Cantilever Decks: Design, Framing, Rules

Building a cantilever deck is a unique way to take advantage of the innate strengths of different structural members. Although beams can cantilever beyond support posts, a cantilever deck usually refers to the joists extending beyond the outer drop beams or exterior wall top plate. However, some specific rules affect the design and framing to ensure the structural safety of a deck that overhangs, extends, or cantilevers past its supports. If you’re unsure what the rules are, we’re here to help!

A cantilever deck may rest on beams that extend beyond support posts as well as joists that cantilever several inches or even feet past exterior supports. The joists may extend up to 1/4 of the backspan beyond the forward support beam. However, the proportion depends on local codes, structural integrity, and balance.

In this guide, we’ll explain what a cantilever deck is, if they are safe, and its advantages and disadvantages. We’ll discuss the maximum cantilever length, the types of materials to use, and how to design a cantilever deck. We’ll also explore building code regulations and framing rules for cantilevering a deck, plus if you can cantilever a free-standing deck. Our aim is to provide you with the information necessary for your cantilevered deck project.

What is a Cantilever Deck?

A cantilever deck can be used to provide extra outdoor living area, shade, decoration, or aesthetic appeal to a building. They are like any other deck attached to a structure but their joists extend beyond the outer support beam to create an overhang. A cantilevered deck is typically a more efficient use of materials as it often maximizes the structural potential of the materials involved.

A cantilever deck may have a drop beam that is cantilevered up to 1/4 span beyond the end support posts. Plus, according to the American Wood Council (AWC), the joists may extend past the drop beam up to 1/4 of their backspan. The actual cantilever distance, however, depends on wood species, lumber grade, lumber depth or dimensions, spacing between joists, load parameters, and local building codes.

To prevent a teeter-totter or spring-board effect in a cantilever deck, it’s important to ensure the backspan is properly anchored and supported. To support the overhang’s weight and maximize the distance cantilevered, it’s common to use stronger building materials, fasteners, and different construction techniques. So, while a cantilevered deck is like any other deck, there are significant differences.

Many cantilevered decks attach to a structure using a ledger board anchored to the building. Joists run from it and are supported by one or more drop beams supported by posts. The outer ends of the joists extend beyond the last support beam by inches or feet, depending on member strengths and design factors.

A cantilevered deck may use upper-story joists to create a small deck or balcony that requires no support posts. The deck may extend 6” to 12” to create a standing balcony or 4’ to 6’ to form a sitting one. Such a deck takes advantage of vistas, provides shade to windows and the ground below, and often has an aesthetic purpose. It also doesn’t require a foundation or posts that may interfere with lower-level views or use.

Are Cantilevered Decks Safe?

Cantilever decks are safe like any other deck if they are built properly and maintained. Some special rules affect the distance it can overhang in proportion to the backspan to maintain a balanced weight. Most cantilever decks are built to support a total load of 50psf, similar to other decks, including your living room floor.

Advantages and Disadvantages of Cantilevering a Deck

Cantilevered decks, like everything, have advantages and disadvantages. Since they are less common than other decks in residential areas, many presume that there are fewer advantages, which isn’t the case.

It’s actually less expensive to build a larger deck surface by cantilevering than a comparably sized deck supported at the end by a beam. However, there are more codes or rules to comply with when designing and building, so more difficult. Below, we identify and explain the advantages and disadvantages of cantilevering a deck.

Advantages

As mentioned, a cantilevered deck extends beyond the support beam, thus maximizing the joist span-to-support ratio. This often results in a larger deck surface to support structure, which usually translates into a money saver. Cantilevering can be very effective for second-story decks or if the lawn area is small underneath since the result is a larger deck but a smaller foundation footprint.

A cantilever may also be used to make a stronger, stiffer deck by moving the outer beam inward to maximize the overhang. For instance, a 14’ deck of 2×8 joists at 12” OC can span up to 14’-2” with a drop beam at 14’. This maximizes the span but results in a springier deck.

