Dormer window
A roof structure protruding from a sloped roof to create vertical wall space for a window, gaining usable floor area and standing headroom in loft conversions.
What is a dormer window and what makes it different from a roof window?
A dormer window (vikier in Slovak) is a roof structure that protrudes from a sloped roof, creating a vertical wall section with a window opening. Unlike a roof window, which is a flat or tilted glazing product installed directly in the roof plane, a dormer adds structural volume and usable floor area. In Slovak loft conversions (podkrovné opatrenie), a dormer solves the standing-headroom problem at roof edges, where the roof slope is too low for comfortable occupation. The dormer's vertical glazing also faces away from rain streams and direct sun angles that cause roof windows to overheat in summer or leak when driven rain hits the tilted glass.
How much headroom and floor area does a dormer gain compared to a roof window?
A dormer typically gains 0.8-1.2 m of additional headroom and 30-60% more usable floor area in a 4-6 m wide attic, depending on the roof pitch and dormer width. The gain comes from two sources: (1) the dormer cheek (vertical wall) creates full headroom above it, and (2) the dormer shifts the usable floor boundary outward, capturing the space under the low eaves that would otherwise be storage-only. A roof window adds daylight and passive solar gains but does not extend the habitable zone; you still cannot stand or use the space under the sloped roof on either side. This difference determines whether an attic can become a full bedroom or workroom (dormer) or remains a partial-height loft (roof window).
What are the main types of dormers and how do they compare?
The four primary dormer types are gable, shed, hip, and eyebrow, each with distinct structural and visual implications:
| Dormer Type | Profile / Roof Shape | Headroom Gain | Visibility Impact | Cost (relative) | Roof-Geometry Disruption |
|---|---|---|---|---|---|
| Gable | Two pitched roof planes meeting at a ridge, forming a triangular pediment | Maximum at center; tapers at sides | Very visible from street; defines roofline | 1.0x (baseline) | Requires cutting two main-roof rafters; new ridge and two new roof planes |
| Shed | Single pitched plane sloping downward away from the house | Consistent across width; less than gable | Moderately visible but reads as roof continuation | 0.85x (simplest framing) | Requires cutting two main-roof rafters; one new roof plane; simpler junction details |
| Hip | Hipped roof on the dormer (slopes on all sides) | Moderate at center; sharp edges | Less visible; blends into roof composition | 1.2-1.4x (complex raftering) | Requires cutting main-roof rafters and adding complex hip geometry; four junction lines to flash |
| Eyebrow | Curved or semicircular roof profile | Maximum at center; highly tapered | Distinctive, often heritage; difficult to build modern | 1.5-2.0x (bespoke framing, curved assembly) | Requires extensive main-roof cutting; curved formwork and joinery; difficult to seal and insulate |
Why is the junction between the dormer cheek and the main roof plane so critical?
The cheek junction is where the dormer's vertical wall intersects the main roof slope. At this line, three surfaces meet: the main roof cladding above, the dormer cheek wall (vertical plane), and the dormer roof starting its slope. Water running down the main roof can pool or divert along the cheek line before the dormer roof collects it. The flashing stack at this junction must channel water outboard, over the dormer roof below, then down to the gutter; any gap, wrinkle, or undersized flashing allows capillary or splash-driven water ingress into the cheek cavity. The design must also accommodate the roof plane intersecting the cheek at a shallow angle (often 45-70 degrees, depending on main-roof pitch and dormer width), making standard flashing products inadequate. Custom metal flashings (usually aluminium or lead, 1.5-2 mm thick) and sealant ribbons (elastomeric tapes) are the industry standard.
What is the thermal-bridge problem at the dormer cheek and how does it affect comfort?
A dormer cheek is typically framed with timber studs or roof trusses that run on the interior face of the insulation layer. Timber conducts heat about 0.1 W/(m2.K), far faster than the insulation behind it (typically 0.03-0.05 W/(m2.K) for mineral wool or expanded polystyrene). In winter, the outside surface of the cheek is cold; the timber creates a direct thermal path to the indoors, so the inside surface temperature of the cheek is 5-10 °C cooler than the main attic air. This temperature drop increases surface condensation risk and creates a perceived draught along the cheek. The solution is to minimize the cheek width (30-50 mm total build-up) and use structural thermal breaks (products like Isokorb, a composite bracket with a resin-composite break) to interrupt the timber bridge. Detailing matters more than insulation thickness here; a poorly detailed thick cavity is worse than a well-designed narrow cavity with a thermal break.
