SS I don't know if this is relevant to your situation, but last summer we fitted several roof windows to give us access to high levels of the roof for chimney maintenance, and at the same tome provide Observation Posts at highest elevation.
We used products like these.
http://www.sterlingbuild.co.uk/product/f...tAodEAIA7Q
They are dead easy to fit and can be set up to hinge on top or side. The ones we fitted are positioned just below the chimneys and are wide enough to take my telescopic alu ladders, so for chimney maintenance we now have reasonably safe access.
An unexpected side benefit of the skylights is that they are so high up that the local deer population are not aware of us when we are up there and regularly approach to within 45ft range. In summer the extra height lets us see them in the vegetation that normally conceals them from sight at ground level. If roof/ chimney access is not needed OPs can be improvised by simply sliding out tiles from the inside of the loft.
UKS and Midnight make good points about clearing fields of fire, but bear in mind your own fire-power. Clearing back too far beyond what your weapons can reach will hand advantage to anyone who's kit can out-reach yours. Also remember that you may need some cover to allow you to move around your property without being seen from afar.
Your idea to take out cover with the chainsaw only after collapse is sound. We have the same plan, as we want to fell trees across the disused lane beyond our house in order to completely block it off, in particular to prevent access by motor vehicles.... If we do this too soon, all that will happen is that opportunists will be attracted to the lane to strip down the felled trees for firewood....that of course would be counter productive on several levels.