There is water running down the inside of a sloped ceiling, or black spotting along the line where the slope meets the low wall at the edge of the room, or a smell of damp coming from behind it. The conversion was signed off months ago. The roof is not leaking, or at least nobody can find where.
This is the most common thing to go wrong in a converted attic after the builders have gone, and it almost always starts the same way: the problem appeared after the insulation went in. That is not a coincidence, and it is not bad luck. It is what happens when a roof is changed from one kind of construction into another and the ventilation is not changed with it.
This guide is built on one document - Technical Guidance Document F - Ventilation (2019), the guidance to Part F of the Building Regulations - and it exists to answer a single question. Is this a defect your builder has to come back and fix, or is it something you are causing by how the room is used? Those have different answers, different costs and different conversations, and almost nothing written about attic condensation tells you which one you are looking at.
Which edition we are quoting, and why we are saying so
Every paragraph number on this page is from TGD F 2019, which applies to works commencing on or after 1 November 2019 and is the edition published as current on gov.ie. There is also a TGD F 2025 public consultation draft in circulation. It is a draft, its cover is marked for consultation, and we do not quote a single figure from it. We are flagging it because Section 2 is numbered identically in both documents - so a citation taken from the draft looks exactly like a citation from the operative guidance, and there is nothing in the paragraph number to tell you apart. Check the cover page, not the numbering.
Telling condensation from a leak, before you spend anything
Do this first. A leak and condensation produce similar-looking stains and completely different bills, and the wrong diagnosis sends a roofer onto your roof to find nothing.
The useful distinction is when and where, not what it looks like.
It follows the weather. It appears during or shortly after rain, and worst in driven rain from one direction. It is local - one patch, one run, usually traceable up-slope to a single point such as a rooflight flashing, a valley, a chimney abutment or a slipped slate. It leaves a hard-edged tidemark, because the water arrived, spread and stopped. It happens in mild wet weather as readily as in cold weather.
One more observation worth making before anyone opens anything up. Condensation in a converted attic is usually seasonal and reversible. If the black spotting disappears in July and returns in November, that is a temperature story, not a hole in your roof.
Why it started after the insulation went in
Before the conversion your roof was a cold roof. The insulation sat on the attic floor, the whole roof space above it was outside the heated house, and it was ventilated at the eaves. Warm moist air that leaked up out of the house met a cold roof space that was constantly being flushed with outside air. Most of the time that was enough.
A conversion moves the insulated line. Over the footprint of the new room the insulation goes up into the slope, and the space underneath comes inside the heated house. That converted part of the roof is now a warm roof. Three things change at once:
The moisture source moved up
There are now people sleeping, breathing, showering and drying clothes at the top of the house rather than below it. The wettest air in the building is now in direct contact with the roof.
The ventilated space got much smaller
The generous, freely-ventilated attic void is gone. What is left between the insulation and the slates is a narrow gap, if there is one at all - and a narrow gap only works if it was deliberately built and deliberately vented.
The airflow path got cut in two
A cold roof is ventilated by air crossing from one eaves to the opposite eaves. Insulating the slope puts a wall across the middle of that path. Air can no longer cross the roof, which is exactly why a converted slope needs venting at the ridge as well as at the eaves.
The distinction the rest of this page depends on
After a conversion your roof is two different constructions, not one. The converted slope is a warm roof. The space behind the low walls at the edges of the room, and the space above the flat part of the ceiling, is still a cold roof. They have different rules, different figures and different failure modes, and the boundary between them is where most real problems sit. Any advice that treats your roof as one thing is not going to help you.
Which material was used in the slope changes how forgiving that build-up is, and it is a decision with consequences well beyond condensation. We set it out in our comparison of spray foam and rigid board.
What Part F actually requires
Part F of the Building Regulations has two halves. One is ventilation for the people in the room. The other is the one that matters here, and it is a requirement in its own right - F2:
"Adequate provision shall be made to prevent excessive condensation in a roof or in a roof void above an insulated ceiling."
Note what F2 covers: a roof and a roof void above an insulated ceiling. Both halves of your roof are inside it. Note also what F2 does not say - it does not name a figure. The figures are in the guidance, and the guidance is how you show the requirement was met rather than being the requirement itself.
Part F binds an attic conversion for the same reason every other Part does: article 11(1)(a) of the Building Regulations 1997 applies the Regulations to all works in connection with the material alteration of a building, and a conversion is a material alteration. We set that argument out in full in our planning permission guide, which also separates planning exemption from Building Regulations compliance. Being exempt from planning has never had anything to do with this.
These are the four numbers. Each belongs to a specific part of the roof, and mixing them up is how a roof ends up half-vented.