Moving the beam back to 12’ and cantilevering the same joists 2’, results in a deck that is the same size, but much less springy. So, although it won’t save money, it results in a stiffer, stronger deck.

Another structural significance is the aesthetic appeal of a cantilevered deck. The smaller support footprint, or seemingly floating aspect of an upper story cantilever, provides a less bulky visual. Additionally, since the outer edge isn’t directly over a support beam, the deck can be more easily sculpted or curved. This not only enhances the home’s façade but accentuates the deck itself.

A cantilevered deck, whether extending from upper-story floor joists or a ledger board, can provide a 180° or better panoramic view. It allows you to see a greater vista than standing at a window, especially at upper windows.

Although more common in high-density apartments, they provide an advantage to single-family dwellings too. Cantilevering a deck can result in a larger or stronger deck at a lower cost, plus it can have a more elegant shape than just a square or rectangle.

Disadvantages

A cantilevered deck, while possible for a DIYer, needs to comply with specific rules and building codes to ensure it is structurally sound and safe. It requires more knowledge of building practices, greater precision, and an understanding and awareness of building materials. The greater complexity makes it more complicated, which may disadvantage some.

Cantilevering a deck may require additional support from the ground or fastened to the building’s exterior. This may increase the budget, as well as the complexity of the build. It may also require the assistance of a structural professional more so than a non-cantilevered deck.

What Is the Maximum Cantilever Length?

The distance any joist can span or cantilever depends on the wood species, grade, dimensions, load factors, the spacing between parallel joists, and beam dimensions. According to Table R507.6 of the 2021 IRC, Southern Yellow Pine (SYP) offers the greatest cantilever.

A #2-2×12 SYP at 12” centers and live loads of 40psf, can span 18’-0” when supported by a drop beam at the end. However, if you wanted a larger deck, those same 2x12s could cantilever 4’-1” beyond that drop beam, resulting in a 22’-1” deep or wide deck. So, the maximum cantilever using dimensional #2-grade lumber is 4’-1”.

Materials Used in Cantilever Decks

The type of materials typically used when building a cantilever deck are similar to those used when building a standard drop beam deck. Concrete footings with galvanized steel brackets to support wooden posts.

Lag screws or bolts, nails or screws to fasten components, and joist hangers for the ledger plate. Pressure-treated dimensional lumber for beams, ledgers, joists, rim or band boards, and blocking, and then the desired decking material to finish it off.

However, a cantilever deck may be an extension of the joists used to form an interior floor and won’t require additional footings, posts, or beams. Alternatively, the deck might use decorative steel structural bracing that attaches to the exterior of the building to support it. Some cantilever decks are poured concrete with steel reinforcing, and others are ornately worked iron or steel.

Engineered wood products are also used for some components such as decking and rim, band, or facer boards due to their flexibility and lower maintenance requirements. Tempered glass, clear plastics, and other clear mediums are also used to improve views too – the Grand Canyon Skywalk or the panoramic glass floor terrace over the Cabo Girao cliff in Camara de Lobos, Portugal for example.

How to Design a Cantilevered Deck

A cantilever deck often begins with a similar design to a regular post and drop beam deck. Always check with the local building department for requirements, or with a Structural Engineer. Start with how and where it will attach to the house, its overall dimensions, and how it will be supported at the opposite end.

The joist spacing, wood species, and thickness determine the maximum span, which identifies the end beam location. The cantilevered deck frame must be able to support the cantilevered section plus people and furniture on it.

If that span results in the desired deck dimension with the drop beam at the end, then you can decide to cantilever the design. Cantilevering the design will result in a stronger, sturdier, less springy deck for the same cost.

Check the Building Code for joist backspan lengths and cantilever maximums (Table R507.6 of the 2021 IRC), and then move the drop beam location in the design back toward the ledger board that distance or less. The result will be a more stable deck of the desired dimensions with a smaller footprint.

