Thread: Trusses spaced 4' apart?

This is a guest house on the property. Several deficiencies in nearly all categories indicate amateur construction. The trusses in the attic are spaced 4' apart. Top and bottom cords are 2x6 and the webbing is 2x4. I have never seen this before and am confident that it is incorrect but I want to make sure. Is there a possible construction method that I am unaware of that would make this correct? Your comments are appreciated.

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Yes, no, maybe.
Really it depends on the design.
From what I can see I would not be concerned.
The strength of the truss will usually be much stronger than is required for the load with the weakest link being the span of the decking.
The span for the OSB in this case would not be determined by the truss spacing but by what I will call the 2x4 purlins.
These appear to be about 2' spacing which may be ok depending on the rating of the OSB.

Thanks for your comment Jim. Like I said I've never seen this before. Wouldn't the number of trusses directly effect the ability to transfer of load to earth? I never lifted the insulation to look but I also wonder about the ceiling coverings. I guess you would have to install bridging between the trusses to hang the drywall? This is important to the client because all of the other deficiencies can be corrected relatively easy and inexpensive but if the roof structure is not right it could be a major expense to correct. There were no signs of failure.

Trusses at 4' centers is not typically a deficiency. It usually only means that each truss will carry a corresponding larger unit load into the exterior walls, than if the trusses were at 2' centers. You didn't mention what the wall framing is, but if there were "no signs of failure," things are probably OK.

The log house I lived in for 5 years in Colorado had massive timber trusses, at (I think) 8' centers. Never any signs of problems, even though the snow load at our 8000' elevation was 110 lb. per square foot.

Thanks Bridgeman. The exterior walls appear to be 2x6.

Originally Posted by Jim Luttrall

The span for the OSB in this case would not be determined by the truss spacing but by what I will call the 2x4 purlins.
These appear to be about 2' spacing which may be ok depending on the rating of the OSB.

OSB is to be installed with the long dimension parallel with the span, i.e., perpendicular to the supports - just like plywood is.

The OSB shown is installed the equivalent of plywood with the face grain perpendicular to the span, i.e., parallel with the support - in other words, the OSB in installed incorrectly using the 2x4 purlins as the supports. This installation also leaves the ends of the OSB unsupported above the trusses but in open air because of the 2x4 purlins.

I would want an engineer to bless this construction. Yes, there are times when trusses are designed for 4 foot centers instead of 2 foot centers, but the OSB creates a problem as it is installed, thus I would want an engineer to the laying on of hands and bless that work, because I am not going to.

There appears to be metal diagonal metal braces installed under the decking secured to the trusses.

Also notice that the gable end wall is not a gable end truss. That means the OSB cannot be installed properly with regard to direction over supports and nailing pattern.

Here's a couple more pics. The vent pipe is servicing the water heater. The house is slab on grade. A single plumbing supply comes through the slab and is distributed to the fixtures through the attic.

Originally Posted by Bill Anglin

Here's a couple more pics. The vent pipe is servicing the water heater. The house is slab on grade. A single plumbing supply comes through the slab and is distributed to the fixtures through the attic.

The water lines should be insulated to protect from freezing.

And not many shingle nails are visible, penetrating the OSB. Nails too short, or did they use roll roofing?

It is a metal roof

This is the sort of framing I see (4' oc with trusses w/ 2x4 purlins) on horse arenas which all have metal roofs.

From JLC-
Whether or not a metal roof can be installed without a solid deck depends on if the metal roof product is structural (high strength) or architectural (lower strength). If you install without a solid deck, the walkability on the roof may be jeapordize. It is best to consult with the manufacturer or supplier of the roof product before installing.

i question the structural integrity of the 2x4's laid flat at 4' o.c. to support the roof loads. provide truss calcs and plans. i don't like the exposed paper on the insulation. i also wonder how the interior ceiling is applied to trusses at 4' o.c.

Originally Posted by Bill Anglin

It is a metal roof

That small tidbit of info makes all the difference. It is customary to install metal roofing on horizontal strapping just like that.

Originally Posted by brian schmitt

i don't like the exposed paper on the insulation. i also wonder how the interior ceiling is applied to trusses at 4' o.c.

Those are good points.
Walking that attic without stepping thru the ceiling would be a challenge as well.