A flat roofed conservatory is really a small single storey extension with a lot of glass in its walls, and it behaves like a flat roof rather than like a conservatory. That changes what matters: the fall to the outlet, the covering, the insulation position and the detail where the roof meets the house.
Conservatory flat roof replacement: why the falls matter more than the covering
A flat roof is not flat and needs a designed fall to an outlet. On a small conservatory roof there is very little depth to build that fall into, which is why so many of them pond. Water sitting on a covering is heavy, accelerates the ageing of the membrane and finds every weakness. Correcting a fall means firrings or tapered insulation rather than a better membrane, and it is the single best thing to spend money on when the roof is being replaced.
Coverings, and which suit a small roof
A one piece rubber membrane suits a small rectangular roof because there are no joints in the field. A liquid applied system suits a roof with several penetrations or an awkward shape. A high performance built up felt system remains a serious specification. Single ply is used where the roof is larger. Rooflights are common on these roofs and each one is a kerb and a detail, so the number of them changes the job more than the area does.
Insulation and where the dew point lands
A warm deck, with the insulation above the structural deck and the waterproofing over it, is the reliable modern build up because it keeps the deck warm and avoids a cold void that nobody can inspect. It raises the roof level, which eats into the height available at the upstands and at the house abutment, and on a low conservatory that can be the binding constraint. Approved Document L sets the thermal expectation when a roof is renovated.
The abutment is the usual leak
Where the flat roof meets the house wall there is a flashing and frequently a shallow box gutter, and that junction causes most of the leaks reported on flat roofed conservatories. It is shallow, easy to block, and often has a small outlet. A replacement that renews the field of the roof and leaves the abutment alone has left the problem in place, so ask specifically what is being done there.
Questions people ask about conservatory with flat roof
Why does my flat roof conservatory pond?
Because it has too little fall to its outlet, either by original design or because the structure has moved. A new covering over the same falls will pond in exactly the same place.
What is the best covering for a small flat conservatory roof?
A one piece rubber membrane is a common and sensible choice on a small rectangle. Where there are several rooflights or awkward upstands, a liquid system detailed in place often suits better.
Can I add rooflights when the roof is replaced?
That is the natural moment to do it, since the deck is open. Each one is a kerb and a weathering detail, so agree the number and positions before work starts.