This home comes with a lake... on the roof!

[Christopher Morin]

I'm looking for anyone with some experiences on intensive water damage to properties... I've already talked to my contractor and I think we've properly accounted for costs, but I don't want to end up in a very bad situation here...

The home was built in 1955, 1400 sqft, and has an almost flat roof, which is completely worn out.  The center of the roof is actually bowed inwards from the weight of a sizable amount of standing water, and the home has been vacant for upwards of 3 years.  The interior ceilings are completely ruined and heavily stained from the water damage

IF we take this on, we're going to replace the framing and trusses and have an engineer put a pitch on the home to increase ARV. I'm concerned that the weight and water may have damaged the structure of the home resulting in MASSIVE costs.

Our current cost estimates include a rework of everything from the ceiling up (10-15K)...  How bad could this get?