Sturdy deck railings are an important safety feature for raised decks and are required by code for any deck 30 in. or more above grade. Building codes define the minimum height and strength of the railing system, as well as the size of gaps in the railing – they must be small enough that a small child cannot slip through.
On many older decks the 4×4 posts were notched and then lagged to the rim joist with 3/8 in. or 1/2 in lag bolts or structural screws such as LedgerLoks. More conscientious builders may have used through bolts. The post-to-joist connections were often strong enough, especially with bolts. The weak link in the system, however, was the rim joist that the post was fastened to. Apply enough force to the top of railing and the rim joist would rotate. This can be addressed by reinforcing the rim joist connection to the floor framing (details, below).
On some decks, it’s possible to use 4×4 or larger posts that run continuously from the railing to the footings, supporting both the deck structure and the railing. As long as the posts are not notched, these easily meet the most stringent deck codes.
Another approach gaining in popularity is to use steel connectors to reinforce the post attachment. By using special framing connectors engineered for this specific application, you automatically get a rugged railing and a code-approved connection. And installation is pretty straightforward once and efficient once you’ve done a few decks this way.
Whatever approach you use, make sure your lags, bolts, or other hardware are suitable for today’s highly corrosive treated lumber. That means heavy hot-dipped-galvanized or polymer coatings approved for use with pressure-treated lumber.
Deck Guardrail Code
The International Residential Code (IRC), adopted by most US cities and towns, requires 36-in.-high guardrails (called “guards” in the 2015 IRC and later versions) for decks more than 30 in. above the ground. The guardrail system must be strong enough to resist the following loads:
- Uniform Load: 50-pound live load (per linear foot) applied in any direction along the top rail
- Point Load: 200-pound concentrated load applied in any direction at any point along the top rail or top of a post
- Infill Load: 50 pounds of force applied horizontally over one square foot of the balusters or other infill material (area below the top rail)
- Safety Factor: Remember to multiply all these loads by 2.5 for the required safety factor.
With the standard safety factor of 2.5 used by engineers, that means railing and posts must be designed to resist a 125-pound uniform load or a 500-pound point load, and transfer that load through supports to the structure. The infill (details below) must resist 125 pounds over one square foot. The code specifies the load requirements, but does not tell you how to meet it.
The main concern is a push outward from people leaning against the railing. Few older deck railings can meet the code requirement, but building inspectors and professional deck builders are paying more attention to this critical detail. Even if you can squeak by with a weaker railing, do you really want to?
In general, deck posts should be spaced no more than 6 feet apart. Beyond that amount, it becomes very difficult to meet the safety requirements.
Attaching Deck Railing Posts
The problem is that the railing post acts like a lever, exerting a very large force on the connection at the bottom. The weakest link is not the post-to-rim-joist connection, but the rim joist to the floor framing.
One practical way to resist this force with 4×4 wood posts is to use specialized steel framing connectors. Both Simpson Strong Tie and USP now make steel tension ties designed for this application. The connectors fasten to a deck joist (or joist blocking) with heavy ¼ x 1½-in. wood screws and bolt through the post with a ½-in. diameter through-bolts or threaded rods.
The Simpson connectors (DTT2Z ) can be purchased prepacked with the approved screws and a washer for the bolt head in the post. The USP version (DTB-TZ) installs with the company’s polymer-coated WS15-GC screws. Both companies also sell stainless-steel connectors and screws, designated SS. As an engineered connection, it’s always best to use the fasteners specified by the manufacturer and to stick with the same company for connectors and screws.
While the connectors are moderately expensive at $8 to $10 each (less by the box), they install quickly and provide a rock-solid connection that is virtually impossible to achieve any other way.
The installation details differ somewhat depending on whether the post is installed inside or outside the deck joists. To avoid confusion we’ll refer to the outer joist that runs perpendicular to the deck joists as the rim or band joist. The outer joists that run parallel to the deck joists are called end joists.
The most difficult part of the installation is accurately drilling two 9/16-in. holes in each post for the 1/2-inch bolts or threaded rod. Most installers use a marking jig to locate the holes. A drill press is helpful to keep the holes square to the post. Otherwise a good eye and steady hand are essential.
In general, the steel connector is used with the upper bolt, about 2 inches down from the top of the joist. The lower hole receives a bolt with washers. For longer connections, threaded steel rod is used rather than bolts.
Outside the rim joist. This is the most common installation and the simplest if the post aligns with a floor joist (see Detail A, below). If the post is between two joists, you will need to use two connectors, one on each nearby joist, and bolt the post to the rim joist between the two connectors.
Outside the end joist. Where the post attaches to an end joist (running parallel to the deck joists) blocking must be added to tie into the second joist. This detail requires two connectors as shows (Detail B, below).
Inside the rim joist. Posts placed inside the rim joist need to align with a perpendicular joist. The connecting joist needs to be reinforced with blocking fastened with 24 10d nails. (Detail A, below).
Inside the end joist. This connection requires double blocking, plus an extra long bolt or threaded rod to transfer the load to the next joist bay (Detail B, below). A single corner post (not shown) can be installed the same way. Remember all hardware must be rated for use with pressure-treated lumber — either hot-dip galvanized or treated with approved coatings.
Connecting a post to an end joist requires blocking in two joist bays plus a long threaded rod. Source: Simpson Strong Tie
Many of the commonly used guardrail details are shown in the Prescriptive Residential Wood Deck Construction Guide, which shows accepted industry practices that have been shown to meet the requirements of the code. Deck hardware manufacturers, such as Simpson, also publish details that have been shown to meet the code through independent testing and the issuance of a “code evaluation report.”
