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Can I modify a Traditional Truss to Scissor Truss?

jetescamilla's picture

Happy holidays everyone, 


I'm new to the forum and would like to get some opinions on a potentional modification to my garage.  Currently I have a garage with standard 8' ceilings and a traditional roof truss.  I would like to "vault" the ceiling to create more height over a small area.  My wife gave me a golf simulator for Xmas and I would like to make room to swing some clubs in my future man cave.  As I said I think I could get by with only making modifications to 4 or 5 truses to create a localized high roof area.  It will also limit my construction efforts.

Before I get commited to doing something like this I like to do a little research and hear from people more knowledgable than myself.  So onto some questions...

 

  1. Would modifying the existing truses into scissor trusses be the easiest route?
  2. Should I sister new trusses next to the existing trusses and then cut the bottom chord?
  3. Is this something a moderate DIY'er could do? 
  4. Has anyone done anything similar?  I'd love to see some before and afters.

Any replies and insight would be great.  I'll see if I can get some photos up sometime later to show existing trusses.

Thanks guys!

Jesse

Do NOT modify trusses without an engineering design. Doing (post #207255, reply #1 of 19)

so is asking for disaster. 

What if said original poster (post #207255, reply #2 of 19)

What if said original poster is a Licensed Professional Engineer?  Even more disaster huh?

I don't design residential, I design bridges.  Very large vehicular and pedestrian bridges.  I can run calcs on simple trusses.  I dont have experience on residential.  I'm just asking what common practice is.  Trust me, I'm not going to do anything that would jepordize the structural integrity of my roof/garage.

Bridgeman (post #207255, reply #3 of 19)

There's a fellow here that is just what you are looking for-a truss designer.

Ron-BossHog.

I'll see if I can ring him up.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Thanks (post #207255, reply #4 of 19)

Thanks Calvin,


I appreciate it.

There is no easy way to (post #207255, reply #5 of 19)

There is no easy way to change flat BC trusses to scissor trusses. How difficult it will be depends on several things - The span of the trusses, the pitch, how steep you want the slope of the ceiling, and your local loading requirements.

Adding new scissor trusses next to the old ones is an option. But how do you plan to get the new ones in place? That's never easy.

 

If you modify the existing trusses I have one suggestion. Rather than trying to make a scissor truss, do a raised center. That way the last few feet at each end of the truss will not have to be modified, and the changes should be less extensive.


If the house is new enough that you know how built the trusses, give them a call.  They probably won't design the changes for you, but they should be able to refer you to an engineer with expertise in wood trusses who can.

Thanks Boss (post #207255, reply #6 of 19)

I appreciate the insight.  In my "google'ing" of raised roof truss options I've came across ideas of a simple raised center as well.  You're right that it would be the least extensive option.  I've called a local truss manufacturer and the sales person assured me that trusses could be modified if designed accordingly.  The truss engineer was out on hoiday leave and will get back to me sometime later.


I didn't buy the house new so I'll have to see who designed the original truss.  I'll probably put together some loads and get some factored stresses to see if this project even seems do'able.  My gut feelign is, yes.  Most residential systems are overly redundant and overly designed...but I won't know for certain until putting some numbers to it.

Bridgeman (post #207255, reply #7 of 19)

Since you are a bridge engineer-do you know Dave Reichman-he works in your field in NYC.  Friend of ours.

and

Have you read Concrete Planet-which raises some questions in the use of concrete in our bridges,buildings, etc.  Just started the book, but it compares the use of concrete by the Romans (among others) and it's seemingly lifelong guarantee and the use in modern times (lasting a whole lot less).  Just curious.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Calvin (post #207255, reply #8 of 19)

Nope, I don't know David.  I am in Arizona and most of my projects are on the West Coast (Arizona, New Mexico, Nevada, Californina & Washington).  Although, through our inter-office work sharing I have worked on one bridge retrofit in Washington DC.  Sorry went on a tangent...I've never met David.  My firm does have offices in NYC so its possible we may cross paths one day.  Bridge design is a small industry.

I also have not read concrete planet, but it looks interesting.  I may have to make an impulse amazon buy.  Thanks for the recommendation.

Jesse

I thought I'd ask about David....... (post #207255, reply #9 of 19)

because as you say, the industry is small and I'd assume there's collaboration or at least shared networks across the country.

rdesigns mentioned the book here not long ago-I picked up one of 6 copies here in the county library. Worth the read for me to just find out how it probably was discovered long long ago.  Robert Courland is the author.

After you get your project figured out, do stop back-though the call for answers on bridge building doesn't come up often.  However, if you can design bridge structure, you should be able to answer some of the engineering questions that come up.

thanks.

edit:  The wife and I are giving some thought to coming to Phoenix for spring training this year-catch a beer?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Phoenix is great in the spring (post #207255, reply #18 of 19)

Have you been before?  You'll love the weather, there's a reason golf prices are so high during the fall and spring.  If you need any info on things to do, what to see, or where to stay I may be able to help you out.  If timing works out then yeah, beer would be great.

