We are putting a new roof on our house (framing and all). The engineer sent me three prints with different truss spacing (16",19.2" and 24" OC). For obvious reasons, I would prefer to use the 24" centers (cheaper material cost, faster construction).

I am wondering what the downfalls are to using 24" centers. Do I have to use heavier sheathing if the trusses are spaced farther apart? The prints call for 1/2" sheathing on all three prints, but I will probably go with 3/4 just because.

FYI, I am in Ohio (stupid snow loads...) and we are using TJI trusses. Any advice is appreciated!

  • Why did the engineer give you 3 options? You might ask him/her if there are other cost ramifications besides just the trusses. I would think the engineer should give you the single most efficient design that meets your needs, there shouldn't really be any choice about snow loads or weight-bearing ability. – Hank Jul 19 '14 at 3:04
  • Different builders have different preferences, subcontractors, and suppliers. Their pricing will reflect those preferences. Therefore, actual market conditions at the time of bidding will determine the cost efficiencies of the alternatives not an engineering equation. – user23752 Jul 19 '14 at 3:24

The primary advantage to closer truss spacing is greater load-bearing ability, which is important if the roof will have heavy stuff on it, like shingles and snow. With 24" OC trusses and a conventional shingle roof, you may need to shovel your roof from time to time to prevent snow buildup from becoming dangerous. On the other hand, you can decrease this irritation by making the trusses' load lighter. One way would be to have a lighter roof material, such as metal. It will be more expensive up front, but a metal roof will last far longer than shingles and probably prevent you from having to worry about snow loads. It will also probably increase your house's resale value, which will not happen if you're simply replacing a failing roof and structure with a new one of the same or similar quality as the old one.

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  • Thanks for the feedback. Unfortunately I don't think we will be able to afford metal roofing this time around. I think we will be stuck with shingles. – BWDesign Jul 18 '14 at 17:37
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    Determining the load-bearing capacity is the job of the engineer. Presumably he would not have offered an option with 24" spacing if it was unsuitable. – Hank Jul 19 '14 at 3:19
  • An engineered design is engineered for the required loads. No additional maintenance is required based on a wider truss spacing. Furthermore, a narrower spacing may create additional deadload requiring upgrading other structural components. It will always require additional locations where the roof system interfaces with the wall system and therefore more opportunities for poor execution of the connection (though the loads passing through the connection will be lower). – user23752 Jul 21 '14 at 16:44

So long as the installation meets the engineered design [and the design was properly engineered], the sole reason to choose one spacing over another is cost. In some cases larger spacing will require an upgrade to supplementary structural components such as framing anchors or increased straps or additional nails. The only way to know if there is a difference is to correctly price out the alternatives.

As an aside, spacing on floors can have noticeable effects because minimum code requirements can overlap with 'bouncy' floors.

The important thing is to make sure you really have a handle on all the differences between the alternatives, not just the first order ones.

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