short answer:

Can "you" just cut/modify some trusses?

Nope.

Can an engineer design and calculate a way to get to your desired end result? Possibly.
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Old 11-28-2011, 12:13 PM   #11
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Default Re: Converting roof trusses to rafters

You really don't have to convert all of them, just the ones above where you plan to install the lift (plus maybe one on each side.) If done correctly and with an engineers blessing, I wouldn't think that it would have much of an effect on the structure.

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Last edited by Ray-CA; 11-28-2011 at 06:32 PM.
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Old 11-28-2011, 01:29 PM   #12
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Default Re: Converting roof trusses to rafters

Quote:
You have to box the perimeter, where the lift will conflict, and convert the loads to vertical. You don't want horizontal loads above the garage door if it can be avoided.
The way I read that is beams on either side of the clear space running wall to wall, with a ridge beam over that clear space with the end ridge beam loads carried down to the cross beams or through posts to the floor. Not cheap and an engineer would need to calc the loads to size the beams. To have that space intersect with the big front door would have really amped up the 16' beam cost. What happens with a ridge beam is that the rafters only rest on the beam - all the roof loads go to the beam, so that beam has to have end support to transfer that load to the foundation. You may still have to sister the upper chord of the trusses to hold up the roof dead load. This is why my shop has no clear span for a high rise lift - money and complexity.
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Last edited by Falcon67; 11-28-2011 at 01:33 PM.
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Old 11-28-2011, 01:58 PM   #13
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Default Re: Converting roof trusses to rafters

Iron-Crow and Ray-CA have the idea.
But a lot depends on your local snow load numbers.

Most times the side loads from rafter construction is taken up by top plate level joists.
But you can put the joists higher up.
Then they are called “collar ties.”
Somewhere in the 1/3 to 2/3 of the top plate to ridge distance.
It depends on your local code how high you can go.
You will need to check on how high you can go.

If you can get the height you need with the collar ties, then in the area you need the extra height, put in rafters and collar ties and then remove the trusses.
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Old 11-28-2011, 02:35 PM   #14
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Default Re: Converting roof trusses to rafters

Couldn't you run the overhead lines/plate above the trusses? Your lifting height may be affected slightly and you could even rig up some kind of "stop extension" to the existing stop bar on the lift.

Or, like someone mentioned, shell out the dough for a Mohawk.
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Old 11-28-2011, 04:01 PM   #15
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Default Re: Converting roof trusses to rafters

Add a 3 foot high crib wall to raise the roof. Cut through the top plate with a demo blade, jack up a little at a time, blocking as you go.
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Old 11-28-2011, 04:45 PM   #16
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Default Re: Converting roof trusses to rafters

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Originally Posted by kbs2244 View Post
But you can put the joists higher up.
Then they are called “collar ties.”
Somewhere in the 1/3 to 2/3 of the top plate to ridge distance.
It depends on your local code how high you can go.
You will need to check on how high you can go.

If you can get the height you need with the collar ties, then in the area you need the extra height, put in rafters and collar ties and then remove the trusses.
Down to 2/3 is a collar tie - it serves no purpose other than to maybe hold the rafters in to the ridge board and keep them from spreading. Some codes ask for them, some don't. Go below that and you have a RAFTER tie - now you have a big deal. The rafter tie height and the rise/run determine how much load gets put on the rafters. Depending on dead and snow loads, you can actually engineer it so that you can't make your rafters big enough to handle the load. I researched this quite a bit. Raising my 24' joists 2' would have increased the rafter loading enough to require a 2x8 rafter on a 12' span. It's like the span increased to 17'.

http://www.nachi.org/collar-rafter-ties.htm

LOL - do the math
http://mathscinotes.wordpress.com/20...d-collar-ties/
Calculator:
http://www.bobharrisdesign.com/struc...rafterTies.php
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Last edited by Falcon67; 11-28-2011 at 04:49 PM.
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Old 11-28-2011, 04:52 PM   #17
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Default Re: Converting roof trusses to rafters

Quote:
Originally Posted by plym49 View Post
Add a 3 foot high crib wall to raise the roof. Cut through the top plate with a demo blade, jack up a little at a time, blocking as you go.
This may very well be the best way to do this. Reworking trusses is quite complicated and easily could have dangerous results. The rafter sizes to replace trusses are much larger and a collar ties must be matched to negate the horizontal thrust.

Paying an engineer will also be expensive because the plans they sign will be excruciatingly detailed to avoid liability. I'll never forget the structures professor I had that started off his class with the saying "you are living dangerously if you cut a beam in the tension zone". The biggest hurdle is always going to be pleasing your building inspector. Might want to get him on board at the start.

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Old 11-28-2011, 05:23 PM   #18
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Default Re: Converting roof trusses to rafters

It only 24" wide. nail 2x6 on the trusses, nail a2x4 stringer from side to side just high enough to clear the roof of you tallest truck. These guys are making a mountian out of a ant hill. Rafters have been around alot longer than trusses.
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Old 11-29-2011, 02:14 PM   #19
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Default Re: Converting roof trusses to rafters

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It only 24" wide. nail 2x6 on the trusses, nail a2x4 stringer from side to side just high enough to clear the roof of you tallest truck. These guys are making a mountian out of a ant hill. Rafters have been around alot longer than trusses.
True - BUT every "joist" (lower truss chord) removed increases the horizontal push on the walls. Over the door looks like very minimal beam support and a wide hole in the wall makes it the weakest in terms of resisting any outward push. It won't happen "boom" but after a few years, maybe "hey, what's with the door frame..."

One thing to consider is a specific opening - say 9' x 20'. Like a giant attic access door. 2x6 sistered rafters in the clear space, double 2x10 or 2x12s on the long edge, and a 2x6 or 2x8 "space frame" sandwich on each end. That's a 2'x9' box with glued and screwed OSB top and bottom with internal 2x ribs which forms a very stiff box to hold the wall in place and spread the load along the walls at each end. That's a real SWAG at a possible solution.
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Old 11-29-2011, 03:47 PM   #20
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Default Re: Converting roof trusses to rafters

If it were my building and I *had* to do it, I'd choose the width, lets say 10-feet. After creating a rigid frame at the door opening with a engineered header, I'd create truss girders on either side of the opening. Using temporary supports, you can remove the unneeded trusses and replace them with purlins designed to be supported by the side truss-girders.

The truss-grider, brace frame, header and purlins will need to be designed for dead and wind loads.
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