Should the joist span not produce a deck of the desired dimensions, check the Code to see how far the joists can be cantilevered. If the added distance generates the deck dimension, then the added cost of longer joists versus an intermediate beam with posts and footings may prove negligible. The cost, along with span and cantilever distances, depends on the joist depth, spacing, wood species, and anticipated loads.

A key advantage to cantilevering joists is that the joist ends can remain the same lengths to form a straight, box-like finish. Alternatively, cutting the joists shorter towards the deck edges from the center will result in a curved deck edge. Thus, resulting in a gentler, more aesthetic finish. Plus, the smaller footprint uses up less of the ground underneath, resulting in more usable or open ground space.

Building Codes and Regulations for Cantilever Decks

Building a cantilever deck typically includes all the requirements for a regular deck, with some additional ones. The 2021 IRC and the AWC provide excellent information and guidelines for cantilever deck construction. However, always check with your local building department or a certified or licensed professional for local restrictions.

For example, while the IRC allows up to a 4’-1” overhang using #2 grade SYP lumber, your local code may not. The IRC and AWC also both identify the maximum joist cantilever as 1/4 of the backspan, many local building departments, though, restrict it to 2’ or less. So, regardless of allowable spans and cantilevers based on wood species, grade, dimension, spacing, or snow load rating, as identified in the IRC or by the AWC, always check locally.

Even so, there’s a lot of helpful information available in Section R507 of the IRC that addresses exterior deck construction. It also identifies the type of materials, fasteners, and connectors that should be used. In addition, it explains different minimal structural and construction aspects. Table R507.6 identifies the maximum joist spans and cantilevers by load, species, joist size, and spacing for #2-grade lumber.

The Code further identifies how blocking at the beam and joist hangers at the ledger board is to be used. It further explains how to fasten the ledger, rim, and band boards, and how to fasten railing posts. It should be noted too, that the ledger board cannot be attached to the rim joist of a cantilevered floor or building frame.

A cantilevered joist also has the same strength as one that is flush with the beam. Plus, the number and type of fasteners used to attach the rim joist to cantilevered joists are addressed in the Code too.

Cantilever Deck Framing Rules

Framing rules for cantilever decks include most of those that apply to decks flush with the beam. However, the joists extend beyond the beam a set distance allowable in the local code, which may be up to 1/4 of the joist’s backspan. The distance the joists cantilever depends on wood species, grade, spacing, beam construction, and loads.

The portion of the joists that cantilever is restricted by the distance of the joist’s backspan, this helps prevent teeter-tottering and bounce. The backspan needs to balance the cantilevered weight. Imagine six people standing at the railing and no one on the backspan, without proper balance, the deck will pivot at the beam. This also means the ledger attachment must follow rigid guidelines.

A cantilever deck often uses a ledger board attached to a building. The ledger needs to be securely fastened to the structure to counter the upward thrust on each joist when loads are applied beyond the beam. So, the ledger fastening requires extra attention to spacing and placement of lag bolts or screws, and even the use of hold-down tension devices.

The ledger must also be flashed to prevent moisture damage to the home’s band joist. Additionally, the ledger can’t attach to a rim joist of a building’s cantilevered floor.

The deck frame must also be built so it is strong enough to support all loads. Depending on geographic location, those loads may exceed 90psf, but they typically are expected to support a live load of 40psf and a dead load of 10psf, or 50psf in total.

It should be noted, blocking where the joists cross the beam plus hurricane straps or brackets to secure the joists to the beam to prevent uplift is addressed in the codes and regulations too.

Can You Cantilever a Free Standing Deck?

A free-standing deck is one that isn’t attached to a structure. It can be cantilevered up to 1/4 of the joist’s back span distance. Plus, beams can be cantilevered up to 1/4 of their support post distance. Thus, the deck could be cantilevered over its support foundation on all four sides, provided all codes and regulations are met to ensure attachment, balance, and structural integrity.

Figure R507.6 of the 2021 IRC depicts deck joists cantilevered over drop beams on two sides of a deck. Many above-ground pools have cantilevered free-standing decks too. It is recommended that all designs be vetted or produced by a licensed professional before being presented for a building permit.