Civil Engineering

Building an exterior deck is an exciting outdoor project and a great way to extend your home’s living space.

Deck construction, while certainly not the easiest of projects, can be done by anyone with a carpentry skillset and decent construction wherewithal. Some homeowners opt to build decks themselves, while plenty more opt to hire a handyman for the job. There are, however, several pitfalls associated with building outdoor decks that you, as a homeowner, should pay careful attention to, whether you’re going the DIY route or hiring a contractor.

This article will focus on five key design aspects relating to deck construction:

Ledger-board Attachment & Water Intrusion Proper Joist & Beam Sizing Beam Connections Guards & Railings Foundations There are a few immediate actions you’ll need to take before diving into the design and construction process. The first step is to verify permitting requirements and any special conditions with your authority having jurisdiction (“AHJ”).

Some local town ordinances, for instance, mandate that any proposed deck with an elevation of 30-inches or greater above an existing grade requires a work permit. To avoid any legal snafus, be sure to contact a building official about any conditions that may apply to your area.

Once you obtain approval from your relevant AHJ, your next step should be to call an NYS Digsafe ticket, if digging is involved. This ticket will verify the location of any underground utilities in your work area, which is essential to your safety and for your deck’s footing placement. This is a free service, but it does have a lead time involved – give yourself at least 3-5 business days of cushion time for this step.

Ledger-board Attachment & Water Intrusion A combination of improper deck ledger attachment and water penetration is perhaps the number one cause of deck failures. Therefore, it is critical that your deck be both anchored securely to the band joist (or other structural member) of your house and properly designed to shed water. If you cannot access your home’s band joist due to a finished basement or other obstruction, your best move will be to install a free-standing deck (which is unfortunately outside the scope of this article).

Ledger-Board Attachment

Your deck’s attachment to your home’s band joist must be accomplished with minimum 1/2” diameter lag screws or through-bolts (toenails or nails subject to withdrawal are prohibited). You can use specialty fasteners but be sure to follow manufacturer’s instruction and check with the building official prior to use. The 1/2” fastener on-center spacing on the ledger will vary based on your joist span distance and exterior sheathing thickness. See Table 1 from the NYS Residential Building Code:

Table 1 – Deck Ledger Connection to Band Joist In this design guide, we’ll use a 16’ joist span as our example. As you can see on the chart, when using 1/2” lag bolts with 1/2” thick sheathing, you must space the bolts 21” apart. In addition, these bolts must be spaced in accordance with Figure 1.

Figure 1 – Placement of Lag Screws and Bolts in Ledgers Note: The staggered installation method helps keep the ledger from cracking.

Figure 2 – Placement of Lag Screws and Bolts in Band Joists Deck lateral loads must now be accounted for according to new code changes. They must be transferred either to the ground or to a structure capable of transmitting them to the ground. This can be accomplished via two methods:

Where a lateral connection is provided in accordance with Figure 3, the hold-down tension devices shall be installed in not less than two locations per deck, within 24 inches of each end of the deck. Each device shall have an allowable stress design capacity of not less than 1,500 pounds. Where the lateral connection is provided in accordance with Figure 4, the hold-down tension devices shall be installed in not less than four locations per deck, and each device shall have an allowable stress design capacity of not less than 750 pounds. Figure 3 – Deck Attachment for Lateral Loads Figure 4 – Deck Attachment for Lateral Loads Water Intrusion

Properly designing and building a ledger board deck that can prevent water intrusion is critical to a successful deck construction project.

Failing to install adequate flashing at your deck’s ledger board can be a very costly oversight. That’s because water intrusion can jeopardize the structural integrity of your deck and its supporting structure. Essentially, this could cause the non-pressure treated band joist of your house to eventually rot out and deck to pull away from the house.

There are several steps involved in flashing a ledger board:

Install a water and ice shield member between the ledger board and band joist Install a Z-flashing over the ledger board to direct water away from the house Install J-channel siding along the decking surface once completed.

It’s always a good idea to seek advice from a licensed professional here. You’ll also want to check with your relevant permitting office to see what approvals are necessary and what possible variations are acceptable.

Proper Joist and Beam Sizing Beam Sizing and Spacing

In a nutshell, a deck beam is a structural member that supports the joists. Joists can either bear on top of the beam (“dropped beam”), or the beam can support the joists via joist hangers (“flush beam”).

In our example, we will use a dropped beam configuration. See figure 5 below:

Figure 5 – Typical Deck Beam Span The allowable beam span is based on the nominal size and lumber species of the beam as well as the joist span. Remember, in this guide, we are assuming a joist span of 16’.

With that info at hand, let’s determine the beam span and the allowable cantilever using (2)-2”x12” Southern Pine.

See Table 2 below, for spacing requirements.

Table 2 – Deck Beam Span Lengths (ft-in) Note : The allowable cantilever is 1/4 of the beam span.

