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attic truss design

attic truss design

attic truss design

(OP)
I'm in the process of having a home built and the roof system has created some debate. The plans call for a traditional rafter and floor joist systems by request to create an open space in the attic for storage and potential future living space. The builder has been pressing hard to go with an attic truss. I asked around and read online and agree this seems to be the best way to go weighing costs and time to get the house weather tight. The house is a simple rectangle (30'x40' outside dimensions) with a designed 5.25/12 pitch gable roof. I've realized after the fact the pitch is far to shallow to achieve the center room desired height of between 6'6" and 7'. What I have read online thusfar seems to point to a 9/12 pitch being typical for attic trusses. I fully intend (since its required by the BI) to have the system engineered and certified, but my questions are: given the outside dimensions of the house (plus a 12" overhang if its important to know), what should be the minimum pitch of the roof to achieve the 6'6" to 7' center room height? and what is the interior width of the space?

RE: attic truss design

You should considered a 2' to 3' heel height to get a higher ceiling with a wider room. It is very easy to do with Metal Connected Wood Trusses.

Garth Dreger PE
AZ Phoenix area

RE: attic truss design

You will probably be dealing with either a scissors truss option or a collar tie option.  In either case your engineer will have to check for lateral spreading on the exterior walls.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: attic truss design

If you want to create a "box room" inside the trusses, you will have to have an increased chord depth, top and bottom.  For the span of the "box" your truss designer will have to consider bending in the chords, not just simple truss action.  In addition, as Mike noted, you might need to consider collar ties.

If you want to create storage area, a scissor truss will not help.

Assuming your truss will span the short distance, you will need a pitch of 5.5/12 to 6/12 to achieve the height you want at the ridge.

RE: attic truss design

wingnutNJ, Attached is a typical attic truss configuration to get a large living space. Talk to a truss designer about room size and cost for the trusses. If you can increase the pitch and decrease the room so that the ceiling and wall meets, it will decrease the cost.

Garth Dreger PE
AZ Phoenix area

RE: attic truss design

Yep.  Looks good to me.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: attic truss design

Agree...just make sure that bottom chord splice is good!

RE: attic truss design

Since you started with a traditional joist and rafter configuration - the truss designeR may be able to use any interior bearing walls to design the trusses with smaller chords  - THEREFORE CHEAPER.

At least ask.  BTW - this type of construction has been used for well over 20 years and is most cost effective and gives good value.

RE: attic truss design

(OP)
Thanks everyone and especially Woodsman88.

The information has proven very informative and useful. The exterior wall construction of the house is ICF and there is a centerline bearing wall that carries down to steel girders in the basement.

A raised heel had been discussed, but in coordination with an open floor truss. The builder's idea is to lay two more courses of ICF (32" of increased height) and construct a knee-wall to the floor framing to tie everything together. The way I understand it from the posts, a typical attic truss with a raised heel can be engineered to achieve what is desired without having to frame a traditional floor (i-joist or dimensional), thus reducing cost due to more concrete and floor framing.

I appreciate the feedback and advisements; I think I can speak a lot more intelligently about what it is we want and possible methods to achieve it.

RE: attic truss design

Be careful using only a 2x6 top chord.  You need enough depth for insulation.    

Jim Houlette PE
Web: www.evstudio.us
Online Magazine: www.evstudio.info
 

RE: attic truss design

(OP)
Thanks for the advisement JHOULETTE.  The initial intention of the space is for coexistence between an AHU and storage.  In the future, it may be converted to an occupied space as an office or other function.  Additionally, we are going with a standing seam metal roof with the possibility much later on down the road of installing a PV system over that.  I understand the entire roof system needs to be engineered to consider all of these factors, insulating the roof deck certainly being one of those.  At this point, we are looking in the direction of about a 1'6" heel height with a 6/12 pitch.  This will give us a center room of a approximately 6'8" high in the center, 4'9" high on the side walls, and a 12' width.  Since it hasn't been engineered yet, I am considering a 2' heel to make the space a little less clostrophobic for someone over 6' tall.

RE: attic truss design

Perhaps it might be good to have 7 foot ceiling height for a "habitable" room as defined by code for later.

Also, we often see or design attic trusses with a 2x6 top cord and then add an packer along the cord for the depth needed to hold the insulation as was mentioned above.

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