What pandamonk said. Bottom line is this though. "Soundproof" is a misnomer. There is no such thing. Only degrees of "transmission loss". Additionally, it makes no sense to modify a building envelope to increase the transmission loss to achieve isolation of 110db sounds at 20' from the building, when the object of your desired isolation is 100' away and resides in a concrete bunker.

Distance and THEIR existing building envelope transmission loss is your friend.
For GUARANTEED results, first off, you would need to define the SPL of the sound you are producing, and then the SPL of that sound reaching your target, and measure the distance between.( Sound pressure decreases by 6db for every doubling of the distance)
Do this by measuring the SPL INSIDE your existing structure(while drums are played), outside at 5' away,and then at the boundary of the target envelope. This will tell you HOW MUCH you have to improve the transmission loss of your existing building, which can be an futile exercise in physics unless you understand HOW to interpret the data in terms of actual effective construction (areas of transmission loss improvement). Usually, this can only be achieved by trained acousticians.
Thats why, for most HR and DIY musicians, effective "soundproofing" is a lesson in guesswork and trial and error. For the most part, BRUTE FORCE from the beginning is the method of choice....ie....seal the weak links, add LOTS of MASS(decoupled mass if budget permits), usually defined by 2 layers of well insulated 5/8" drywall(depends on existing transmission loss), and detail articulated door/jamb/seal assembly improvement(i.e...SOLID CORE Doors(or two) with caulked jambs/thresholds/casings with good weatherstriping seals. Sound propagation from AIRLEAKS is the point here.) In essence, you AIRPROOF the building interior envelope from the outside world, while adding enough mass to keep transmission loss through the building envelope assemblies at a minimum. Thats why pandamonk addressed your roof and window.
However, once you SEAL an enclosed space, you effecitively have AIRPROOFED it, and you know those musicians...they like to breath.

...ie..you have to VENTILATE the enclosed space while keeping sound from propagating through the "system" to the outside world. Thats a whole 'nuther ballgame.
To FULLY address your project, you really need to tell us the complete circumstances . ie...distance to target, what the target of isolation is, and all the details of your existing building...type of music, how hard you play, etc etc.
fitZ