rafter spacing and sheathing

[Bwagner]

We are putting a roof on a home. When we removed the origional roof the raters have odd spacing. they are 50 inches apart. on top of the rafters they have horizontal 2x6's that are nailed on every two feet. They are true trusses not hand framed. We are going to sheet the roof and put on a new compostion roof. We sheet houses with skip sheathing all the time, but i want to make sure on how to do this. From looking at the code tables it looks like we may need to put two layers of t and g (3/4")

is that correct? or can we just sheet it and clip it?

thanks,
Matt Swanson

[Jerry Peck - Codeman]

Hi Matt,

We are putting a roof on a home. When we removed the origional roof the raters have odd spacing. they are 50 inches apart.

That tells me the trusses are not the typical engineered trusses designed for spacing of 24 inches on center.

Those trusses would have to have been specifically designed for that spacing and the loads (dead load and live load) considered within the design parameters. Those trusses would, basically speaking, have been designed for twice the loads of the typical 24 inches on center trusses - each truss carries the load of 48 inches plus (50 inches) which is twice the load carried on 24 inch spaced trusses.

My recommendation is to get a structural engineer, starting with the truss manufacturer's engineer if the truss manufacturer is known (there should be a stamp or label on the trusses with the manufacturer's name/logo/identifier), then going with another truss engineer if the manufacturer is no longer in business or is not known, or a structural engineer.

on top of the rafters they have horizontal 2x6's that are nailed on every two feet.

Presuming that those are perpendicular across the trusses, those are nailers/supports for the plywood/OSB sheathing which is intended to be installed on 24 inch on center supports.

Whenever something which is "highly unusual" is found (and trusses at 50 inches on center are "highly unusual", although not unheard of, as engineered trusses can be designed to meet almost any load and spacing the designer wants), it is best to verify that it is as it was intended - such as ... you do not want to install a roof on those trusses only to find out later that those trusses were designed for 24 inch on center spacing and were installed at that 50 inch spacing.

I definitely would get a structural engineer to review that, or review the permit file and the original approved plans at the building department to verify that was what was intended and approved at the time of construction ... before doing any work on that roof.

It may well turn out to be "as intended and as designed" ... but it might not too.

[Jerry Peck - Codeman]

A correction/clarification:

You said "rafters" and I used the term "trusses" - regardless, 50 inch spacing is unusual ... especially with "rafters".

Now, if you are referring to heavy timber construction or old post and beam construction, then a spacing that wide or wider would not be uncommon (albeit 50 inch spacing is odd as it is not an evenly multiple number to work with - 100 inches (is not 8 feet), 150 inches (is not 12 feet), etc, and spacing are typically based on a larger number having been divided up for an easier working solution.

So that 50 inch "rafter" spacing is odd, and, yes, I would still recommend a structural engineer, just drop the 'truss engineer' part.