Re-Roofing a Tar & Gravel roof with Asphalt Shingles

I need to re-roof our house it currently has a Tar & Gravel roof on it. The pitch is good for shingles.
Do I have to remove all the T&G roof prior to putting on the shingles or can I remove all the loose gravel and then apply the new shingles?
I'm doing this myself and just thought it would protect the roof while I slowly get it all shingled.
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It is an unusual house. Shingles generally require a minimum of 3 in 12 pitch. Hot tar can be purchased as steep asphalt and can be applied to a maximum of 3 in 12. There is a special steep which can go to 6 in 12.
Leaving the existing tar roof leaves the weight on the roof. You will be adding 235 lbs per square or more. If the framing can take it, go for it. If the roof leaks now, it serves little function.
The major problem you will have is removing the gravel well enough to get a smooth or semi smooth surface to begin the shingle work. There will probably be quite a bed of stone buried in the asphalt. One tool that can help is a power roof spudder, a gas engine spinning chains at the roof to break out the gravel. They work much better when the roof is cold. Here is an example: http://www.constructioncomplete.com/RoofingEquipment/RoofSpuddingMachines/RoofSpuddingMachines.html
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The pitch of 95% of the roof is 3/12 with one small area of almost flat roof (small porch/shed roof). I'm now thinking of stripping the T&G off and putting 1/4" Densdeck on to make the surface smooth on all the 3/12 area prior to the asphalt shingles. Then putting Metacrylics roof system on the nearly flat area. I don't see how I could use that machine on a 3/12 roof pitch for removing the old T&G roofing? So, am I left with using a roofing shovel and such to remove the old T&G? Good news is the weather has turned colder so it should be brittle.
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