A cantilever beam is a beam that is supported at only one end, or that runs past its last support, so the free end hangs in the air with nothing under it. Balconies, deck overhangs, canopies, bay window floors and crane jibs all rely on cantilevers, and they all share one rule: the fixed end has to resist a large bending moment that a simply supported beam never sees.
This guide explains how a cantilever carries load, why it bends far more than a beam supported at both ends, the deck overhang rules used in house construction, and what an engineer checks before a steel cantilever is built. Sources were checked on October 31, 2026.
How a Cantilever Beam Carries Load
An ordinary beam sits on a support at each end. Load pushes it down in the middle, and each support pushes back up with part of the load. A cantilever has no second support. The fixed end has to do two jobs at once: hold the beam up (shear) and stop it from rotating (moment). That rotational restraint is what makes a cantilever possible, and it is also what makes the fixed end the most heavily stressed point.
The bending is the reverse of a normal beam. In a simply supported beam the bottom flange is stretched and the top flange is squeezed. In a cantilever the top of the beam is in tension and the bottom is in compression, with the highest stress right at the support. That reversal matters for steel beams, because the compression flange is the one that can buckle sideways, and on a cantilever it is the bottom flange.
Two Ways to Build a Cantilever
Fixed (built-in) cantilever
The beam is welded or bolted to a column or wall with a connection rigid enough to stop rotation, and nothing else holds it up. Steel balcony brackets and canopy arms that stick out from a column are typical. In steel framing this needs a fully restrained connection; our guide to the moment connection explains how flange welds or end plates carry that moment into the column. A simple shear connection with a pair of clip angles cannot hold up a cantilever.
Back-span (overhanging) cantilever
The beam rests on two or more supports and keeps going past the last one. The part between the supports, the back span, acts as a lever arm that balances the overhang. Deck joists running past a beam, floor joists carrying a bay window, and roof rafters forming eaves all work this way. The support next to the overhang takes more than the full load of the overhang, and the far end of the back span can try to lift, so it may need a hold-down connection.
Why Cantilevers Bend So Much More
The standard textbook formulas for elastic beams show the difference clearly. For a beam of length L with stiffness EI (modulus of elasticity times moment of inertia):
| Load case | Maximum moment | Maximum deflection |
|---|---|---|
| Cantilever, point load P at the free end | P × L | P L³ / (3 E I) |
| Simple span, point load P at midspan | P × L / 4 | P L³ / (48 E I) |
| Cantilever, uniform load w along the length | w L² / 2 | w L⁴ / (8 E I) |
| Simple span, uniform load w | w L² / 8 | 5 w L⁴ / (384 E I) |
Under the same point load and length, the cantilever sees four times the moment and sixteen times the deflection of a simply supported beam. Under a uniform load it is four times the moment and 9.6 times the deflection. Deflection also grows with the cube or fourth power of length, so a cantilever that is 50% longer deflects about 3.4 times as much under a tip load.
A worked illustration with a real steel shape
Take a W10x12 beam, which the AISC Shapes Database v16.0 lists at 9.87 inches deep with a strong-axis moment of inertia Ix of 53.8 in⁴. Use E = 29,000 ksi for structural steel and hang 1,000 lb at the end of a 6 ft (72 in) cantilever, ignoring the beam’s own weight:
- Moment at the support: 1,000 lb × 6 ft = 6,000 ft-lb.
- Tip deflection: 1,000 × 72³ / (3 × 29,000,000 × 53.8) = about 0.080 in.
- Same beam spanning 6 ft between two supports with the 1,000 lb at midspan: 1,500 ft-lb and about 0.005 in.
This only shows the ratios. It is not a capacity check: it leaves out self-weight, load factors, lateral-torsional buckling, the connection and the supporting column. If you want to compare stiffness between shapes, our I-beam moment of inertia calculator gives Ix for standard sections.