How does airtightness affect a dormer installation?
A dormer conversion introduces four airtightness challenges: (1) the junction of the dormer wall base to the main attic floor must be sealed with polyethylene tape or spray foam to prevent air leakage from the attic into the dormer cavity; (2) the cheek junction with the main roof plane is a risk for air movement between the dormer cavity and the roof space, requiring gaskets or sealant around flashing penetrations; (3) the window itself is a potential leak path at the frame-to-wall junction, calling for triple-glazed units and careful framing with sealant tape; and (4) the dormer roof-to-wall transition must have a continuous air barrier. In practice, many dormer retrofits ignore the dormer cavity air sealing because the work is hidden, leading to year-round air circulation that shortens roof material life and increases moisture in the timber frame. A modern dormer design treats the cheek cavity as a separate pressure zone, sealed from the attic and the main roof space, with a dedicated small ventilation duct to prevent pressure imbalance.
How does a dormer compare in cost to a roof window for providing usable attic space?
The cost comparison is typically per square metre of usable living area, not per square metre of glazing, because the dormer provides both window openings and new floor space.
| Element | Single Roof Window (Skylight) | Single Dormer Window (1.2 m wide, gable type) |
|---|---|---|
| Glazed area (typical) | 0.6-1.0 m2 | 1.0-1.5 m2 (dormer window only) |
| Material cost | €400-700 | €800-1200 (dormer window frame, flashing kit, structural lumber) |
| Structural/framing cost | €200-400 (rough opening cutting, sill reinforcement) | €2000-4000 (timber framing, roof restructuring, cheek wall) |
| Flashing and waterproofing | €150-300 (built into window kit) | €400-800 (custom metal flashings, multiple junction details) |
| Insulation and thermal break | €100-200 (roof opening insulation) | €300-600 (cheek insulation, thermal break, cavity fill) |
| Labour (installation) | €800-1500 (1-2 days) | €2500-5000 (5-8 days for carpentry, flashing, sealing) |
| Total per window opening | €1650-3100 | €6000-12000 |
| Usable floor gain | None (daylight only) | ~3-6 m2 of new headroom floor (makes 20-30% of attic usable) |
| Cost per m2 of new living area | Infinite (no floor area) | €1000-2000/m2 of new habitable space |
At €1000-2000/m2 for new dormer floor area, a dormer is expensive but competitive with some renovation costs. A roof window is cheaper upfront (€1650-3100 per unit) but solves a different problem: daylighting and passive solar gain, not habitable space expansion. The choice is therefore use-driven, not cost-driven. If the goal is to make an attic bedroom viable, a dormer is the only answer. If the goal is to brighten a storage attic, a roof window is faster and cheaper.
What are the planning and neighbourhood considerations for adding a dormer?
In Slovakia, the roof is the most visible component of a house. Adding a dormer changes the roofline, silhouette, and street presence. The building act (25/2025 Z. z., in effect since 1 April 2025) treats a dormer as a structural alteration to the roof and often as an extension (because it adds floor area and may increase the building footprint). Authorization typically requires a building permit and design documentation (architectural and structural drawings). Local zoning rules (territorial-planning regulations) may restrict roof changes in historic districts, listed neighbourhoods, or areas with height or setback controls. Practically, dormers often provoke neighbour concerns because they are conspicuous and perceived as adding storeys or bulk. Before designing, check with the local building office (stavebný úrad) and review the local territorial plan (územný plán obce) and construction regulations (stavebný poriadok). In some Slovak communes, a single dormer on a pitched roof requires public notice or neighbour notification.
What are common construction defects and how are they prevented?
Water leakage at the cheek junction is the most common defect, caused by inadequate flashing, sealant breakdown, or poor detailing. Prevention: specify a site-built flashing design reviewed by the structural engineer, use elastomeric tape (e.g. Illbruck or equivalent) over all sealant joints, and flash the uphill (upper) cheek edge with an L-shaped metal flashing that diverts water onto the dormer roof, not into the cavity. Thermal bridging and surface condensation come second, especially if the cheek is more than 80 mm wide or lacks a thermal break. Prevention: minimize cheek thickness to 40-60 mm, use a structural thermal-break bracket (like Isokorb) if the cheek carries roof loads, and ventilate the dormer cavity with a small duct to manage humidity. Air leakage at the base of the cheek (where it meets the attic floor) and around the window frame is often ignored because it is concealed. Prevention: seal the base junction with polyethylene tape or expanding foam, wrap the window frame with continuous airtightness tape, and test with a blower door after completion. Insulation voids and compression in the shallow cavity occur if standard blanket batts are forced into a 100 mm cavity; they compact over time and lose R-value. Prevention: use rigid insulation boards (XPS or rigid mineral wool) cut to size and friction-fit, or specify low-density batts and over-insulate to compensate for expected settling.
