The size of the joists, the type of wood and quality of lumber used, as well as the distance between the joists all play a role in determining how far your joists may cantilever or overhang a drop beam securely. Cantilevers on joists are frequently capped at a maximum of 24 inches in several construction departments. When you are attaching the rim joist to the ends of your joists, you should always aim to utilize at least five framing nails for each attachment. Before constructing any decking design, it is strongly suggested that you always contact your local code department to ensure that all code and safety criteria are being met. This may be done by calling your city or town's building department. Cantilevers are only allowed to take up a maximum of one-fourth of the total span of the joists, according to the American Wood Council.
The overhang appears to be floating in midair without any support. Is it a solid object? Do you think you could walk on it? It is risk-free so long as the construction is done correctly. There are a variety of distinct forces at play in this location, and you need to adhere to some specific guidelines.
Read more: Deck Blocking & Bridging
The length of the cantilever. The species of wood, the grade of the wood, and the size of the wood all contribute to the overall strength of the cantilever, but the distance between the joists is the single most important factor. The cantilever that extends outward from our structure measures three feet and is supported by two-by-eight pressure-treated southern yellow pine (SYP PT) joists that are spaced 12 inches apart. Cantilevers with a live load of up to 40 pounds per square foot (psf) are permitted according to the Building Code (IRC 2009 or IRC 2012); however, your engineer may permit larger cantilevers. If you upgrade to joists that are 2 by 10, the building code will allow you to cantilever for over 5 feet.
Backspan. A deck's proportions must be such that the weight distributed across its cantilever is balanced, in addition to the overhang limit that must be adhered to. It is imperative that each joist's backspan, which refers to the distance from the beam back to the house ledger, be sized appropriately. According to the Building Code, the back span of a deck must be at least twice as long as the cantilever extension. (see also footnote 4) This ratio of 2:1 is liberal; however, I like the more conservative (and safer) ratio of 3:1. (Please see the note that follows this one for further information on backspan standards.)
Ledger attachment. When weight is applied downward on the deck cantilever, an upward thrust is applied to the joist (the beam acts as the fulcrum of this lever). At the opposite end of the joist, where it is attached to the house with a joist hanger, there should be some resistance applied to this upload. In this particular illustration, a complete design load that is applied to the cantilever results in an upload of 148 pounds per individual joist. When securely attached to a PT ledger, a Simpson LUS 2x8 hanger has the ability to withstand uploads of up to 1,335 pounds. (reference number 5)
Read more: How to Properly Space Deck Boards
Think about the alternative if this piece of engineering appears too difficult to understand. In order to move a deck out from a home by 17 feet using the conventional girder method of construction, all of the joists and the ledger would need to be upgraded from 2x8s to 2x10s. This brute force approach would add around $4 extra to the cost of each joist, which would result in an additional $100 for a deck that is 16 feet wide (taking into account the 17 joists, ledger, rim joist, blocking, and joist hangers at the girder).
Cantilevers do not always result in cost savings for decks. You may extend a frame by 14 feet by using 2x8 joists and a straightforward girder structure. The cantilevered version of this deck still requires 2x8 joists (2x6s will not be sufficient), but the result is a structure that is far more robust. A girder that is 14' 0" long is very near to reaching its full potential because the maximum span for SYP PT 2x8s 12" o/c is 14' 2"2. On the other hand, a cantilever that is only two feet long extends the beam only twelve feet, which is significantly less than the limit. As a result, the cantilevered deck produces a platform that has increased rigidity.
Therefore, by cantilevering the joists, it is possible to construct decks that are either larger at a lower cost or stronger. That's already quite fantastic, but it keeps getting better. Cantilevers can provide two additional advantages. They move the beam and the supporting columns to the side, which gives the deck a more relaxed appearance.
Maximum allowable overhang cannot exceed 1/4 of the actual main span.
|
Joist Spacing (o.c.) |
|
12" |
14" |
16" |
|
Species |
Size |
Allowable Overhang |
|
|
|
Southern Pine |
2x6 |
1'-0" |
1'-1" |
1'-4" |
|
|
2x8 |
1'-10" |
2'-0" |
2'-4" |
|
|
2x10 |
3'-1" |
3'-5" |
2'-10" |
|
|
2x12 |
4'-6" |
4'-2" |
3'-4" |
|
Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir |
2x6 |
11" |
1'-0" |
1'-2" |
|
|
2x8 |
1'-8" |
1'-10" |
2'-2" |
|
|
2x10 |
2'-10" |
3'-2" |
2'-9" |
|
|
2x12 |
4'-4" |
3'-11" |
3'-3" |
|
Redwood, Western Cedars, Poderosa Pine, Red Pine |
2x6 |
9" |
10" |
11" |
|
|
2x8 |
1'-5" |
1'-7" |
1'-9" |
|
|
2x10 |
2'-5" |
2'-7" |
2'-9" |
|
|
2x12 |
3'-7" |
3'-9" |
3'-1" |
Maximum allowable overhang cannot exceed 1/4 of the actual main span.
FAQs about Deck Joist Cantilever Rules and Limits
How far can I cantilever deck joists?
-
By code, a joist overhang is limited to the lesser of the table value (LO) or L/4 (one-quarter of that joist’s backspan between supports). Example from DCA-6 Table 2: a 2×10 DF/HF/SPF @ 16″ o.c. allows ≈3′-2″ overhang (still capped by L/4). Always use your species/size/spacing line in the table.
Does the cantilever count toward the “joist span”?
-
No. Prescriptively, joist span is measured bearing-to-bearing (ledger to beam centerline, or beam-to-beam) and doesn’t include the overhang past the beam.
Do I need blocking at the beam when joists overhang?
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Yes. DCA-6 shows full-depth blocking at the beam for overhanging joists; many city handouts echo this and allow skipping blocking only when the overhang ≤ 24″. Follow your local detail sheet.
Are there hard local caps like “no more than 24 inches”?
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Often, yes. Many AHJs add a simple cap (e.g., max 24″), even if the code tables and L/4 would allow more. Treat 24″ as a common local rule of thumb—verify locally.
What backspan-to-cantilever ratio should I assume?
-
Under the DCA-6/IRC L/4 rule, your backspan is at least 4× the cantilever. Some jurisdictions publish additional heuristics (e.g., ≤3′ overhang only when backspan ≥6′). Your AHJ’s deck sheet governs.
Do beams have cantilever limits too?
-
Yes. Prescriptively, deck beams may cantilever up to 1/4 of their actual beam span (LB/4). Check your beam tables and posts before using a beam overhang.
What connections handle the uplift a cantilever creates?
-
Cantilevers push the joist up at the ledger side; use listed joist hangers and follow nail schedules. DCA-6 also requires joist-to-beam connectors (or clips) with ≥100 lb uplift/lateral capacity at the beam. Attached decks must also meet the IRC lateral-load connector provisions.
Do decking material/orientation change anything?
-
They change joist spacing, which indirectly affects allowable spans/overhang tables. Typical handouts: 16″ o.c. for perpendicular decking and 12″ o.c. when boards run at ~45° (composites often require this). Always match your brand’s chart.
