Glossary
The vocabulary
of a dormer project.
Quotes and council letters are full of jargon. This page collects the terms you will meet most often, in plain English, with the Dutch original where it helps.
In het kort
Vier vuistregels
- Vergelijk minimaal drie bedrijven op meer dan alleen prijs.
- Beoordeel materiaal, isolatie, montage en garantie afzonderlijk.
- Controleer altijd vergunningseisen bij uw eigen gemeente.
- Een lage prijs zonder specificatie is een waarschuwingssignaal.
Terms
Types and shapes
- Dormer (dakkapel)
- A vertical extension built out of a sloped roof to create headroom, light and an extra usable space inside the attic.
- Flat-roof dormer
- A dormer with a flat roof finished in EPDM, bitumen or zinc. Most common type because it adds the most headroom for the width.
- Pitched-roof dormer
- A dormer with a small pitched roof (mono-pitch or gable). Often required by aesthetic rules in historic streets.
- Prefab dormer
- A dormer built in a factory as one piece and craned onto the roof in a single day. Faster install, less site work.
- Traditional dormer
- A dormer built up plank by plank on the roof. Slower, but easier to fit unusual sizes and rooflines.
Terms
Materials
- PVC cladding
- Plastic cladding panels. Low maintenance, affordable, but limited colour and finish options.
- Composite / HPL panel
- High-pressure laminate panels (e.g. Trespa, Keralit). Durable, sharp finish, more expensive than PVC.
- Polyester shell
- A glass-fibre reinforced polyester body — common for prefab dormers because it is light and watertight.
- EPDM
- A rubber roofing membrane used on flat dormer roofs. Long lifespan, seamless, repairable.
- Lead apron
- The lead flashing that seals the dormer to the existing roof tiles. The single most common leak point if done badly.
Terms
Insulation and energy
- R-value (Rc)
- How well an insulation package resists heat loss. Higher is better. New dormer insulation typically aims for R ≥ 4.5 m²K/W.
- Thermal bridge
- A point in the build where insulation is interrupted (e.g. at a frame, lead flashing or corner) and heat escapes — often causing condensation.
- HR++ glazing
- High-efficiency double glazing with a low-emissivity coating and a gas-filled cavity. The standard for new dormers.
- Triple glazing
- Three panes with two gas-filled cavities. Better thermal and sound performance, heavier and more expensive than HR++.
Terms
Planning and rules
- Permit-free (vergunningvrij)
- A dormer that meets all national criteria (mostly rear-roof position, modest size) and does not need a planning permit.
- Omgevingsvergunning
- The Dutch all-in-one building permit. Required for any dormer that does not qualify as permit-free.
- Aesthetic committee (welstand)
- A local council committee that judges whether the design fits the street and neighbourhood.
- Bouwbesluit / Bbl
- The Dutch building code (now Besluit bouwwerken leefomgeving). Sets minimum technical requirements for insulation, ventilation, safety and structure.
Terms
Process and warranty
- Quote (offerte)
- A written, itemised price for the work. Should list materials, glazing, insulation, lead work, permit handling and transport separately.
- Handover (oplevering)
- The formal moment the work is accepted. Defects noted at handover must be fixed by the company.
- Trade warranty
- A warranty backed by a trade body (e.g. BouwGarant). Covers completion or defects if the original installer is unable to repair them.
- Service-desk response time
- How long the company takes to reply when you report an issue after installation. A practical measure of after-sales quality.
Conclusie
If you meet a term in a quote that is not on this list, ask the company to define it in writing. A supplier who cannot explain their own terminology is not the one you want fitting a dormer to your house.