 

Jesse

Jesse (post #207255, reply #19 of 19)

The last time in Phoenix-maybe '72.

We visited an old friend that had a small adobe place, down a "lane" in the sand, past a parked 50 something Caddy pickup (like what would haul the flowers to the cemetary) with a camper built on the back and various other "structures" tacked on.............

A few other squatters in shacks, a small trailer or two...........

This was January I think-

The area we were in was called "Scottsdale"..............way b/4 what it is today. 

 

We went over to Tempe one day-rained so hard we couldn't get back to the shack-seems when it poured the dry "gulches" (that what they were called?) ran water so deep, you couldn't get through. 

Spent a couple days and nites in a stupor, fled back to Ohio.

 

So yeah, if we do end up going I'll certainly be in touch-we do intend to get over to the Grand Canyon as we've not seen that yet and it'd be foolhardy to miss the opportunity.

Right now, plans are to maybe stay using points at a Marriott property near Goodyear, spring home to the Indians-perhaps meet up with another couple-Grand Canyon visit............beats me-I'm not much of a tourist.  But as we've done in the past-have lunch/a beer with a BT acquaintance if possible.

It takes the mystery out of the internet.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Glad to be of some (post #207255, reply #14 of 19)

Glad to be of some help.

I'd be interested in seeing whatever you come up with, if you don't mind posting it.

You can't practically modify (post #207255, reply #10 of 19)

You can't practically modify a conventional truss into a scissor truss.  Basically you'd end up cutting away all but the top chord and then building a new truss.

You could do that, though -- build new trusses and cut away the existing.  But as an engineer you can appreciate that the connections are the critical part of a truss, and hard to get right outside of a factory.  And keep in mind that the center bottom connection of a scissor truss is subjected to substantial stress.

And understand that, for a residential roof truss, you're not simply supporting a point load at the peak, but rather load distributed along the top chords.  A conventional residential truss uses a relatively thin (generally 2x4) top member, but it's supported at multiple points by intermediate cross pieces.  A scissor truss generally has less opportunity for effective cross-bracing and hence may require a stiffer top member.

It might be worth considering a cable truss design, for ease of retrofitting.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Modifying trusses (post #207255, reply #11 of 19)

I don't have any idea as to the size and configuration of your garage, but you could possibly  install beams as required to support the top chord of the trusses and then totally remove the bottom chords. 

In keeping with residential types of materials, you could use engineered lumber ( LVL's)  which have Mod. of Elas. of 2,000,000 psi and Flex. Stress of 2800 psi.  Much higher than standard lumber.  Check with your local lumber yard. 

They could be installed by opening up one of the gable ends and sliding them into place.  If you are not eliminating the garage door(s), you may need to beef up the existing headers a little.  As usual, an explanations like this, is an over simplification but as an ehgineer it should not be a problem for you. 

Just a thought.

Without intermediate braces (post #207255, reply #12 of 19)

Without intermediate braces the top chords would sag.  They would need to be reenforced.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Intermediate braces (post #207255, reply #16 of 19)

Dan, please note, I said beams (plural), not simply a single  beam at the ridge.  A beam at the ridge and  where the existing struts intersect the top chords.  Since I was talking to an engineer, I didn't think I needed to go into a lot of detail. I assumed since he designs bridges,  he could deal with simple beam and rafter span calculations. 

IMHO this would not be anymore difficult than trying to modify several exiting trusses on site and in place.

jetescamilla, (post #207255, reply #13 of 19)

I have a customer who is not an engineer but a really great wood worker that has done exactly what you propose. He sheathed parts of the trusses in plywood and then cut out some to make room for his boat build.

I think I would lay a glulam on its side and bolt to the top plates and then stick frame some rafters to the top chord. Could use some steel angle instead of glulam.

 

KK

You can't modify the trusses (post #207255, reply #15 of 19)

You can't modify the trusses but you can leave the trusses in place while you insert traditional rafters next to them. Once you do that the trusses are no longer needed and you can cut them flush with the bottom of the rafter.

Florida Licensed Building Contractor, 40 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Thanks (post #207255, reply #17 of 19)

Thanks for all the recommendations all.  The more I hear I dont think a scissor truss modification is going to be feasable...might as well rip the roof off and repalce them.  I like the idea Bosshog had said of lifting up the bottom chord...everything will have to be beef'ed up and a redesign done, but it sounds simpler to me.


I'm not planning on doing anything soon, I've got a kiddo due any time now and my wife would kill me if my garage project was sitting undone for an elongated period of time.