Calling all builders!

[Bill McNiel]

Install the sheets horizontally with an 1/8" gap.. The main reason for sheathing a building is shear resistance. That is the resolution of horizontal forces, mainly seismic but wind as well. OSB and plywood will swell with moisture absorption.

[Jeff Caulk]

OSB should be 1/8" undersize.
You can use H clips on the edges to help get the 1/8" offset without taking time to align and measure.
There should be two different styles of sheathing.
One has the primary strength axis running along the long (8') direction, the other running along the narrow (4') direction.
Here they are edge painted orange and green to distinguish them from one another.
The strength axis arrows are printed on the sheet.
We use the orange long oriented sheathing installed vertically on the walls and the green short oriented sheathing installed horizontally on the roof.
Definitely use H clips on the roof, you need the strength between sheets to lock them together.

[Dennis McCullen]

Update - I began framing about 6 weeks ago and I am about to install the sheathing on the roof. I bought a GP thermostat with foil-facing in 7/16". The guys at the building materials store have "20 yrs experience" and never heard of "sized for spacing" which is printed on many osb sheets. I measured these and found them to be 1/16" shy of 4 ft and also of 8 ft. The material I bought is a full 48" by 96" and has the recommended spacing of 1/8" printed on the sheet. I will install horizontally on the roof and have it covered with 4 sheets, therefore 3 gaps between them. I can start the first sheet 1/8" shy of the middle, next sheet hits the middle, the third sheet is 1/8" beyond the middle so I'll be fine. I wanted this particular product because of the attached foil (radiant barrier) for the roof but I will definitely try to find a sized for spacing OSB for the walls where the OSB will be installed vertically (twice as many gaps between sheets). I believe this is not a case of "overthinking" by me but perhaps underthinking by the manufacturers who seal the edges of the OSB sheets and then print on the sheet that you should space it 1/8". A quick field cut solves the problem of not hitting the rafter but exposes an otherwise-sealed edge now that it has been cut. I only build for myself, but have no call-backs in my history!