Posted by bostondecksandporches | Dec 27, 2014 | Deck cost, Deck Structure, Deck Terms, Homeowner, Joist | 0
Cantilevers come to us from bridge architecture, where engineers have to span wide rivers with few supports. We can apply their cantilever¹engineering to deck construction to good advantage.
Cantilever Bridge under Construction
Forth Bridge, Edinburgh Scotland
Say you are designing a large deck and want to use materials efficiently. Standard deck structure is simple: extend the joists from the house out to a girder at the front edge.
Girder Deck – 3D
You can enlarge this deck by moving the girder further out, but only to the limits the joists can span — to 12′ 10″ for 2×8 joists 16″ on center or to 14′ 2″ if you place the joists 12″ on center.² More? You want more? Then spend the money to upgrade all the joists to 2×10. Or you can use some clever engineering: cantilever the joists.
Cantilevered Deck Frame
Instead of a girder at the end of the joists, you’ll need to put a beam under the joists, so they can extend past that support. This framing style is called post and beam, and the overhang is a cantilever. Now your deck can extend 17′ out: a 14′ span from house to beam plus a 3′ cantilever.³ Nice. For a thorough explanation of post and beam deck construction and comparisons with alternatives, see my article “What is Post and Beam and Why would your Deck Care?”.
17 ft Cantilevered Deck
The overhang by itself seems unsupported. Is it solid? Would you walk on it? It is safe — if you build it properly. There are different forces at work here, and you must follow some special rules.
- The cantilever length. How far out a cantilever can safely overhang is determined by the strength of the wood – the species, grade, and size – and by the spacing of the joists. Here we show #2, 2×8 Pressure Treated Southern Yellow Pine (SYP PT) joists, spaced 12″ on center to support our 3′ cantilever. The Building Code (IRC 2009 or IRC 2012) allows a 40 ½” cantilever for a live load of 40 pounds per square foot (psf); your engineer may allow more. Upgrade to 2×10 joists, and the Code says you can cantilever almost 5 feet.³
- Backspan.In addition to the limit on a cantilever’s overhang, a deck must also be proportioned to balance weight on its cantilever. The backspan of each joist – the distance from the beam back to the house ledger – must be sized properly. The Building Code requires a deck’s backspan be at least twice the cantilever extension. (footnote 4) This 2:1 ratio is liberal; I prefer a more conservative (safer) 3:1. (For more on backspan standards, see Note B, below.)
- Ledger attachment. A downward weight on the deck cantilever causes an upward thrust on the joist (the beam acts as the fulcrum of this lever). This upload must be resisted at the other end of the joist, where it attaches to the house with a joist hanger. In our example, a full design load on the cantilever causes an upload of 148 lbson each joist.³ A Simpson LUS 2×8 hanger, properly fastened to a PT ledger, can resist uploads up to 1,335 lbs. (footnote 5)
If this engineering seems complicated, consider the alternative. To extend a deck 17’ from a house with traditional girder construction, you’d need to upgrade all the joists and the ledger from 2x8s to 2x10s.² This brute force approach would add approximately$4 moreper joist; for a deck 16ft wide (including the 17 joists,ledger, rim joist, blocking, and joist hangers at the girder) that amounts to an additional $100.
Cantilevers do not save money on every deck. You can build 14’ out using 2×8 joists on a simple girder frame. The cantilevered version of this deck still wants 2×8 joists (2x6s would not suffice)², but it creates a stronger structure. The maximum span for SYP PT 2x8s 12” o/c is 14’ 2”², so a girder at 14’ 0” is close to the maximum. Conversely, a two foot cantilever puts the beam only12’ out, well short of the limit. Thus the cantilevered deck creates a stiffer platform. (footnote 6)
So cantilevering the joists allows you to build larger decks at lower cost orbuildstronger decks. That’s good — andit gets better. Cantilevers offer two other benefits. They pull the beam and supporting columns back, which softens the look of a deck. Consider:
Tall Cantilevered Deck in Sudbury, MA
Best of all, by divorcing the deck edge from the supporting structure, cantilevers give you the freedom to creatively shape your decks.You can efficiently build decks like the following without support columns at every corner.
Two Tiered Deck
Front Entry Deck
In all, a cantilevered deck can be larger, stronger, cost less — and be more interesting and even elegant. A winner.
Want to see more? This gallery displays a wide range of deck designs that all benefit from cantilevers.
Grand Canyon Skywalk
Now, here’s an impressive cantilever: the Skywalk over the Grand Canyon. Would you walk out there? Really? To the far edge?
