roof water proofing

huangyhg

roof water proofing
a flat roof of building is leaking. buidling is old so do not want to place some heavy water proofing material on it.
can you recommend some good material. also slab is depressed at middle (may be 3 inches). is there some thing we can place thicker in middle and thins out to ends to mock a sloping roof.
have you considered installing a new steel sloped roof on above it the flat roof? i am seeing this more often, the patching seems to only work temporarily on flat roofs.
plainsboro-
what caused the low spot?
if you can tear off the old roof, consider using tapered insulation or a non-structural self leveling compound to fill in the low spot.   
if the roof structure is underdesigned there are several types of unballasted single ply membranes available that you could use to re-roof and keep your dead load down.
what kind of roof deck do you have?  does the existing roof pitch and drain properly, other than the low spot?
regards.
sarnafil is a good long life covering, but you will have to address the low spot. assuming that you find the roof at that point structurally sound, you may want to add an additional roof drain at that point.
when you have lemons, make lemonade.
plainsboro...as said, check the reason for the middle deflection.  if structurally ok, then you can use lightweight insulating concrete (lwic) or tapered insulation (more expensive and requires more installation skill) to provide the slope you want, then put most any type of membrane on top of that (remove existing membrane).  my preference is a modified bitumen membrane with granular surfacing.
lwic and membrane will weigh about 8 to 10 psf for reasonable thicknesses, but can easily jump up to 12 to 14 psf if the lwic gets too thick.  tapered insulation and membrane will come in at 6 to 8 psf.