I always redo the chimney flashing. Chimneys are the most common source of leaks, and the only way I know to make sure it’s done right is to pull up the old metal and redo it afresh. I don’t rely on the older flashing from earlier installations.
I apply copious amounts of ice and water shield. This is a rubberized membrane that adheres directly to the wood decking. It acts as a barrier preventing water from reaching the wood. I apply it in all areas of the roof that could be trouble later. This includes valleys, eaves (along the gutters where ice damns build up in winter), around chimneys, skylights, and other roof penetrations.
Once the shingles are removed, I inspect the wood decking for compromised wood. All wood that is rotten or badly warped is replaced.
Debris often accumulates in gutters during chimney installation. I make sure all the gutters on your house are cleaned once the roof is installed.
A good roof installation includes a good clean-up. I make sure all the loose debris is removed from the yard and the lawn is swept with magnetic rakes for nails. And we tarp over shrubbery on down slope areas to protect it from sliding shingles.
We use the best asphalt shingle manufacturers. Our preferred brand is Landmark by Certainteed, but if you prefer some other brand (e.g., Owens-Corning, GAF, IKO), there is no extra cost to use them.
And finally, and this is the most important consideration, I will personally spend most of the day on site while the job is being done. That way, I can make sure everything is done the way it is supposed to be done.