Water – How to inspect and repair a roof with moss and other greenery on it

The best way I've found to check for damage isn't the way you want to hear: pull everything out, open it all up, and expose any areas that could have been damaged. Faced with that, most of us ignore the problem until there is a visible sign (rotting wood or water damaged drywall). But that's not to say this is the right choice, since mold and mildew will spread and make the problem worse.

If the problem is behind a few sheets of drywall, the best option may be to simply pull them down and replace them with a mold and mildew proof board. Do a thorough inspection for damage while the drywall is out.

The probe sounds like a wood moister tester. A high reading is bad, but a low reading doesn't mean you're ok (the water could have gone somewhere else). If you can't access the affected area from the attic or crawl space, then you may want to see if you can rent a borescope, drill some holes near where you think the water could have gotten in, and see what you can find.

The bleach mix will be dilute enough that it will not be of harm. I've used it myself at least a few times to kill off algae that has accumulated.

You can also use things like Oxy-Boost and Stain Solver, a bleach substitute which I believe contains some hydrogen peroxide. It will also work against light moss and algae, yet perhaps be less harmful to plant life underneath.

More importantly is to fix the source of the moss. Roofs that gather moss tend to be too shaded, under big trees that stop it from drying out. You may need to open things up, just by a bit, but enough to let it dry out. Yes, those trees are a natural source of air conditioning, but trees tend to grow. So trim off branches that overhang the roof. Cut a few others out to let the sun in.

For roofs that have only a bit of green, you may find it is sufficient to add zinc strips along the top. The zinc leaches out, keeping the roof clean of moss accumulation. There are also shingles with a moss inhibiting additive that prevents moss from growing. Copper strips should also work, and copper flashing is easily available. I've even read that you can glue (use silicone) copper pennies just under the edge of the top row of shingles. They are mainly zinc anyway.

If the moss had accumulated heavily, the problem is your roof likely needs repair, as it will have been too wet for too long. The shingles will need to be stripped off and replaced, but probably the roof decking will be soft and need some repair too. This can best be judged by a professional who will inspect the roof carefully.