Why Tear Off the Old Roof?

As the big bad wolf said to Lille Red Riding Hood: the better to see you. Yes, to see the deck is to know the deck. Rotten or weak spots cannot always be telepathically known and even though attic inspections tell of lots of thing, nothing is better than seeing. Seeing is believing. Show me.
Take plank decks. They always have knot holes and cracks and it is my belief they should always be replaced but they feel firm under foot. The darn things crack later and it is considered a warranty issue unless properly addressed. The most proper way is to demolish it and install a new fangled radiant barrier OSB deck from Techshield with edge support ( H clip).
Most roofers don't know how to do a butt-up or nested roof job. That is where you trim the edges so it looks like one roof and then use the old roof as a pattern for the application of the new roof. This leaves a tell-tale short course on the bottom. The only roof that should ever be overlaid is a three tab and you need to use the same size shingle.
We use to use the $15.00 per square for tear off to upgrade to a 25 year Timberline. These days the tear off is $30.00, usually, and so is the upgrade to the better warranty 30 year shingle, which is the same shingle.
Most insurance companies won't take a home with two roofs and most mortgage companies won't finance a home with an overlay. If the insurance takes the policy there is usually a penalty.
Reroofs are more easily hail damaged and more costly to replace so there are disincentives to do it.
In a practical world it is best to remove all roofing but there are exceptions. If a house has five roofs on it and you pull all off then I bet the windows and doors, which have been adjusted, will not work properly. Prudence dictates that if you pull off all five you put back a very heavy roof like CertainTeed Tri-laminate Presidential. even this won't stop the house from taking a deep breath and stretching Code which limits homes to two roofs. In the past a wood roof was treated like a deck so two roofs could be installed over a wood roof. Not anymore. Do that and the Dallas building official will hunt you down like a dog. Municipal building officials are law enforcement officers and can have you arrested or do it themselves. Most building officials first names is Mister.
Do yourself a favor and spring for the tear off. It doesn't cost that much and it protects your investment. Plus the roof usually looks better.

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Jon Wright Roof Dallas