What is a penthouse?
A penthouse is the dwelling on the top storey of an apartment building. Its defining feature is that the top storey is set back from the facade below, creating a generous roof terrace on one or all sides.
The term comes from American high-rise jargon: originally a “penthouse” was the structure on the roof housing lift machinery and services. From the 1920s onwards those spaces in New York buildings were converted into luxury apartments, and that meaning has stuck.
Building characteristics
- Set-back facade. Often several metres, creating the terrace and making the building look lower from street level. Planning policy sometimes requires it explicitly.
- Lightweight construction. Because the top storey is set back, the load-bearing walls do not always sit above those of the storey below. A transfer slab or additional beam is then needed.
- Large areas of glass. Solar shading and overheating deserve attention: a penthouse has a lot of facade and a lot of sunlit roof, with relatively few adjoining heated spaces.
- Plant on the roof. Heat pumps, air handling units and solar panels usually sit on the same roof. Watch acoustic and vibration isolation down into the dwelling below.
Where it typically goes wrong
The roof terrace is the point at which things most often fail in construction terms. What you want to see:
- A roof waterproofing layer that runs on underneath the terrace paving, with pedestals or ballast above — not slabs laid straight onto a vulnerable cap sheet.
- Enough upstand height at the door threshold, with a channel or grating in front of the glazing so driving rain does not blow in.
- Accessible rainwater outlets and an emergency overflow, so the terrace can discharge outwards if an outlet blocks rather than loading the roof.
- A balustrade or parapet at the required height and without climbable footholds for children.
Legally a penthouse is an ordinary flat within a management company or owners’ association. The roof terrace is often common roof with an exclusive right of use — read the lease or deed before you start altering it.
Related terms
- Flat roof
- Parapet
- Roof terrace
- Loggia
- Access gallery
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.