Pitched vs flat-roof dormer windows & lead flashing
Whether your home has traditional pitched-gable dormers at the front or wide flat-roof box dormers at the back, replacing the window frames costs €450 to €950 to €850 to €1,750 per opening, while constructing a brand-new structural dormer costs €3,500 to €7,500.
Comparing gable, hipped, flat-roof and shed dormers on Irish homes, and how the critical lead apron junction under the window cill works.
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Calculate your dormer window & insulation cost
Select whether you are replacing existing dormer windows, upgrading a whole dormer bungalow, insulating cold dormer cheeks, or building a new attic dormer.
Estimated supply & fit guide
€7,400 to €15,600
2 new attic dormers at €3,500 to €7,500 per structural dormer opening
Complete structural build and glazing for 2 new attic dormers with colour-foiled triple-glazed uPVC or flush sash windows: rafter trimming, insulated stud cheeks, pitched slate or warm GRP flat roof, Code 4/5 lead saddle and apron flashings, and internal plasterboarding.
Ask a local specialist for a quoteFour dormer styles on Irish roofs compared
| Dormer type | Where you see it in Ireland | Window & flashing detail |
|---|---|---|
| Gable / pitched-roof dormer | Front elevation of 1.5-storey Irish dormer bungalows | Two sloped roof verges meeting at a ridge; takes a standard rectangular or arched-head casement window |
| Flat-roof / box dormer | Rear attic conversions on urban semi-Ds and terraces | Maximises full standing headroom across the back roof; takes wide 2- or 3-pane casement windows under a GRP, EPDM or warm-deck roof |
| Hipped-roof dormer | Period houses & rural planning-sensitive sites | Three pitched roof planes hip back into the main roof; reduces visual bulk for county council planners |
| Half-dormer (eaves dormer) | Country cottages & story-and-a-half builds | Window cuts halfway through the eaves wall and halfway into the roof slope |
How the window cill meets the roof lead flashing
Nine out of ten leaks below a dormer window are caused not by the glass unit, but by a perished lead apron or a replacement window fitted without a proper stub cill drip over the lead tray:
- Code 4 or Code 5 milled lead apron: A dressable lead sheet turns up behind the bottom timber rail of the dormer opening and laps 150mm–200mm down over the roof slates or tiles below.
- 150mm or 185mm projecting external window cill: The new dormer window sits on a welded external cill with an anti-capillary drip groove that throws rainwater clear of the vertical lead upstand so wind cannot blow water uphill under the frame.
- Side cheek soakers and step flashings: Where the dormer cheek meets the pitched roof slates, interleaved lead soakers and a cover flashing prevent water tracking down the corner post of the window frame.
What dormer windows cost in Ireland
Our supply-and-fit price guide for single dormer windows, full dormer bungalows, cheek insulation and new attic dormers.
| Job | Typical cost | What moves the price |
|---|---|---|
| Replace single dormer window (existing dormer, white uPVC) | €450 to €950 per window | Includes roof access, lead apron check & Part B egress stays |
| Replace dormer window in coloured triple uPVC or flush sash | €650 to €1,250 per window | 0.8 W/m²K triple glazing · stops upstairs cold draughts |
| Replace dormer window in aluminium or alu-clad | €850 to €1,750 per window | Ideal for high-wind coastal and upland dormer roofs |
| Full dormer bungalow replacement (8–12 windows), uPVC | €5,200 to €10,500 | After €4,000 SEAI detached grant: €1,200 to €6,500 |
| Full dormer bungalow replacement (8–12 windows), aluminium / alu-clad | €9,500 to €17,500 | After €4,000 SEAI detached grant: €5,500 to €13,500 |
| Insulate & re-clad dormer cheeks & reveals (while window is out) | €650 to €1,450 per dormer | PIR insulation board, breathable membrane, uPVC/zinc cladding & lead check |
| Build new structural attic dormer (carpentry, roof, lead & window) | €3,500 to €7,500 per dormer | Structural rafters, Code 4/5 lead flashing, slate or GRP roof & window |
Real examples and installation detail
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Questions dormer homeowners ask
How much does it cost to build a new flat-roof or pitched dormer in an attic?
Forming a new structural dormer opening in an existing pitched roof—including doubling the rafters, building and insulating the cheeks and roof, lead flashing, slating/GRP roofing, plastering and fitting an A-rated dormer window—typically costs €3,500 to €7,500 per dormer.
Will replacing my dormer window damage the roof slates or lead flashing?
No. A specialist window installer unclips the old frame from the timber trimmer studs above the lead upstand and beds the new external cill directly over the existing Code 4 lead tray without lifting surrounding slates. If the old lead is cracked, it can be renewed while the frame is out.
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Replacing cold, draughty or rotten dormer windows?
One vetted window company covering your county inspects the dormer frames, lead apron and side cheeks, and gives you a written price.