Fault diagnosis

Roof Condensation Versus a Leak: Telling Them Apart

Condensation in a roof void is warm moist household air reaching a cold surface and depositing its moisture there, and it is corrected by ventilation and vapour control rather than by anything done to the covering. Compare the pattern on your own slopes against roof condensation, then book a survey to settle it. What it covers: Interstitial condensation · Roof void condensation · Leak.

Interstitial condensation is moisture depositing on a cold surface inside the construction rather than water arriving from outside. Telling it from a leak is the whole diagnosis, and it usually costs nothing to fix.

Condensation Uniform droplets across the underside of the underlay, worst in cold still weather.
A leak Follows rainfall, localised to one point, and dries between events.

An extract fan ducted into the void: It discharges moisture straight into the roof space. Bathing, cooking, drying laundry indoors and unvented tumble dryers are the sources.
An unsealed loft hatch: A direct moisture path from the house into the void.
Insulation pushed into the eaves: It blocks the inlet and stops the cross-flow the void depends on.
Impermeable 1F felt above: Bitumen underlay traps vapour that a breathable membrane would release.

Where the cause is a fan, a hatch or blocked eaves, the remedy is not a roofing job and we will say so.

How do you tell condensation from a leak?

Condensation deposits uniform droplets across the whole underside of the underlay in cold still weather. A leak wets one localised area and tracks rainfall. Distribution and timing separate them without any need to open the roof.

Interstitial condensation is moisture depositing on a cold surface inside the construction rather than on a room surface. The leak has a differentiation of follows rainfall, localised to one point, dries between events.

What causes condensation in a roof void?

Four things drive void condensation: household moisture rising through gaps and an unsealed loft hatch, extract fans ducted into the void, blocked eaves stopping cross-flow, and impermeable bitumen underlay trapping vapour that cannot escape.

An extract fan is ducted into the void rather than through the roof, discharging moisture directly above the ceiling.

Household moisture source enumeration: bathing, cooking, drying laundry indoors, unvented tumble dryers

Why did it start after the loft was insulated?

Insulation laid over the eaves blocks the low-level air inlet that the void depends on. The roof space then becomes colder and less ventilated at the same time, which is precisely the condition condensation needs.

Insulation at eaves is insulation pushed into the eaves blocks the inlet and stops cross-flow. Impermeable 1F felt traps vapour that a breathable membrane would release. The remedy of ventilation is continuous low-level inlet plus high-level outlet restoring cross-flow.

Does condensation damage a roof?

Yes, when it persists. Sustained high humidity wets rafter feet, battens and the underlay itself, initiating decay and supporting mould growth. Occasional droplets on a few cold mornings do not cause lasting harm.

Prolonged void condensation wets rafter feet and battens and initiates rot.

Warm moist air rises into the void meets a cold underlay and deposits a moisture.

How is it fixed?

Clear the eaves inlets, fit high-level ventilation to restore cross-flow, duct extract fans through the roof rather than into it, and seal the loft hatch. Replacing impermeable underlay is considered only when the covering is being renewed anyway.

An insulation blocking the eaves stops a cross-flow and raises a void humidity.
Impermeable bitumen felt traps a vapour that a breathable membrane releases.
Sustained void condensation wets rafter feet and initiates decay.

Some of this is not a roofing job.

What to change indoors first

Condensation wets the whole underside evenly; a leak wets one place. The distribution is diagnostic and costs nothing to observe. Laundry dried inside, an unvented dryer, or a fan ducted into the void — none of which a roofer can fix by touching the roof.

Getting the void assessed

Topping up loft insulation over the eaves blocks the inlet, and a well-intentioned energy improvement creates the damp. Impermeable 1F bitumen underlay traps vapour that a modern breathable membrane would pass, so pre-2000 roofs are disproportionately affected.

Interstitial condensation, and 6 more

interstitial condensation
roof void
underlay
1F bitumen felt
breathable membrane
cross-ventilation
vapour control layer

7 symptoms route to their own diagnosis: A damp patch on an upstairs ceiling, Water coming in with no visible damage outside, Slates or tiles slipped out of line, Water spilling over the gutter, Damp in the loft with no rain, A ridge line that dips in the middle and Moss across the slope.

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