A. For these design examples, I selected a live load (and snow load) of 40 pounds per square foot (psf) and dead load of 10 psf. Your local design loads, especially snow loads, may vary. (For a detailed discussion of snow loads on decks, see my blog article “Will Snow Damage my Deck?”) The specifications from the International Building Code 2009 that I reference here may seem aggressive. Check with your local building officials or your engineer; they may want more conservative parameters.
B. IRC 2009 and 2012, Table R502.3.3(2) address balconies, but that is the closest they come to deck cantilevers. Other backspan standards vary. The American Wood Council publishes a deck construction guide, Design Code for Acceptance, DCA 6, which specifies deck standards intended to augment the IRC. Because DCA 6 prescribes generic standards safe for any deck, without specific engineering, it is very conservative. DCA 6 prescribes a 4:1 backspan ratio.
C. For a discussion of other deck terms, see my article aboutunderstanding your deck contractor.
- The term cantilever first appeared in English the 1660s, as a combination of the word cant from the Old French, meaning “corner” or “edge”, plus lever, from Old French levour, “to raise“. From Webster’s New World Dictionary, second edition, 1986.
- From the International Residential Code 2009, Table R502.3.1(2), for #2 Southern Yellow Pine 2×8 joists, 16″ and 12″ o/c, supporting 40 psf live load + 10 psf dead load.
- Ibid, Table R502.3.3(2), interpolated for 40 psf live load.
- Ibid, Table R502.3.3(2), Note b.
- From Wood Construction Connector Catalog from Simpson Strong Tie, online, the table for LUS hangers fastened with SD screws
- Actually for this cantilevered configuration, you could save a few joists and a little money by setting the joists 16” o/c rather than 12” o/c.
1. Encyclopedia Britannica, online, http://www.britannica.com/EBchecked/topic/93144/cantilever.
2.2013-2014 Wood Construction Connector Catalog from Simpson Strong Tie, online, http://www.strongtie.com/ftp/catalogs/c-2013/C-2013-p071-073.pdf.
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Cantilevers provide a clear space underneath the beam without any supporting columns or bracing. Cantilevers became a popular structural form with the introduction of steel and reinforced concrete. They are used extensively in building construction, notably in: Cantilever bridges.Should you cantilever a deck? ›
The Many Benefits of a Cantilever Deck
If elevated and structurally licked to building walls, a cantilever deck allows unobstructed panoramic views that safely support the occupant loads. One of the main benefits of cantilever deck design is that the joists and the beams could run past the solid piers.
The American Wood Council states that cantilevers are limited to 1/4 the span of the joists. Joist Spacing (o.c.) Maximum allowable overhang cannot exceed 1/4 of the actual main span.What is the cantilever rule? ›
The general rule of thumb is that the back-span (the portion of beam supported below) should be twice the distance of the cantilever. The cantilevered beams then extend out to catch a W10x77 steel beam at the perimeter of the cantilever.What is a cantilever on a deck? ›
In simple terms, cantilever is the part of a deck that extends past a structural beam. A cantilever allows for enlarged square footage of the deck and also more creative design.How much weight can a cantilever support? ›
Cantilever columns run from 500 to almost 60,000 pounds capacity.What are the disadvantages of a cantilever? ›
Although cantilever bridges typically last for years, there is a higher chance of failure when bridges are not attached to the sturdier teeth found at the back of the mouth. Damage may occur. Because they are only anchored on one side, there is a slightly higher risk of cracking or debonding than with other bridges.Are cantilevered decks safe? ›
It is safe — if you build it properly. There are different forces at work here, and you must follow some special rules. Backspan. In addition to the limit on a cantilever's overhang, a deck must also be proportioned to balance weight on its cantilever.What do you put under a cantilever? ›
Insulating foam sheathing and/or house sheathing or siding should be attached to the underside of the cantilever floor joists as a protective covering. Air sealing and insulation materials may be installed by framers, insulators, and/or siding installers.What caused the cantilevered deck to collapse? ›
This is what happened to the deck – the joists rotted sufficiently to reduce the strength required to support the cantilever, and when a group of people gathered on the deck, it collapsed.
