Flat roof screeds laid to falls that actually fall, membranes taken up the parapet and dressed under the coping, and pitched roofs in steel or timber with insulation where the client wants a room under it.
Every leak we have ever been called back to look at, on our work or anybody else's, was a detail rather than a material: an upstand too short, a pipe penetration sealed with the wrong thing, a gutter with a fall of nothing. So we photograph every detail before it is covered and the photographs go with the handover file.
We also do waterproofing to bathrooms and planters, which are the two places in a Maldivian building most likely to ruin a ceiling below.