Truss Design Question

Dave Richards

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A friend of mine is in the process of building a 24x24 building for his new woodworking shop. He wants to build the trusses with raised chords something like in my sketch. We're trying to sort out how high he can place the chord to gain as much ceiling height as possible. Any suggestions would be appreciated. The sketch is only intended to illustrate the general look and not as a construction plan. FWIW, he's going to use 2x6s on 24 in. centers for the wall framing.



Thanks in advance
 
Well Dave i cannot offer solution as to his issue but çan tell you that what i did in my previous shop was to have a scissors tuss designed which allowed for max ceiling height. I have pics on here somewhere and if not can send you some if he wants to see it.



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Typically, building codes in our neck of the woods require the "Collar Ties", also called "Collar Beams", to be located 1/3 down from the peak to the top plate of the supporting walls.. The Collar Ties are usually installed at alternating roof rafters, and without the additional trusses shown in your sketch, although personally, I would prefer to have them as they would add greater stiffness/rigidity to the structure.

Since building codes vary by state, you should check what the local requirements are.
 
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thanks guys. I suggested scissors trusses but he wants a mostly flat ceiling. I could see modifying scissors trusses with a horizontal member added in.

Al, what you are describing would involve adding a ridge beam, correct? Does the code there specify the size of the rafters?
 
Yes, typically the rafters are 2" X 10" X 16 O.C. with a 12"-14" ridge beam. The size of the ridge beam is usually sized to accommodate the angular face of the rafter that is joined to it. Longer span roofs require larger ridge beams, so ridge beam can also be made from "Engineered Lumber". The rafter sizes depend upon on the roof span & pitch & load (snow/ice) spec criteria. Although some building inspectors just consider the horizontal span, as do some "construction manuals", and do not consider the roof pitch, I think that is wrong as the steeper the pitch the loading becomes more vertical over the supporting walls and do not have to carry as much live/dead load as snow will not accumulate as much - slides off more readily that from a flatter roof. Where a 2" X 10" X 16 O.C. may be typical, a 2" X 8" X 16" O.C. may be adequate. Also, the 16" O.C. can be 12" O.C. if 2" X 1" rafter are marginal. Many variables involved.
 
Thank you for that.

This friend of mine is in the UK and lives in an area that sees little if any snow. It never sticks around as it does here, when it does fall. He wants to make trusses instead of just rafter and a ridge beam although maybe I ought to try changing his mind.

FWIW, I've drawn the rafters on these trusses as 2x8s, the collar tie as a 2x6 and the bracing as 2x4 although as I said, they are just to give an idea of the look.
 
Thank you for that.

This friend of mine is in the UK and lives in an area that sees little if any snow. It never sticks around as it does here, when it does fall. He wants to make trusses instead of just rafter and a ridge beam although maybe I ought to try changing his mind.

FWIW, I've drawn the rafters on these trusses as 2x8s, the collar tie as a 2x6 and the bracing as 2x4 although as I said, they are just to give an idea of the look.

I noticed that, wasn't being critical of your sketch. I forgot to mention that typically around here the Collar Ties are 2" X 8" when tied to 2" X 10" or larger rafters. I sure wish I did enough "project" type work to justify learning the SketchUp software I bought from you. My woodworking projects generally relates to fix-it types, or small trinket type work, i.e. jewelry boxes, etc, which don't need SketchUp.
 
Al, I didn't think you were being critical at all. I appreciate the information muchly.

I wonder, considering essentially no snow load and the pitch of the roof, if what I've drawn is close enough. His big worry I think is the length of the rafter between the collar tie and the wall. He found a site where a guy sistered a 2x4 to the bottom edge of the rafters between the collar ties and the wall. He cut the ends of the sisters at the roof pitch angle so they were horizontal and replaced the birdsmouth cut outs.