50 mm
Minimum free air space between the roof deck and the insulation, converted slope (TGD F 2019, §2.1.16)
25 mm
Eaves ventilation, on two opposite sides, converted slope (TGD F 2019, §2.1.14)
5 mm
Ridge ventilation, where the insulation follows the pitch (TGD F 2019, §2.1.17)
10 mm
Eaves ventilation for a pitched roof space that was not converted (TGD F 2019, §2.1.11)
"The void should have a free air space of at least 50 mm between the roof deck and the insulation. Where joists run at right angles to the flow of air, a suitable air space may be formed by using counter battens."
This is the clause that insulating tight up to the underside of the slates breaches, and it is the single most common thing missing in a converted attic. Fifty millimetres is not a tolerance or a nice-to-have. It is the gap the air has to travel along, and rafters in an older Dublin house are often shallow enough that fitting the required insulation thickness and keeping 50 mm behind it means the rafters have to be deepened. That costs money at first fix, which is why it gets skipped.
Source: Department of Housing, Planning and Local Government, Technical Guidance Document F - Ventilation (2019), requirement F2 and Section 2, "Condensation in Roofs". Read the document - read 15 September 2026.
The rule almost everyone gets wrong
Here is the sentence that decides which set of figures applies to your attic, and it is the one clause on this subject that Irish contractor pages consistently do not mention. From §2.1.6:
"However, if the ceiling of a room follows the pitch of the roof, ventilation should be provided as if it were a flat roof, irrespective of the slope of the roof."
A converted attic is precisely a ceiling that follows the pitch of the roof. That is the definition of the room. So the guidance for pitched roofs - §2.1.10 to §2.1.13, the familiar 10 mm at the eaves - does not apply to the converted part of your roof. The flat-roof guidance does, §2.1.14 to §2.1.19, and that is where the 25 mm, the 50 mm and the 5 mm come from.
TGD F says the same thing twice, from both directions, which is a fair indication of how often it is missed. §2.1.10, sitting at the top of the pitched-roof section, says only: "If the ceiling follows the pitch of the roof, see Paragraphs 2.1.14 to 2.1.19." And the heading over the flat-roof section reads "Roofs with a pitch of less than 15° and roofs of any pitch where the ceiling follows the pitch of the roof".
The practical consequence is worth being blunt about. Your roof might be a 40° pitch. It does not matter. For the part of it with a room underneath, the pitch is irrelevant to the ventilation figures, and a builder who ventilated your converted slope to the pitched-roof numbers ventilated it to the wrong standard. This is the clause to name if you need to have that conversation.
The space behind the low wall is still a cold roof
Almost no conversion turns the whole roof into a room. What is left is the triangular space behind the low walls at each side of the room - the knee walls - and usually a void above the flat part of the new ceiling, and whatever is left at a hip or a gable end. Those spaces are still cold roof. They were not converted, nothing about them became a warm roof, and they keep the ordinary pitched-roof figures: eaves ventilation on opposite sides equal to a continuous 10 mm opening (§2.1.11).
That sounds simple, and then it stops being simple, because the two constructions share an edge and they share an eaves.
The same run of eaves has to feed the 50 mm void behind the converted slope at 25 mm (§2.1.14) and ventilate the leftover cold void at 10 mm (§2.1.11). They are not alternatives and one does not satisfy the other. In practice this means the eaves detail is more demanding after a conversion than it was before it, and an eaves that was adequate for the old cold roof is not evidence that it is adequate now.
Keep this separation in your head when anyone inspects the job. "We checked the attic and the ventilation is fine" is an answer about one of your two roofs, and probably not the one causing the problem.
A membrane is not a substitute for the void
The most common defence when a homeowner raises this is "there is a vapour barrier in there". TGD F answers it directly, in §2.1.4:
"Use of a vapour control layer at ceiling level, on the warm side of the insulation, will assist in limiting vapour transfer, but cannot be relied on as an alternative to ventilation."
So both are needed, and one does not buy off the other. The same paragraph is also the clause that says a vapour control layer should be used "where the roof pitch is less than 15°, or the insulation follows the pitch, or where the shape of the roof is such that there is difficulty in ensuring adequate ventilation, e.g. room-in-the-roof". That last phrase is the only place in Section 2 where a converted attic is named outright, and it is naming it as the difficult case.
§2.1.4 also deals with the holes, and this is the half that gets ignored. It asks that transfer of water vapour to cold roof voids be limited as far as practicable, and that care be taken to seal around all penetrations of pipes, ducts and wiring through the ceilings, including an effective seal to the attic access hatch.