How does a dormer compare to raising the knee wall (raising the roof line) as an alternative?
Some older attics are framed with a knee wall (a short load-bearing wall installed on the attic floor, raising the roof plane and interior headroom without protruding from the roof). A raised knee wall is cheaper than a dormer and does not change the external roofline, so it may avoid planning scrutiny. However, it sacrifices usable floor area in the sloped eaves (the knee wall consumes the space where a dormer would capture full headroom), and the interior volume gained is still bounded by the roof slope. The trade-off table below shows the geometry:
| Approach | Visible from Outside | Usable Floor Area (for attic 6 m wide, 30° roof pitch) | Max Headroom Gain | Cost (relative) | Planning Risk |
|---|---|---|---|---|---|
| Do nothing (unmodified attic) | No | ~50% (only center strip) | None | 0x | None |
| Add roof windows only | Slightly (small glass areas on roof plane) | ~50% (no new floor area; light only) | None | 0.2x | Minimal (cosmetic) |
| Install knee wall (1.2 m high) | No (internal only) | ~70% (knee wall occupies ~1.2 m of eaves width) | 1.2 m at knee top, then slopes | 0.5x (framing + internal finishes) | Low (no external change) |
| Add dormer window (1.2 m wide, gable) | Yes (visible roofline modification) | ~85% (captures full-height floor over dormer width) | Full standing room over dormer; tapers at edges | 1.0x (baseline dormer cost) | Moderate-to-high (roofline change requires permit and may need neighbour notification) |
The knee wall is a compromise: more usable area than roof windows alone, less visible than a dormer, but still bounded by the roof slope. A dormer commits to the roofline change but provides the most usable floor. The choice depends on planning constraints, budget, and whether the goal is a full bedroom or a partial workroom.
Frequently asked questions
- What is the main difference between a dormer and a roof window?
- A dormer is a protruding roof structure with a vertical window that provides standing headroom and more usable floor area at roof edges. A roof window (skylight) is a flat or tilted glazing opening in the roof plane itself, offering less headroom and more heat gain in summer. Dormers are more visible from outside and structurally invasive but solve attic conversion limitations that roof windows cannot.
- What are the main types of dormers?
- Gable dormers have a triangular pediment and are structurally simple but most visible. Shed dormers have a single pitched roof sloping down and maximize floor area. Hip dormers have a hipped roof for a more integrated appearance. Eyebrow dormers have a curved or rounded profile, common in historic properties. Each type trades cost, visibility, and roof-geometry disruption.
- Why is the flashing detail at a dormer's junction so critical?
- The cheek (vertical wall side) where the dormer meets the main roof plane is where three surfaces intersect: the main roof, the dormer cheek, and the dormer roof. Water can pool or redirect along these lines if flashing is inadequate, and the narrow build-up (often less than 150 mm to the insulation layer) makes airtightness and thermal continuity difficult. This junction governs whether a dormer conversion stays dry and comfortable for 30+ years.
- Do dormers require a building permit in Slovakia?
- A dormer involves altering the roof structure and adding habitable floor area, so it requires authorization under the building regulations (zákon o výstavbe 25/2025 Z. z., in force since April 2025). The process and thresholds vary by municipality and whether the work is deemed a modification or an extension. Consult the local building office (stavebný úrad) and review local zoning rules; roof alterations are highly visible and often generate neighbour concerns.
- What is a thermal bridge at a dormer cheek and how does it affect comfort?
- Where the dormer cheek wall rises from the main roof plane, the structural support is often a timber frame that runs inside the insulation layer, creating a high-conductivity path from inside to outside. This thermal bridge (weak point in the insulation) causes the interior surface of the wall near the cheek to be cooler, increasing surface condensation risk and perceived draught. Minimizing the cheek width and using structural thermal breaks (like Isokorb) mitigates this.
- Why do dormers cost more than roof windows for the same glazed area?
- A dormer requires rebuilding the roof structure in that zone, adding timber framing, waterproofing, flashing, and the dormer roof itself. A roof window is a single product that slots into a rough opening. A dormer easily costs 2-4 times more per square metre of glazing than a roof window, but provides standing headroom and usable floor that a roof window does not. The comparison is not per-glazing-area but per usable living-area gained.