As to the SketchUp stuff, maybe something will change and you'll find uses for it. ;)
 
One thing to consider, if you haven't already, is to contact a truss company in the area. When I was planning my shop addition a few years ago, then my 20x12 lawn equipment building, I went to a local truss company. They engineered and built my trusses for about what it would have cost me to buy materials.
 
  • First of all that is a relatively flat roof pitch which indicates more structural framing members.
  • There may still be a "minimum" building code requirement in the UK which would determine the rafter size.
  • I would still be inclined to use 2" X 10" rafters if the decision is to use a ridge beam. Otherwise, the 2" X 8"s should be adequate* with no additional loading.
  • "sistered a 2x4 to the bottom edge of the rafters between the collar ties and the wall" - I wouldn't feel comfortable with a 2" X 4" running only from the "connection point" of the Collar Tie to the wall as the "connection point" would still be the "flex point". Better to add a longer piece to the side of the rafter to support the "connection point" as well, and larger than a 2" X 4". Frankly, I think in the example given this is a jury-rig approach to beef up a "minimal" truss work.
  • Walls usually have a double top plate & single sole plate.
  • The (3) top trusses are in compression so the 2" X 4" framing should be adequate. A truss design is strong for its light weight, but there are situations where extra wood is preferred.
  • Example: the 2" X 8" Collar Tie is in tension/bending, so should there be any "extra loading", say from a hoist later on, you may want to have a few larger sized Collar Ties, or doubled up Collar Ties specifically located for lifting things on the hoist.
  • With a 24' span across the building, and no other apparent interim wall for support, a mid-span support(s) with beam may be required anyway, even with the truss design, & this would better serve a hoist as well. This may conflict with the arrangement he has in mind, so he may have to reconsider how best to support this long span. If no mid-span supports are permissible for his application, then a Laminated Veneer Lumber (LVL) of appropriate size (depth) may be the best choice.

*So much depends on the local building code requirements. Actually, an understanding of these codes should be known first before designing/constructing a building, otherwise it's a best-guess situation. And when applying for a building for a permit, it's always better to have it pass review the first time around, rather than having to go back a second/third time which is usually under more scrutiny.


PS Good suggestion from Bill!
 
dave i have two suggestions, first one is if he wants more head room make the walls higher and use normal truss design, and secondly contact your local lumber yard for the engineered truss style for what your after. they will make you a drawing of what they suggest and then you could use that to do your drawing.. as i see your drawing right now it would work like it is. but the other two suggestions is the best way to deal with it. when making your own truss you need to use metal plates preferably rather than plywood gussets
 
Just semantics, but I think we are talking about rafter ties here instead of collar ties. I always thought the collar ties were to resist ridge separation either under unbalanced snow loads or wind uplift. Rule of thumb for rafter tie placement is no more than 1/3 up from the bottom. The higher the tie the more bending load it places on the rafter, to the point it forces the walls apart. Here are some interesting discussions:
http://mathscinotes.wordpress.com/2010/11/29/the-mathematics-of-rafter-and-collar-ties/
http://www.nachi.org/collar-rafter-ties.htm
http://www.finehomebuilding.com/how-to/qa/rafter-ties-different-from-collar-ties.aspx
http://meaddesign.com/inspectorsblog/?p=63
 
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The right answer as far as I'm concerned is that he needs to go the his local building department find out what they want & do it. They are the ones to give the final inspection & he has to pass that inspection. It doesn't make any difference what someone else thinks.

As far as strength you can easily span a 24' area with a 2x4 truss with a 4 in 1 pitch with plenty of strength to spare & in a low snow area you can do it with a 3 in 1 pitch.

I know where there is 1 buildings connected in a medium snow load area 1 has a 3 in 1 & the other has a 4 in 1 the trusses are constructed out of 2x4's the buildings are 32' wide. The 3 in 1 is approximately 50 years old & the 4 in 1 is approximately 40 years old They have survived all these years with no problems.