Assuming a dropped beam configuration and the use of three support posts, the total width of the deck is approximately 18’-8”,if we use (2)- 2”x12” nominal lumber. It is important to realize this setup assumes certain loading conditions (40 psf snow & live, 10 psf dead loads). Always verify any exceptions with your AHJ.

Joist Sizing and Spacing Joist sizing and spacing information is available in the joist table as shown below. In most cases, the allowable joist span depends on the thickness of the decking, the nominal size and lumber species of the joist, and the center-on-center spacing.

Using our 16’ span criteria, check the chart below to find the required spacing of the deck joists using 2”x10” lumber. To achieve our 16’ distance, we would need to space the joists 12” on center, with a maximum cantilever of 3-4”.

Table 3 – Typical Deck Joist Spans (ft.-in.) Figure 6 – Typical Deck Joist Spans Note : If you wanted to reduce the number of joists needed, you would have to increase the size of the joist to 2×12. These could then be spaced every 16”. This would also allow you to have a longer cantilever, at 4’-2”.

To sum up, based on the specifications for our example deck, we have determined:

Minimum 1/2” through-bolts for ledger board attachment spaced every 21” in a staggered fashion as per figure 1. To achieve a 16’ span with a 3’-4” overhang, we would have to use 2x10s spaced every 12” on-center. The deck beam would consist of (2) 2x12s in a dropped beam configuration. Overall deck dimensions 18’x8” x 19’-6” If you happen to be installing composite decking, it’s important to review the manufacturer’s installation instructions. Depending on the thickness of the boards (e.g., 1” or 2”), some composite decking may not allow the joists to be spaced greater than 16” on-center.

Guards, Railings, and Stairs The consensus among building codes is that all decks greater than 30” above grade are required to have a guard. Most municipalities have adopted this requirement as well.

Always discuss your deck specifications with your building official and follow applicable permitting requirements.

Below is an example guard diagram, courtesy of the American Wood Council:

Figure 7 – Example Guard Detail Note some of requirements:

Minimum nominal 4×4 post (with notching prohibited, as notching reduces the structural strength of the post). Minimum nominal 2×8 rim or outer joist. Minimum 1/2” diameter through-bolts and washers Other methods and materials may be used for guard construction when approved by your AHJ. Handrails and guards shall be designed to resist a single concentrated load of 200 pounds.

Residential codes also stipulate certain requirements for handrails and guards, including:

Handrails shall be provided on not less than one side of each flight of stairs with four or more risers . Handrail height, measured vertically from the sloped plane adjoining the tread nosing, or finish surface of ramp slope, shall be not less than 34 inches and not more than 38 inches A stair guard is required for stairs with a total rise of 30” or more Figure 8 – Handrail & Stair Guard Requirements See Wood Deck Construction Guide and 2020 Residential Code of NYS for further information regarding these requirements.

Foundations According to the relevant codes, decks shall be supported on concrete footings, as shown in Figure 9. Other systems, however, can be used if they’re accepted by your building official.

Deck footings must be sized to carry the imposed loads from the deck structure to the ground. In addition, they need to meet footing frost depth requirements – generally abound 48” in the Central NY area.

And remember, always use pressure-treated (“PT”) lumber when ground contact is anticipated.

Figure 9 – Deck Posts to Deck Footing Connection Footing sizing is based on the tributary area of the supporting footing, soil bearing capacity, and the highest load case (Dead + Live) or (Dead + Snow). The tributary area is the portion of the deck weight a particular footing will be carrying (i.e., the load).

Table 4 – Presumptive Load-Bearing Values of Foundation Materials Let’s assume our tributary area is 80 sq. ft., and let’s further assume it will be bearing on a sandy gravel type subbase (3,000 lbs./sf). Based on the table below, we can use a 15”x15” square footing (or 17” round) that is 6” thick. In most Upstate New York regions, the ground snow load is 50 psf.

Table 5 – Minimum Footing Size for Decks And don’t worry, you need not be a geologist or soil scientist to confirm your subbase type. Normally, these can be identified through inexpensive methods or even just a visual inspection.

Concluding Remarks

A safe, durable deck can add value to your home while providing a wonderful setting for outdoor enjoyment. If you’ve got some intermediate carpentry experience, constructing a deck may well be a viable – and even fun – DIY project.

And if you’re not confident in your own construction abilities, there are plenty of handymen and contractors who would be happy to take on your project.

Regardless of the route you choose, you’ll want to make sure that the five main pillars of proper deck construction – ledger-board attachment and water intrusion prevention; proper joist and beam sizing; beam connections; guards and railings; and foundations – are adequately addressed.

Having these five pillars properly dealt with will save you lots of time, money, and headache down the road.

Structural Disclaimer: This article is intended for information purposes only. It is not intended to relieve the user of this article of responsibility for a proper condition assessment and structural evaluation by an experienced licensed qualified design professional.

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