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 Railing Code
The International Residential Code (IRC), adopted by most US cities and towns, requires 36-in.-high deck railings for decks more than 30 in. above the ground. The railing must be strong enough to resist the following loads:
- 125 pound uniform load (per linear foot) applied horizontally or vertically to the railing
- 200 pound point load applied at the top of the post in any direction, or at any point along the railing.
With the standard safety factor of 2.5 used by engineers, that means railing and posts must be designed to resist a 300-pound uniform load or a 500 pound point load. The main concern is a push outward from people leaning against the railing. Few older deck railings can meet this requirement, but building inspectors and professional deck builders are starting to pay 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. The only practical way to resist this force with 4×4 wood posts is to with specialized steel framing connectors. Both Simpson Strong Tie and USP now make connectors 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 not 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 outer joist (called a rim or band joist) and whether the rim joist runs perpendicular or parallel to the deck joists. To avoid confusion, we’ll refer to parallel rim joist as an end joist.
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 to post to the rim joist between the two connectors.
Outside the end joist. Where the rim joist runs parallel to the joists (sometimes called an “end joist”) blocking must be added to tie into the second joist. This joist 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.
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 joist, you can use two corner posts, as shown in the detail above. With two corner posts, the top rails extend beyond the posts and meet at the corner. Another option is to use a single post at the corner, installed the same way as the inside posts in Detail A, above.
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.
If using steel tension ties seems like overkill, there are simpler methods for attaching guardrail posts. 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 the fast code-compliant connections, use timber screws instead of through-bolts. In general these don’t need pre-drilling and can provide structurally ratings that will meet deck codes. The detail below, from Simpson StrongTie, is specifically for their SDWS Timber Screws, but TimberLok screws or other structurally rated timber screws can work as well.
RAILINGS & BALUSTERS
To meet code standards, the railings that span from post to post should be as sturdy as the posts themselves. To achieve the necessary strength, it’s best to use at least 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 rainy 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.
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.
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.
Cable railings. 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 (max. space 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.
Manufactured Railings. 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 stand 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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