How much does it cost to replace a roof?
There is no simple answer to how much the bottom line will be for your roofing project. A bid from your contractor should be based on square footage, the pitch of roof, accessibility, type of roofing material needed, 1- or 2-story home, removal of old roof (if necessary), roof permit and city license, and labor. The average cost to replace a roof runs from approximately $2.000.00 to $12,000.00 depending on the size of the roof and the quality of the workmanship and materials. The average square foot price for a residential shingle job can be between $1.80-$2.75 per square foot. (Take the square footage of your base floor plan under roof. Multiply the total by 1.3 to get a estimated square foot. Multiply this number by $1.8 to $2.75. this is the estimated range your shingle roof could cost)


Can I roof over the top of my existing roof?
                                                                            
Yes, you can. But there are some things you need to know. Especially with the Florida Weather.


What's the minimum slope I need to install asphalt shingles?
Don't go lower than 2:12 which means two inches of vertical rise for every twelve inches of horizontal run. Anything between 2:12 and 4:12, you should consider installing two layers of #15 felt underlayment <http://www.roofhelp.com/nailing2by15.htm>, reducing the exposure of the shingle - the exposure being the part of the shingle that remains visible after installation, or both. In lieu of both of these, you may want to install a self-adhering ice and water underlayment over the entire deck surface.

What kind of felt underlayment should I use?
Felt paper is a vapor barrier plus keeps the oils in the shingles from leaching out and into the wood substrate (i.e. plywood or OSB roof deck), so #15 or #30 will work for any shingle roof. For slopes less than 4/12, I recommend 2 layers of #15. Helps reduce the possibility of leaks. The felt overlaps in "shingle fashion". Slopes 4/12 to 6/12 I recommend one layer of #15. Slopes greater than 6/12 I recommend one layer of #30. The steeper the slope, the easier it is to slip and tear the felt. #30 is heavier so on steeper slopes, you don't slip and tear it as easily.

If you want some installation instructions proving that it's required, visit the following pages of three popular material manufacturers.
   
http://www.gaf.com/Products/Residential/timber_e.pdf
http://www.certainteed.com/pro/roofing/ctroof/pdfs/hall_appl.pdf
http://www.tamko.com/pdf/wrapper/heritage3040.pdf