So I've been reframing my 115 year old "garage" and its time to put my attention towards the roof. In addition to replacing the rafters I will be installing new sheathing.

The roof is a slanted low slope roof with a dimension of 21x21 and a 1.5/12 pitch. For discussions sake I will be using 9/16 OSB on 24" OC rafters. A rubber roof will be installed over the sheathing.

So....since I am not a roofing expert by any means I figured I would ask if there are proper ways to install sheathing? A quick search on the net laid out some rules that I was not aware of:

1. Sheathing gets installed perpendicular to rafters. i.e. if the rafters are oriented North/South the sheathing goes East/West.
2. There should always be 1/8" space between all sides of the panels.
3. H-clips should be used on the 8' side of the sheathing.

Is that true or did I read some overkill article written by the plywood police? This is an unheated "garage", not a home after all.

I would appreciate your advice because my mental plan going into this was to do the exact opposite of what I laid out above. Sheathing running same direction as rafters and all edges touching.

 

The plywood/osb is stronger when it's laid crossways on the rafters/joists. A subfloor is installed the same way. Leaving a small gap between sheets allows for expansion/contraction that might cause the plywood to buckle a little. The clips give added support when you have rafters on 24" centers.

 

Well I'm glad I asked....because I was about to do the exact opposite.

While we are on the topic...I'll ask two related questions.
1. I was planning on 19/32 sheathing. Although 19/32 clips are technically available, no one seems to carry them locally....only 5/8 clips are stocked. Oddly enough, true 5/8 sheathing isn't really available locally but 19/32 is. So does everyone just use 5/8 clips with 19/32 sheathing?

2. Since I'm a total Newb with plywood clips....is there anything I need to do to cover them up aside from normal? Will they produce a bulge when used with underlayment and EPDM?

Thanks again!

 

I assume the 5/8" clips will work although the only ones I've ever used were for 1/2" They don't show on a shingle roof but I don't know if they would be an issue or not with your rubber roof. I'm just a painter - the roofers/carpenters should be along later.

I don't think anyone has made true size plywood in years ..... maybe they save money making plywood a 1/16" thinner than they used to

 

Not a pro but when I built my 26x40 garage I didn't use the H clips and I have two small bulges between the trusses. I had the clips but they kept falling out so I tossed them.

I've since learned where the sheets meet just stick a nail between them to make a small gap, nail down the sheet and remove your spacer nail. I'm quite sure you want a gap on the 4' edge also.

 

After doing plenty of internet reading (so then it must be true...right?) I've settled on 19/32 PLYWOOD instead of OSB. Even though it is almost going to cost me double, plywood is stiffer and will probably be a better long term solution.

Being a detached outdoor garage with a dirt floor (for now) and subject to high humidity, with 24" OC rafters the OSB may sag even with clips....years from now. A rubber roof would probably make this even more obvious to the naked eye. If it were 16" spacing I'd give OSB a shot, but with 24"....I'm just going to go with plywood.

So I will amend my previous question.

When roofing with 24" OC rafters and using 19/32 Plywood....do you need to use clips?

 

If it were 1/2" ply/osb the clips would be a must but the 19/32" ply is a lot stiffer so IMO [non professional] it would be ok without the clips. While plywood is better than OSB, I wouldn't be scared to use the OSB.

 

As long as you are replacing rafters I would do something to increase the pitch as well. Ponding is one of the worst things that can happen to a roof and with that low of pitch ponding is almost guaranteed.

For myself, the cost difference between OSB and plywood is not great enough to take a chance with the OSB. Add 2x4 nailers as necessary keep the edges fully supported and there would be no need for the H clips. I personally would be wary of H clips used with a rubber membrane.

 

Thanks for the feedback everyone!

So I'm learning more about rubber roofing, and I talked to the roofing buddy of mine that will be helping with the roof.

In 100% brandy new construction the rubber membrane can be applied directly to the roofing deck. The sheathing has to be installed with this in mind. Usually screws instead of nails to prevent nails from popping out....and NO H-clips. Everything has to be installed perfect.

On retrofits or re-roofs generally there is a "fiberboard" that gets installed between the sheathing and the rubber membrane. It helps hide imperfections, protects against nail heads, slight defects in the sheathing, etc. The fiberboard is more or less rigid foam board that is more durable but less insulative.

Since my garage is 115 years old, my "new" roof will still have plenty of imperfections despite my greatest efforts. Plus, I am handy but not a professional so there will undoubtedly be blemishes here and there. For this reason some type of fiberboard will be used.

 

The fiberboard between the sheathing and the rubber membrane is more to protect the membrane from damage than it is to present a better appearance to the finished roof.