What can I use to seal section of concrete roof?
I would suggest a two part epoxy similar to this:

Mult-E-Poxy

This is a two part system and very durable. I have it on my garage floor. 

To prep the surface you are going to want to clean the space with muratic acid (it's nasty stuff).  The acid cleans out the pores in the concrete and etches the surface to ensure good adhesion of the paint. 

Once the acid does its thing (follow the directions on the bottle) rinse the surface with water to remove the acid and any crud it removes. 

Ensure you wait until the concrete is fully dry.

Once dry (a full day in the sun is a safe bet unless you live somewhere super humid) apply the epoxy.