The most difficult challenge of a cantilever balcony is maintaining a weather tight joint where the joists pass through the wall. Joists will move with changes in moisture and temperature, as the result of expansion and contraction. The result is unseen rot and water damage, with a great safety risk.How do you support a cantilever deck? ›
Cantilever decks also function to save more money. One example is a second-floor cantilever deck. Instead of putting up a foundation on the ground, specifically footings, and piers, you simply use two large brackets connected to the house's exterior in a diagonal position.Can you attach a deck to a cantilever? ›
Decks typically shouldn't be attached to cantilevered floors*. The image above comes from the Prescriptive Residential Wood Deck Construction Guide.How far can you cantilever a 2x6 deck? ›
A Wood Council document (here) gives some very high-end detail about decks, but the take-away as far as cantilevers go is almost exactly the same: 400mm/16” for 2x6 or 2x8 lumber, and 600mm/24” for 2x10 or 2x12 lumber.How far can a floor be cantilevered? ›
The recommended maximum extension for a cantilevered exterior deck is four feet. In this situation, 2X joists should extend back into the floor at least eight feet, using a recommended 2:3 ratio. It is possible to extend beyond four feet but each situation must be examined and analyzed for structural capacity.Is cantilever expensive? ›
Architectural carports: A longer cantilever is an expensive feature since it takes substantial engineering and proper load calculations, but the effect is truly dramatic.Why is it called a cantilever? ›
Cantilever bridges were originally called cantlapper bridges—from cant, or "slope," eventually combined with lever, from the Latin levare, "to raise."What is an example of a cantilever? ›
A balcony protruding from a building would be an example of a cantilever. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete.What does a cantilever look like? ›
A cantilever is a rigid structural element that extends horizontally and is supported at only one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cantilever can be formed as a beam, plate, truss, or slab.How do cantilevers stay up? ›
Cantilevers are rigid structures, such as beams, which are fixed at one end and free at the other end. Some cantilevers can be supported throughout their length by trusses or cables. When a load is applied to the cantilever the cantilever transfers that load to the fixed end by bending.
According to IS code 456:2000, the maximum size (length) of the cantilever depends on the effective depth of the cantilever. * The span to depth ratio for a cantilever slab should not exceed 7. * Also maximum size of cantilever provided should be 10m i.e ;length of the cantilever should not be more than 10 meters.How much does a cantilever cost? ›
How Much Does a Cantilever Bridge Cost? Cost estimates for a cantilever bridge range from $500 to $2,700. Costs vary depending on your location, insurer, and the tooth or teeth being replaced. For a better idea of what a cantilever bridge might cost, consult your dentist.How much cantilever is safe? ›
The deflection limit for cantilever beams set by most design codes is L/180 for live load and L/90 for combined dead & live load. The maximum span depends on the material of the beam (wood, steel or concrete).Can you cantilever a freestanding deck? ›
Free-standing deck joists can also cantilever beyond drop beams at each end, extending the reach of a joist. For example, using DCA 6, 2×10 southern-pine joists at 16 in. on center have an allowable span of 14 ft. between beam faces.How does a cantilever affect building design? ›
Cantilever construction allows overhanging structures without additional supports and bracing. This structural element is widely used in the construction of bridges, towers, and buildings, and can add a unique beauty to the structure.What is the maximum cantilever for a balcony? ›
According to the new span tables and IRC provisions, cantilevers can extend up to one-fourth the backspan of the joist. This means that joists, such as southern pine 2x10s at 16 inches on-center, spanning 12 feet are allowed to cantilever up to an additional 3 feet (see illustration, below).What is the difference between overhang and cantilever? ›
Overhanging: A simple beam extending beyond its support on one end. Double overhanging: A simple beam with both ends extending beyond its supports. Continuous: A beam extending over more than two supports. Cantilever: A projecting beam fixed only at one end, while the other end has no support.What circumstances should cantilever foundation be used? ›
Cantilever footings are used when the column is constructed near the boundary of the plot, and it cannot be extended further. When the distance between two consecutive columns are very large, and it is not possible to provide combined footing. In that case, Cantilever footing is used.How do I strengthen my cantilever? ›
Reduce the load (w), move the load closer to support of cantilever beam if it's a point load, distribute the load more if distributed load. Decrease the length L. Increase the stiffness EI factor.What is the most common cause of deck collapse? ›
The most common cause of total deck collapse involves the deck “pulling away” from a house. This type of failure typically occurs because the ledger board was not properly attached to the house. Structural failures in decks also occur involving the support posts and deck joists.
The question arises when people decide they want to extend the length of their deck. Certainly it is easier to just add longer joists rather than move an existing supporting beam and post structure. The short answer is yes a four foot cantilever is not out of the question.Why is it important to know if a balcony is cantilevered when you observe wood rot at the joists? ›
One of the biggest concerns with cantilevered balconies is ensuring the joints on the wall remain free from moisture collection and wood rot. Additionally, drastic weather changes can expand and contract the wood weakening the connection between your balcony joist and the house or apartment building it's attached to.What is difference between cantilever and balcony? ›
Balconies differ from cantilevers like they don't generally have an open space below and they serve a mainly as an internal element. Whereas cantilevers are mainly an external element which can be used as an internal element also serving a dual purpose.How far can you cantilever a second floor? ›
The setback distance will be twice the length of the cantilever, but the cantilever cannot exceed 4′. But 4′ is sketchy. Use 2′ and you should have no problem.How does a cantilever balcony work? ›
A cantilevered balcony is a structure that extends outward away from a wall in a home or other building. It uses a cantilevered design — that is, the beams that hold up the balcony are secured only at one end, meaning the other end that extends outward from the structure is unsupported.Do deck boards need to overhang? ›
Allow for Sufficient Overhang of Decking
A one to two inch overhang beyond the outer fascia board is sufficient. This means the installed deck boards should have at least 1.75 to 2.75” overhang before the fascia is installed.
