The short answer
A solid conservatory roof needs its moisture managed, and the right method depends on how it is built. Some build-ups are cold roofs with an air path under the covering, while others are sealed warm roofs with a vapour layer on the room side. The Planning Portal expects a new conservatory roof to be properly ventilated, so ask for the maker's detail before work starts.
A solid roof traps moisture in a way glass never did. The build-up decides whether it needs air paths or a sealed design.
Why a solid roof needs moisture control
A glass or polycarbonate roof lets the room shed its damp in plain sight, and condensation shows up as drips on the glazing. Replace it with a solid roof and the moisture has somewhere new to go: into the layers of the roof itself. A conservatory is also a damp room by habit, with plants, wet washing and people drinking tea in it all year round, so the load of moisture is real.
What is the risk inside a solid roof?
Warm, damp air from the room finds its way into the insulation and meets a cold surface, where it turns to water. Over a few winters that wets the insulation and the timber around it, and the damage stays hidden until stains or a musty smell appear. This is why ventilation matters in the roof, not just in the room.
The Planning Portal's guidance on conservatory roofs puts ventilation alongside structure and energy efficiency as things a new roof must get right, so your building control body will want to know how the moisture is handled. In Taunton that body is Somerset Building Control, on 0300 303 7790. Ask them before the ceiling is closed, not after.
What BS 5250 asks of a roof
BS 5250 is the British Standard that guides how designers deal with moisture in buildings. BSI lists the 2021 edition as current, with a review under way. It guides designers; it is not a recipe that fits every roof.
How much air does a cold roof need?
Marley gives figures for tiled cold roofs laid with a non-breathable HR underlay: an opening of 25mm along the eaves for pitches of 15 degrees or less, and 10mm for steeper pitches. Marley adds that where a breather underlay sits above a well-sealed ceiling, the 3mm gap in ordinary eaves can do the job, if a third party has certified the product. Those numbers are for standard pitched tile roofs.
A conservatory roof system comes with tested details from its own maker. Treat the standard as the reason to ask, and the manufacturer's data sheet as the answer. Nobody can fix a ventilation problem on site if the system never allowed for one, so the questions belong at the choosing stage, when changing course costs nothing. File the data sheet with your deeds and bills; a later buyer's surveyor may ask to see it.
Cold roof, warm roof and sealed systems
There are two broad ways to build a solid conservatory roof, and each handles moisture differently.
What is the difference between a cold roof and a warm roof?
In a cold roof, the insulation lies at ceiling level and a ventilated void sits above it, so damp air is carried off through gaps at the eaves and the top of the roof. A warm roof turns this round: the insulation lies above the structure, the timbers stay at room temperature, and a vapour control layer on the room side stops damp air entering the build-up. Many ready-made solid conservatory roofs are designed as one or the other, with a sealed lining inside and the covering on battens outside.
Some systems lean on a membrane that lets moisture pass while stopping water, and these still need the maker's rules followed to the letter. You cannot lift parts from one design and use them with another. A vent in the wrong place, or a membrane laid the wrong way up, can turn a dry roof damp. The guide to conservatory roof condensation shows the same physics at work on a glazed roof, and why it matters on a solid one.
What to ask before a solid roof goes on
You do not need to master the physics, only to ask good questions and see the answers on paper.
Which questions should you put to a roofer?
Ask which build-up is being fitted and whether it is a cold or warm design. Ask for the maker's installation guide and look at where the vents, membranes and vapour layer sit. Ask who deals with building control, and whether the roof will be signed off. Ask what happens to the ceiling, because a downlight punched through a sealed layer can let damp air into the roof.
We read the existing frame first, give you photographs of the real roof, and quote one written price. The ventilation detail comes from the system we are fitting, and the paperwork follows the survey. If you are weighing a solid roof against an insulating lining, read our answer on insulating a conservatory roof, and see the conservatory roofs page for the full set of choices. For a first step, tell us how big the room is and what covers it now.
More on solid roof ventilation
Should a tiled conservatory roof have vents?
It depends on the system. Some designs use eaves and ridge vents to ventilate a cold roof, while others are warm roofs that are sealed and need none. Never add vents or block them without the maker's detail. Ask your roofer to hand over the data sheet for the exact roof being fitted.
Can a solid roof cause damp in the conservatory?
A poorly built one can. Warm damp air that reaches cold insulation condenses inside the roof, where nobody sees it. A vapour control layer, correct ventilation and good detailing at the wall and eaves reduce the risk. Building control looks at the build-up before the ceiling is closed.
The full picture: Conservatory roofs in Taunton ยท More short answers