The short answer: one question decides most of it: is the deck under the membrane still dry? A dry roof with local faults can be repaired or overlaid; a roof with saturated insulation or a damaged deck needs stripping back, whatever its surface looks like. Everything else is refinement of that fork.
What is the dry-deck test?
It is the difference between a roof with problems and a problem roof. Membranes fail at points: a split here, a tired lap there. Decks and insulation fail as a body: once water has spread beneath the waterproofing, it sits in the build-up, feeding damp, rot and further failure from below. Point faults can be fixed at the point. A wet build-up cannot be fixed from above, only sealed in, which is no fix at all.
Moisture below rarely shows on the surface, which is why the test is done with instruments and, where needed, core samples, not with a glance and a quote.
When does a localised repair win?
When the faults are few, findable and surrounded by sound material: an isolated split, one failed detail, storm damage on an otherwise healthy roof. In those cases a targeted repair restores the roof for a small fraction of any alternative, and choosing more would be spending for its own sake. The proviso is honesty about frequency: a roof needing its third repair in two years is telling you something.
When does an overlay win?
When the surface has aged past repairing but the test says dry underneath. The old membrane becomes the substrate, a new system goes over it, and you skip the strip-off, the skips and much of the disruption. It suits roofs that aged honestly: ultraviolet wear, surface crazing, general tiredness without underlying moisture. Think of it as a one-time renewal card each roof holds; playing it on a wet roof wastes the card and the money.
When is strip and renew the only honest answer?
When moisture has colonised the build-up, when the deck itself is damaged or rotten, when previous overlays have already stacked coverings too high, or when failure is so widespread that patching would never end. Stripping costs most and disrupts most, and there are situations where recommending anything else is either ignorance or salesmanship.
What are the two false economies?
They mirror each other. The first is patching a saturated roof: every patch works briefly, because the real problem is under the membrane, not in it, and the money spent patching is subtracted from the renewal that is coming anyway. The second is stripping a sound roof: a dry, bonded covering is an asset to build on, and ripping it out to start again buys nothing the overlay would not have delivered cheaper. One error wastes money slowly, the other all at once.
What if only part of the roof is wet?
Then only part of the roof is finished, and the decision can zone itself accordingly. Moisture mapping often shows saturation confined to one bay or the run below a failed detail, with the rest of the build-up testing dry; there, the honest scope strips and rebuilds the wet zone while the sound majority is repaired or overlaid as its own case. The join between old and new work becomes a designed detail rather than an accident. What zoning cannot survive is wishful boundaries: the wet area is where the readings say it ends, not where the budget hoped it would.
Does the age of the roof matter at all?
Less than people assume, and never on its own. Age is a prompt to investigate, not a verdict: a twenty-year roof that was detailed well and kept clear of standing water can test dry and carry on, while a seven-year roof installed badly over a leaky vapour layer can already be saturated. Treating birthdays as evidence leads to both errors at once, condemning sound roofs and trusting failed ones.
Where age genuinely tips the scales is at the margins of a repair decision. Spending on a precise fix makes obvious sense on a roof with years of demonstrable life ahead; the same spend on a covering visibly at the end of everything else becomes a bridge payment, worth making only with eyes open and a renewal plan behind it.
How does a survey settle it?
By replacing opinion with readings. A proper condition survey gathers the four facts the decision runs on:
- Moisture: is the build-up dry, damp in patches, or saturated?
- Bond: is the existing covering still fixed to what is beneath it?
- Structure: can the deck and joists carry what is proposed?
- Details: are the upstands, edges and outlets rescuable or spent?
Each is photographed and put in writing. From there the repair, overlay or renew decision mostly makes itself, and you can see why each option was or was not offered. If a contractor reaches a verdict without any of that, remember whose interests guesswork usually serves.
And keep the report. Whichever option you choose, the survey that justified it becomes the baseline the next decision is measured against, which is how a roof stops being a recurring argument and becomes a managed asset.