Cellulose insulation can be installed in the cantilevered floor joists two ways – through access from the crawl space or by drilling holes through the bottom of the overhang. Installing cellulose via the crawl space is fairly simple, as it is just blowing the material into the floor joists.How do you size a cantilever? ›
Calculate the bending moment due to the weight of the load. This equals the load's center of weight times its distance from the beam's support. For example, if 10 kg rectangular flower bed sits on a beam at between 15 and 20 m from the support, its induced bending moment would be: 17.5 m * 10 kg = 175 kg-m.Should I use 2x6 or 2x8 for deck? ›
Commonly used joist sizes are 2-by-6, 2-by-8 and 2-by-10. For example, when you space joists 16″ apart — which is standard for residential decks — a deck spanning 9 1/2′ would require 2-by-6 joists, a 13′ deck would need 2-by-8 joists and a 16′ deck calls for 2-by-10 joists.How far can a 2x8 deck joist span without support? ›
A rule of thumb is 1.5 times a joist's depth but in feet when spaced at 16” centers. In general, a 2×8 will span 1.5 x 8, so 12-feet. Based on all factors though, a 2×8 joist span is 7'-1” to 16'-6”, and a rafter 6'-7” and 23'-9”. What is this?
A 2×10 southern yellow pine joist can span 16 feet and 1 inch without support. A doubled 2×10 beam can span 11' without support for a deck that is 4' wide. Lengths of 2×10 joists and beams vary depending on the application you are using them for as well.Where are cantilever used? ›
Cantilevers are employed extensively in building construction and in machines. In building, any beam built into a wall and with the free end projecting forms a cantilever.Why is cantilever beam important? ›
When subjected to a structural load at its far, unsupported end, the cantilever carries the load to the support where it applies a shear stress and a bending moment. Cantilever construction allows overhanging structures without additional support.Why do people use cantilever bridges? ›
A cantilever dental bridge might be a good option if you have a tooth only on one side of your missing tooth – or when there aren't two abutment teeth healthy enough to support a bridge. Your dentist would fit the pontic in the gap, then anchor it to the remaining or healthy tooth on one side.What are the advantages of a cantilever roof? ›
Advantages of Cantilevered Beams and Trusses
Cantilever beam is simple in constructions. It does not require a support on the opposite side. Cantilevered structure generates a negative bending moment which counteracts positive bending moment of back-spans. Cantilevered trusses use less material.
A balcony protruding from a building would be an example of a cantilever. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete.What are the two disadvantages of cantilever bridges? ›
Although cantilever bridges typically last for years, there is a higher chance of failure when bridges are not attached to the sturdier teeth found at the back of the mouth. Damage may occur. Because they are only anchored on one side, there is a slightly higher risk of cracking or debonding than with other bridges.How far can a cantilever extend? ›
The recommended maximum extension for a cantilevered exterior deck is four feet. In this situation, 2X joists should extend back into the floor at least eight feet, using a recommended 2:3 ratio. It is possible to extend beyond four feet but each situation must be examined and analyzed for structural capacity.What is the difference between a cantilever and beam? ›
Continuous: A beam extending over more than two supports. Cantilever: A projecting beam fixed only at one end, while the other end has no support. Below we will analyze some classic loading examples of these beam categories. The simple beam above has two supports and a center load.What are the advantages and disadvantages of a cantilever bridges? ›
Cantilever Bridges experience a high amount of tension during the construction, which is known as a Negative moment. Cantilever bridges have to take care of its stability by balancing between compressive as well as tensile forces acting on it. Cantilever bridges are heavy, so it required larger and stronger support.
The main advantage of a cantilever bridge is that support is required only on one side of each cantilever. Simple columns can be used as support structures. Such bridges do not require falsework (temporary, supporting structure during construction), except for the pier.What are the disadvantages of a cantilever roof? ›
Disadvantages Of Cantilever Beam:
2. Generally results in larger moments. 3. You either need to have fixed support or have a back span and check for the uplift of the far support.