A cantilever is a construction in which part of a building extends horizontally beyond its supporting points — without additional legs or columns on the outside. The overhanging part appears to “float in the air” and is held in place by the structure behind it.
Examples of Cantilevers
| Application | Description |
|---|---|
| **Overhanging roof** | Roof eave extends beyond the wall face |
| **Balcony** | Floor slab extends from the wall without columns |
| **Upper floor overhang** | First floor extends over the ground floor |
| **Patio cover / pergola** | Roof attached to the wall without front posts |
| **Floating stair treads** | Treads built into a wall with no visible support |
How Does a Cantilever Work?
A cantilever works through counterbalance and anchorage:
- The inner part (anchor zone) is heavier or more firmly anchored than the overhanging part
- In concrete construction, reinforcement bars extend inward
- In timber construction, main beams and bearing length provide adequate anchorage
Rule of thumb: The inner anchorage must be at least as long as the overhang outside.
Cantilevers in DIY Projects
As a DIYer, you encounter cantilevers in:
- Patio cover attached to the house — beams fixed to the wall without front posts
- Floating shelf or worktop — wall-mounted without legs
- Carport overhang — single-sided attachment to the wall
- Decking with overhanging boards — boards extend past the joist
Maximum Cantilever for Timber
| Beam (depth x width) | Max. overhang (approx.) |
|---|---|
| 145 x 45 mm | approx. 0.6 m |
| 195 x 45 mm | approx. 1.0 m |
| 245 x 45 mm | approx. 1.5 m |
| 295 x 70 mm | approx. 2.0 m |
Always calculate based on load, timber species, and span. Consult a structural engineer if in doubt.
Planning Permission?
A small overhanging roof or beam is usually permitted development. Larger structures (e.g., a balcony extending more than 1.5 m) may require planning permission. Always check local regulations.
> More guides and building plans: fredsdiyplans.com
Related Terms
- Roof truss — structural roof system distributing loads
- Beam / joist — the load-bearing horizontal member
- Carport — example of a cantilevered structure