Flat Roofing information needed

I have a flat roof with 1 x 8 boards and covered with tar and gravel. I just got a quote from a professional roofer to remove the tar and gravel and re-roof with Dibiten.

Since the 1x8 boards have spaces (½ to 3/4 inches between them) should a new surface such as plywood be installed before the Dibiten?

The online specs for Dibiten state "all exposed surface products must be coated" and then "Periodic maintenance and recoating"

Does this mean that I will need to regularly coat and maintain the Dibiten roof?

Finally, the roofer said that he would guaranty the new roof job for six years. Why does the Dibiten web site say that the guaranty is "up to 20years" for "labor and material?"

 

Dibiten authorized installers can get the extendedd warranty.
An approved insulation board can be used to overlay the planks without the expense of a layer of plywood.
Modified roofs need periodic maintanance. Onless a granulated cap ply is also installed.
Was he going with a single, double or 3 ply roof?