First, in regard to your own insurance, you should be covered so I would start some vigorous complaining. Go back to your broker, review the basics of your coverage, and get the broker to go to the insurance company for you. Also get on the phone with your insurance company to find a supervisor who knows condos. Google DC insurance commissioner and follow the advice on the official website for filing a complaint.
Please post the exact wording from your bylaws (or declaration) that says the association is not responsible for water damage in units.
As I understand it, the association is not responsible for damage that results from bad venting / slab as long as they did not know there was a problem in your unit or similar units and they could not reasonably be expected to know of the problem. They are now responsible for future damage, assuming that is actually the cause.
Here are some of the points in the bylaw that might be associated with my issue:
1. The association shall be responsible for the maintenance, repair and replacement of the following: (1) The common elements, whether located inside or outside of the units
2. The Association shall not be liable for any failure of water supply or other services to
be obtained by the Association or paid for as a Common Expense or for injury or damage to person
or property caused by the elements or resulting from electricity, water, snow or ice which may leak
or flow from any portion of the Common Elements or from any wire, pipe, drain, conduit, appliance
or equipment.
3. Each Residential Unit can be individually utilized and has its own exit to the common
elements of the Condominium. Each Residential Unit consists of the space enclosed horizontally by
the innermost surface of the drywall which comprises the perimeter walls enclosing the Unit and
vertically by the lowest surface of the lowest wooden floor or subfloor in the Unit, and by the
uppermost unexposed surface of the lath or drywall ceilings in the Unit. (note, I don't think concrete slab is "subfloor," right?)
4. The General Common Elements are those poliions of the Common Elements used by all of the unit owners in general. They consist of all Common Elements other than Limited Common Elements, and include, without limitation (i) the
Land; (ii) the foundations, roof (not including roof decks, if any), slabs, floors, ceilings, perimeter
walls, structural interior walls.....
Thanks for your advice!