Where those penetrations are in a converted attic
Recessed downlighters in the sloped ceiling and in the flat part of it. The soil vent pipe and waste runs from an attic ensuite. The extract duct from that ensuite, which is carrying the wettest air in the house and is frequently terminated into the roof void rather than through it. Cable drops into the new room. The eaves storage door and any remaining hatch. Each is a route for warm wet air into a cold space, and collectively they are usually a bigger opening than anything anyone designed.
If you are planning the work rather than repairing it, the two decisions that create these holes are made at first fix, and we cover them in the electrics guide and the ensuite guide.
One further note on membranes, because it cuts the other way too. TGD F §2.1.2 accepts that limiting condensation can be achieved by other means, including vapour permeable or breathable roofing membranes, and points at Part D of the Building Regulations - proper materials and workmanship - for how such a product has to be specified. A breathable membrane is a real and accepted mechanism. It is not a reason to have left out the void unless the whole build-up was designed around it.
When not ventilating is the right answer
Everything above is the ventilated route, and it is not the only compliant one. It would be dishonest to present it as though it were, and a reader who has been told their roof is a warm deck build-up needs to know that this is a legitimate design rather than a corner someone cut.
§2.1.5 is short and unambiguous:
"Roofs where the moisture from the building cannot permeate the insulation, e.g. warm deck roofs or inverted roofs, need not be ventilated."
The logic is straightforward. Ventilation exists to carry away moisture that has got into the construction. If the construction is built so that moisture cannot get in - the insulation above the deck, continuous, with the vapour control on the warm side of it - there is nothing to carry away and no void to carry it through.
TGD F goes further and treats this as the answer in one specific situation. §2.1.18 says that where the edges of the roof abut a wall or other obstruction such that free air paths cannot be formed to promote cross ventilation, "an alternative form of roof construction should be adopted", pointing back at §2.1.5. That situation is not exotic in Dublin. A mid-terrace house with party walls at both ends, or a hipped roof where an eaves run is short and interrupted, can be a roof where the ventilated route genuinely cannot be built.
The thing to check before accepting this answer
An unventilated roof is only safe if it is actually one of the constructions §2.1.5 describes - moisture genuinely cannot permeate the insulation. That is a property of the whole build-up, designed as a whole. A ventilated design that was built without its ventilation is not a warm deck roof. It is a ventilated roof with no ventilation, and the two get described with the same words by people who would rather not open it up. Ask which one it is, and ask who designed it that way.
What to do now
Work through it in this order. The first two cost nothing, and they decide whether the third is needed.
Use the timing test at the top of this page. If it tracks rain, it is a leak. If it is there in warm dry weather in one spot, it is plumbing. Neither is what the rest of this page is about.
One thing we are not going to claim
We cannot tell you whether your roof complies. Nobody can from a web page, and anyone who does is guessing about a construction they have not seen. What this page can do is tell you which clause governs, which figure belongs to which part of your roof, and what question to put. If it comes to a dispute, the person who settles it is an engineer or an architect who will inspect and sign - which is its own subject, covered in our certificates guide.
So, the decision this page exists to settle. Three outcomes, and by now you should know which is yours.
It is a defect, and it goes back to the builder
The converted slope was built as a ventilated construction, and it has less than 50 mm of free air space behind the insulation, or no ridge ventilation, or an eaves packed solid. Name it precisely: the ceiling follows the pitch, so §2.1.6 of TGD F 2019 puts it under the flat-roof guidance, and §2.1.16, §2.1.14 and §2.1.17 are the clauses it falls short of. That is a workmanship and compliance conversation, and it is a much shorter one when you can cite the paragraph.
It is the boundary, and it is fixable without a fight
The slope itself is built correctly, and the damp is behind the knee wall or around the penetrations: unsealed downlighters, a leaky eaves door, an ensuite extract discharging into the roof rather than out of it. This is the most common case of the three. It is real work and it is somebody's responsibility, but it is sealing and ducting rather than rebuilding a roof, and §2.1.4 is the clause it sits under.
It is the moisture load, and it is yours
The build-up is right, the room has the openable area §1.2.4.6 asks for, the penetrations are sealed - and the rooflight has not been opened since March, the extract fan is switched off at the isolator and there is a clothes airer under the slope. It happens, it is not a defect, and the fix is free. Being honest with yourself about this one before you ring the builder is worth doing.
And if you are reading this before the work starts rather than after, the whole page reduces to one sentence to put in front of whoever is pricing it: the converted slope is ventilated as a flat roof per TGD F 2019 §2.1.6, and we want the 50 mm void (§2.1.16), the 25 mm at the eaves (§2.1.14) and the 5 mm at the ridge (§2.1.17) shown on the drawing. It is a cheap thing to ask for at that stage and an expensive thing to retrofit.
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