Can you define the roof of a building on a hole as OB, but if you disc lands under the building it is not OB and you can get a 1m relief from the building without any extra strokes?

Are they declaring the roof OB or is the 2m rule in effect for that roof? As far as making the area under a no-stroke OB area, while the rules don't explicitly allow defining areas that way, special conditions are used for this all of the time.

You could define the area under the roof as a "no penalty" buncr or casual relief area with relief back on the line of play, and landing on the roof could simply be a 2m penalty.

The rules Q&A makes it clear that where there are overlapping playing surfaces one can be o.b. and the other not.

If you wanted to play strictly by the rules, you should make under a casual relief area. See, 803.05C

The rules Q&A makes it clear that where there are overlapping playing surfaces one can be o.b. and the other not.
However, you don't get the 1m relief outside the "shadow" of the OB surface (as desired) when landing in the stacked IB area if using the stacked OB option.

I think the 1m is getting in the way, what are the options available?

1. Declare nothing about the building.
All discs on or below the building play immediately behind the large solid obstacle of the building with no penalty.
2. 2m is in effect nothing else.
All discs on the roof are penalized for above 2m and all discs play immediately behind the building.
3. Declare the building OB.
All discs on or below the building are penalized and get 1m relief from the edge of the building.
4. Declare the roof OB.
Same as 2 but the player on the roof now has the option of throwing from the previous lie with penalty.
5. Declare the roof OB and under the building casual.
Same as 4. (note: that up to 5m to clear the obstacle does not mean you can always take 5m)
6. Use 2m and declare under the building a special play area.
Discs on top follow 2 and discs below are played in accordance with special area definition.

Problem:
I am not sure that a roof can be declared OB since only playing surfaces can be declared OB.
Declaring the roof OB adds the option of throwing from previous lie with penalty.
There is no 1m relief from the side of the building without OB unless the special area rules are used, and then a drop zone is probably a better option.

Problem:
I am not sure that a roof can be declared OB since only playing surfaces can be declared OB.

Solution: Declare the roof to be a playing surface. Then declare it o.b.! :D

well, if a fenced-in porch of a building can be a playing surface (see Atlanta), then a roof certainly can be.

But that's stupid.

At Prairie Center for Worlds, the restroom building just off 18 was OB, painted at the bottom of the building. So, the roof was OB, EXCEPT for the eaves that overhung. The eaves were 2m ruled, and painted with a dotted line under the edge of the eave. Maybe that's not an official arrangement but it certainly worked, was easy to understand, and didn't come into play at all during the tournament.

But it's happened during leagues there. So, it was worth it.