Traditional roofing “felt” is actually a bitumen compound reinforced with hessian or some other fibre. It comes in a bewildering variety of types, of greatly varying cost and quality, so it is not surprising that an unsuitable grade of felt may get used on a cheap job, keep the rain out for a few years, but be unable to withstand the hydraulic pressure of ponding water. In these circumstances the natural human temptation is to get the leak patched up, but this is invariably a bad move and will result in more expensive repair work later on. Some specialist firms now offer to cover your leaking flat roof completely with fibre-glass or glass-reinforced polyester (GRP). Again, this is a tempting option but almost certainly the wrong one; the fault that was causing the roof to leak in the first place was not the pinprick hole in the felt but the sagging and ponding etc, which the new super cover-up material will do nothing to fix.
In fact, like many hi-tech wonder products, these instant roof-repair systems can sometimes do more harm than good. A flat roof has to perform a number of functions – structural support, keeping the rain out, keeping the heat in – and sticking a new impermeable coating on top, without simultaneously adjusting the ventilation and the position of the insulation, can lead to condensation in the room below. The least damaging effect of this will be a spot of black mould growth on the ceiling; the worst will be condensation on the timber roof joists themselves, leading to wood rot and possible collapse. Timber flat roofs need ventilation between the joists to prevent this.
The best modern way of re-covering a flat roof is to use a single-ply polymer membrane, with extruded polystyrene roof boards on top. These can be weighted down with paving slabs to provide a convenient seating area if you have access. Having the insulation on top of the membrane means that the whole roof structure is kept warm, so avoiding the usual condensation problems.