creating attic access

My house was built about 1950 and one thing that always struck me as odd (well, since I noticed it anyway) was that there was no access to the attic. While doing some roof repair I took the opportunity to look down through a vent hole and I figured out why. When this house was originally built, it appears that it had a flat roof. There is a layer of whatever gravel/rock aggregate they used for flat roof covering at that time on the floor of the "attic" space. At some point another cottage style roof was built on top of that one but no access to the created attic was put in.
Now I have to take down the chimney and would like to insulate the attic as well so I have to figure out a way to get at it. Since there are no gables I can't go through from the side and going through the roof wouldn't be a permanent solution, so I have to come up through the ceiling.
To make a long story slightly shorter, does anyone know what that rock aggregate is made of, how thick it might be, and what I might use to cut through it? The ceiling underneath it looks like 2x6 joists and then on top of that planks about 6 inches wide and I would guess a half inch thick if they're anything like the planking in the rest of the place, and then on top of that I assume is the flat roof covering.
Possible to get through from underneath?
thanks gm
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Nothing a Sawzall with a long blade couldn't handle. If it scares you get a carpenter to cut the hole and frame it out so you can hang a pull-down attic stair.

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One thing I'd check first is where the wiring goes. If it goes through the ceiling or sometimes strung across joists, you want to be careful not to cut into it. I have a similar situation and will be making an opening. First I'll cut through the drywall with a keyhole saw by hand and then after making sure there aren't any wires, I'll have at it with the Sawzall.
Bob

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thanks all, sawzall did the trick. Messy job but it got it done.

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If it was my roof, I'd cut a hole between the joists in a closet or other out of the way place and then decide whether it's worth the trouble to do what you want. Make it large enough to fit through and you should be able to pull yourself up and get a much better idea of what you are working with. I definitely wouldn't try to go through the roof as roof repairs are more troublesome than patching a ceiling.
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