The clerestory is the windowed wall zone in the nave of a church, above the roofs of the aisles. Because the nave is taller than the aisles, a stretch of wall is left clear there and can hold windows — the principal light source of the nave.
The Latin-derived form is clerestorium; in Dutch it is called a lichtbeuk, in German an Obergaden.
Why does the clerestory exist?
A basilica is distinguished from a hall church precisely by this difference in height: tall nave, low aisles. In a hall church all the vessels are (roughly) the same height, so there is no free wall zone above the aisle roofs and light enters only through the outer walls of the aisles. A basilica gets its light from high up, which makes the nave brighter and the space feel taller.
| Basilica | Tall nave, low aisles; the clerestory above the aisle roofs lights the nave from high up, making it brighter and feel taller. |
|---|---|
| Hall church | All vessels roughly the same height; no free wall zone above the aisle roofs, so light enters only through the outer walls of the aisles. |
Structurally that comes at a price. The tall, pierced nave wall stands exposed to wind and carries the vault as well. Those lateral forces have to be taken down — in Romanesque building through heavy walls, in Gothic through flying buttresses and piers reaching over the aisles. The larger the clerestory windows, the heavier the buttressing outside.
In the elevation of a Gothic nave wall the clerestory sits at the top: the arcade to the aisle below, sometimes a triforium above that, and the clerestory above all.
The modern clerestory
The principle is still in use, just without the ecclesiastical vocabulary: high-level windows that bring daylight deep into a room. You see it in industrial halls, sports halls, schools and in houses with a raised roof section.
The appeal is the same as it was in the Middle Ages:
- Light reaches further in. As a rule of thumb, daylight penetrates roughly one and a half to two times the head height of the window into the room. A window set high in the wall therefore lights far deeper than an equally large window set low.
- No overlooking and no loss of wall space. A cupboard or a bed can stand under a high window without any problem.
- More even light. Light from above produces less contrast and less glare than one large window in a single elevation.
Points to watch
- Solar shading south and west. An unshaded south-facing clerestory causes serious summer heat gain. Build in an overhang, external shading or solar control glass — and count it in the heat balance.
- Access. Windows four metres up have to be cleaned and maintained. Think through the route, the tower or the operating pole in advance.
- Ventilation. Warm air collects precisely at high level. An opening light in the clerestory works extremely well for summer purge ventilation.
- Detailing the junction. The junction between the low roof and the rising wall with windows is a classic leak point; provide adequate upstand height and a properly formed flashing.
Related terms
- Basilica
- Aisle
- Bay width
- Daylight factor
- Continuous rooflight
- Light hood
- Light transmission
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.