John Kenney from Cotney Consulting Group said:
Whenever you add a layer to the deck, weight should be a concern. The only weight you can verify that your structure can support the additional weight is by having it verified by a structural engineer. Below is the weight per sqft you would be adding for some sizes used for deck overs.
OSB Panel Thickness (inches) | Approximate Weight (psf)
3/8 | 1.2
7/16 | 1.4
15/32 | 1.5
1/2 | 1.7
A ½” gap in your planking is acceptable by most manufacturers, but there is a catch. Any nail in a crack will most likely work back out and eventually through the shingles into plain air. Missing all those cracks might or might not be reasonably achievable. The trick is orienting the shingle’s nailing zone so it doesn’t overlap the no-nail zone on the roof deck (the width of the gap between boards plus 1/2 in. on either side of the crack). It would require a large nailing zone shingle where the zone is 1.5 to 1.75” ” which would allow for nail placement adjustment to miss the gaps and only fasten into the solid plank. Different manufacturers offer these types of shingles.