You can use other attachment methods as long as they are approved by the local inspector, who may require an engineers stamp. If using steel tension ties seems like overkill, some inspectors allow simpler methods for attaching guardrail posts. For example, two ½ in. bolts through the rim or end joist can meet the building code as long as the rim joist is adequately braced against rotating.
Depending on the local code and its interpretation by the building inspector (e.g., the safety factor required), the detail shown below should be acceptable in many locales. If in doubt, have an engineer calculate the specific size and number of lag bolts or structural screws to meet the local code.
For faster connections, designers have developed post details that use approved timber screws instead of through-bolts. In general timber screws don’t need pre-drilling (except into end grain) and can provide structurally ratings that meet deck codes. The detail below, from Simpson StrongTie, is code-tested with their SDWS Timber Screws, but TimberLok screws or other structurally rated timber screws can work as well, subject to local code approval.
In an effort to comply with modern codes, many deck builders now use two posts at each outside corner, set several inches back from the corner. While very sturdy, this approach is complex and costly. Newer one-post corner designs that comply with code are discussed below. Outside corners pose special problems with post brackets. To keep the bolts from hitting one another, they need to be offset vertically about 1/2 inch, and additional blocking is often required.
Posts on the inside. With posts on the inside of deck joists , you can use two corner posts, as shown in detail A above. With two corner posts, the top rails extend beyond the posts and meet at the corner.
Posts on the outside (set back from corner). With posts outside the joists, the simplest approach is to use two posts at each corner, held back far enough to line up with the second joist, about 12 to 16 inches from the corner. The top rails extend beyond the posts to meet at the corner (see detail, below left).
Posts on the outside (close to corner). If you prefer the corner posts to be close together, you can use the same steel brackets with blocking on one side to position and support the second bracket (detail, below right). Also, you must offset the top and bottom bolts by 1/2 inch vertically. Note: You can view both both the upper bolt, with bracket, and lower bolt with washers in Detail A and B above.
SINGLE-POST OUTSIDE CORNERS
To help deck builders assemble faster and simpler railings, fastener manufacturers have developed and tested one-post corner designs built with no extra hardware and no pre-drilling required. These designs are certified to meet the requirements of the IRC through code Evaluation Reports (ICC-ES), which use third-party independent testing to establish code compliance. [Note: This is the standard approach used by manufacturers to establish code compliance.]
One approach (illustration below) uses Simpson’s SDWS Timber Screws, but other comparable timber screws should work subject to approval by your local inspector. The second approach uses FastenMaster’s proprietary no-drill through-bolts called Thru-Loks. For more detailed information, review the technical guides from Simpson Strong Tie and FastenMaster.
RAILINGS & BALUSTERS
To meet code standards, the top railings that span from post to post should be as sturdy as the posts themselves. To meet code, the top of the guardrail must resist a 200-pound load applied in any direction. To achieve this strength, it’s best to use at least a 2×6 railing, either flat or vertical. Use the longest pieces available, with a continuous length for each side of the deck, if possible. Posts should be no more than 6 to 8 feet apart, depending on local codes.
In wet climates, the top railing should be either sloped or chamfered at the edges to shed water. A drip cap below overhanging edges is also recommended (see illustration).
At inside and outside corners where two railings meet, I often use a galvanized steel angle or flat brace to support the joint . This is cheap, but effective, insurance against the corner separating over time. I typically use similar steel hardware to reinforce where the railing connects to the main structure.
Where the guardrail is used at stairways and landings at least 30 in. above grade, you also need to include a code-approved handrail. Since flat 2x4s and 2x6s do not meet the handrail code, a separate handrail is often attached with metal brackets or blocking.
Code requirements for balusters. For child safety, the balusters or other decorative infill must be spaced less than 4 inches apart. In code-speak, a 4-in.-diameter ball should not pass through the spaces. For railings along stairs, the spaces can be a little larger, but less than 4-3/8 in.
The balusters or other infill between the posts should be able to resist a load of 50 pounds applied over one square foot. Using the engineer’s safety factor of 2.5, the test load would be 125 pounds. To comply with code, and for common sense, you don’t want to fill between your sturdy posts and top rails with flimsy infill.
Baluster and infill options. On a site-built wood railing, balusters can be metal, steel cable, or any sturdy material, but the most common are 2×2’s, nailed or screwed directly to the rim joist or attached to the bottom rail. Three common baluster layouts are shown in the illustration above.
With 2x2s, choose good stock and reject any pieces with large knots or other defects that might weaken the baluster. Use stainless steel, coated, or hot-dipped galvanized nails or screws that are rate for use with today’s pressure-treated wood. At a minimum, use either one screw or two spiral-shank nails top and bottom on each baluster.
A common detail has the balusters nailed to top and bottom 2×4 rails, with the ends of the rails toe-nailed to the sides of the vertical posts. Toe-nails tend to split the 2x4s and are not likely to meet the code requirements for the infill area. Steel brackets, like the RailLok, can simplify the connection of the top and bottom railings to the posts.
Horizontal infill made from stainless-steel cable is a popular choice on high-end decks, especially where the owners want unobstructed views. Like all infill options, these need to comply with the 4-inch rule (less than 4 in. between cables) and structural codes. Manufactured systems are available with all the components including steel posts. It is not too difficult to build your own cable railing with components now readily available from specialty manufacturers (photos below). Still plan on some extra time until you learn the particular system – each has its quirks.
Most major manufacturers of composite and plastic decking offer manufactured railing systems that can be used with their decking or as a standalone system. Most prefab systems go together pretty quickly and are engineered to meet the strength requirements of the building code.
They also offer the promise of low maintenance, although this is less of a concern than within decking. Vertical components, such as railing systems, get less sun exposure and less wear and tear than decking, so they tend to hold up well over time. A flat 2×6 used for the top railing that is neither sloped or chamfered is, however, prone to the same type of cupping and checking that affects decking boards.
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