Cantilever Rules for Deck and Floor Joists
Most homeowners meet cantilevers as deck joists that run past the beam. The American Wood Council’s Prescriptive Residential Wood Deck Construction Guide (DCA 6), based on the 2015 IRC, lets joists extend past the beam up to the lesser of a tabulated allowable overhang or one fourth of the joist span (L/4). It also lets deck beams extend past the post up to one fourth of the beam span (LB/4). Its Table 2 bases the overhangs on a 40 psf live load, 10 psf dead load, No. 2 lumber, wet service, and a 220 lb point load at the tip with a deflection limit of the cantilever length divided by 180.
For Southern Pine joists, DCA 6 Table 2 lists these allowable overhangs (checked October 31, 2026):
| Southern Pine joist | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 2×8 | 1 ft 10 in | 2 ft 0 in | 2 ft 4 in |
| 2×10 | 3 ft 1 in | 3 ft 5 in | 2 ft 10 in |
| 2×12 | 4 ft 6 in | 4 ft 2 in | 3 ft 4 in |
The one-fourth rule is not universal. Glenn Mathewson, writing in Fine Homebuilding (September 2024), explains that the 2015 IRC used a 1:4 cantilever-to-back-span ratio, the 2018 edition added a separate maximum cantilever column, and the 2021 IRC dropped the ratio in favor of maximum cantilevers listed by joist span, with no change in 2024. He also points out that the old “2 ft maximum” rule of thumb is not in the code. Which table applies depends on the code edition your jurisdiction has adopted, so check with your building department.
Floor joists inside a house are covered by separate IRC tables for cantilevers that support a bearing wall and roof. Those apply to bay windows and second-floor overhangs, and they are more restrictive because the load at the tip is far larger than deck traffic. Our floor joist spacing guide covers the spacing side of that framing.
What an Engineer Checks on a Steel Cantilever
Lateral-torsional buckling
Because the bottom flange is in compression, a slender cantilever can twist and kick sideways long before it reaches its bending strength. Bo Dowswell’s paper Lateral-Torsional Buckling of Wide Flange Cantilever Beams (AISC Engineering Journal, second quarter 2004) notes that the standard AISC buckling equation was derived for simply supported beams, and that applied to cantilevers it is overly conservative when the tension flange is braced but can be unconservative when the compression flange is braced. Where the bracing sits, and where the load is applied (top flange or shear center), changes the answer.
Deflection and bounce
A balcony can be strong enough and still feel lively underfoot. Deflection limits for cantilevers are often stated against the cantilever length, and vibration can govern the size of a long steel balcony beam. Because tip deflection depends on I, a deeper section usually helps far more than a heavier one of the same depth.
The connection and the back span
For a fixed cantilever, the moment connection and the column or wall it attaches to must take the full support moment. For an overhanging beam, the interior support takes a larger reaction than the overhang load alone and the back-span end may need to be held down against uplift. Thermal bridging is another detail on exterior steel balconies, because a continuous steel beam through the building envelope carries heat out.
Tables such as our steel beam span chart are written for simply supported beams. Do not use simple-span tables to size a cantilever. A licensed structural engineer has to size every cantilever that carries people, a wall, a roof or equipment, and design its connections and supports.
Cantilever Beam FAQ
What is a cantilever beam in simple terms?
It is a beam held at one end only, or one that sticks out past its last support. The free end has nothing under it, so the fixed end has to stop the beam from both dropping and rotating.
How far can a cantilever beam extend?
There is no single limit. For wood deck joists built to DCA 6, the overhang is capped at the lesser of a tabulated length or one fourth of the joist span. Steel cantilevers are sized by calculation for the actual loads, deflection limit and bracing, and that has to be done by a licensed structural engineer.
Where is the maximum stress in a cantilever?
At the fixed support, where both the bending moment and the shear are largest. The top of the beam is in tension there and the bottom is in compression.
Is the 2 ft deck cantilever rule in the building code?
No. It was a rule of thumb. Recent IRC editions give tabulated maximum cantilevers that depend on joist size, spacing, species and span, and the allowable overhang can be more or less than 2 ft.
Can I add a cantilever to an existing beam?
Not without an engineer. Extending a beam changes the loads on its supports, can reverse the bending in the back span, and may lift the far end. The existing beam, posts, footings and connections all have to be checked.
Reference information only, not engineering advice. Disclaimer.