The other thing about building your own trusses is you can build them with good quality straight material the first time. We have had to refuse trusses because they just slapped them together & the material was anything but straight. In Washington state the trusses now days have to have a engineers stamp on them just remember that doesn't mean the are a good quality truss.

The 3 in 2 trusses have plywood plates with weld wood glue & staples joining the joints & the 4 in 1 has steel plates. Actually I prefer the plywood plates because of the glue & the staples are longer & have friction glue on them then the prongs on the steel plates.
 
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Ted you're absolutely correct in that I used the term "Collar Ties" incorrectly in this specific application instead of the more accurate term "Rafter Ties". It is typical to have ceiling joists for the upper level of houses in this area and they serve as rafter ties, so the term rafter tie is seldom used by the builders/contractors. For those structures with an higher ceiling concept, such as tray ceilings, then rafter ties are used to tie the rafters together and are used in conjunction with the collar ties.

Bart, trusses can actually be used to over 60', but if additional loading beyond snow load is required, as previously stated, then further design considerations must be factored into the final design. Low pitch roofs are commonly referred to as "Shed Roofs" in our area, and building inspectors are very concerned about rafter sizes & O.C. spacing; mid-span supports; and truss design & sizes when used.
 
When I built my shop 26x26 I went to Home Depot and looked at the trusses they had to offer. They were stamped with engineering certified stamp. You can get a drawing with ALL the details. The Trusses are designed by architectural engineers.

When I built my roof I did not use the exact design because I was not happy with the design. I used all the same dimensions but bigger wood. They said 2x6 - I used 2x8. They said 24 OC - I did 16 OC.

What I have assurance about is the design, more than passes the engineering certification. I know that!!! I started with an engineering certified drawing.

Even when I got my contractors license the training I got stated that contractors are NOT designers and it is a dangerous thing for a contractor to recommend a design feature - particularly when it is structural. Ohhhhh mayy contractore do - but it is NOT because they KNOW the math, forces, or strength of materials. It is because maybe they say it done, or they used something that was engineered. or perhaps they are just guessing.

I would NOT trust the building inspector either. MOST of the time the building inspector is ABSOLUTELY ** NOT ** a design engineer. He is a contractor that got elected, or just someone that got certified to be a building inspector.

Roof trusses are designed by licensed, certified architectural engineers. NOT - contractors or inspectors.

Home depot does not design trusses - they just sell them. BUT - they are engineered, and the designs are worthy - and FREE - FREE.
 
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Roof trusses are designed by licensed, certified architectural engineers. NOT - contractors or inspectors.

Home depot does not design trusses - they just sell them. BUT - they are engineered, and the designs are worthy - and FREE - FREE.

Your right Leo

My Dad wore out 3 books about the size of a check book over 35-40 years that had the certified drawings designs & requirements for trusses & rafters. I have 1-2 of them around held together by rubber bands.

As you said it is important to get up to date prints & designs approved in your area with an engineers stamp of approval.
 
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"Roof trusses are designed by licensed, certified architectural engineers. NOT - contractors or inspectors."

I never said they did. I simply said the builders/contractors must have their designs pass inspection to get their building permits. The building inspectors use architectural drawings from licensed, certified architects & approved by the state that have architectural seals & stamps of approval. They are made available to the local level for their use in evaluating house designs. If a truss design is not consistent with what they have, inspectors would have an architectural firm, at local or state level, review the drawings for approval. Frankly, I would never trust a contractor to develop an architectural house design even if he/she had long experience building them. That would be putting the fox in the chicken coop!
 
Al, I hope you did not think I posted anything to be in conflict with anything you posted. I had no intention of doing that. Maybe I came off that way and if I sounded like that I am sorry. I did not mean to be offensive to you - or anyone else.

I don't know anyone's background here and I am sure some postings can sound like an attack.

I was simply posting my thoughts on the subject.
 
No offense taken Leo. Appreciate your concern. Your comments were correct. I just wanted to clarify my previous statements and I believe we share the same thinking on this matter.
 
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