Cable railing spacing: the 4-inch rule, post spacing, and tension

Why cable runs are spaced 3 inches apart, how post spacing and tension keep the 4-inch sphere out, and what changes on stairs.

Most cable railing systems space horizontal cables no more than 3 inches apart and place a post or intermediate support at least every 3 feet. That combination, plus proper tension, is what keeps a 4-inch sphere from passing through the guard when someone pushes on the cables. Cable flexes in a way that rigid pickets do not, so spacing, post layout, and tension have to be planned together.

The model code limits guard openings to a 4-inch sphere.

Under the 2021 International Residential Code (IRC), Section R312.1.3, required guards must not have openings that allow a 4-inch-diameter sphere to pass through. The same section allows slightly larger openings in two places on stairs:

• Guards on the open sides of stairs may allow a 4 3/8-inch sphere to pass.
• The triangular opening formed by the riser, tread, and bottom rail may allow a 6-inch sphere to pass, but not larger.

Guards are required where a walking surface is more than 30 inches above the floor or grade below, measured within 36 inches horizontally of the open edge (IRC R312.1.1). Commercial and multifamily work under the International Building Code uses the same 4-inch rule in Section 1015.4. Your jurisdiction may have adopted a different edition or local amendments, so always confirm with your local building department before you order.

Cable is spaced at 3 inches because it deflects under load.

A picket stays where you put it. A cable does not. When a child or a pet leans into a cable run, the cable bows, and the gap between two adjacent cables can grow. If you spaced cables a full 4 inches apart, almost any push would open the gap past the limit.

Cable railing manufacturer Feeney recommends spacing cables no more than 3 inches on center. Feeney notes that this allows up to about an inch of deflection between two adjacent cables while still meeting the 4-inch requirement. Feeney also allows the top cable to sit up to 3 1/2 inches below the top rail, and the bottom cable up to 3 1/2 inches above the deck or bottom rail. Always follow the manufacturer’s current installation guide for the system you buy, since each system is tested to its own layout.

Post spacing and intermediate supports control how far a cable can bow.

The longer the unsupported span, the more a cable can deflect at midspan. That is why manufacturers set a maximum free span along with the cable spacing. Feeney’s wood and metal frame requirements call for a post or vertical spacer at least every 3 feet. Feeney also allows posts up to 6 feet apart when an intermediate picket or spacer sits at midspan.

Intermediate pickets and spacers are usually non-structural. They do not hold up the top rail. Their job is to keep the cables in line and cut the free span in half. Structural posts still have to carry the top rail and resist guard loads. The IRC requires guards to resist a 200-pound concentrated load applied in any direction at the top (Table R301.5).

These figures come from Feeney’s published frame requirements. Other systems, including our own Axiom South Cable Railing, may use different limits, so check the guide for the hardware you are using.

Tension and frame strength go together.

Spacing only works if the cables are tight. Feeney states that properly installed cables typically carry 200 to 300 pounds of tension each. On a 42-inch rail with 11 or 12 cables, that adds up to thousands of pounds pulling the end posts toward each other. If the end posts or top rail flex, the cables lose tension and the gaps open.

• End and corner posts must be stiff enough to hold full tension without bending. Feeney recommends a 4×4 minimum for wood and notes that softer woods may need larger posts, especially at 42 inches.
• A rigid top rail tied to every post keeps the posts from pulling inward.
• Tighten from the middle cable outward, alternating up and down, so the posts load evenly.
• Use a tension gauge if the manufacturer offers one, and recheck tension after the first few weeks and periodically after that.

Cable diameter changes the look, not the spacing rule.

The most common cable size is 1/8 inch, usually 1×19 strand construction in 316 stainless. Feeney also offers 3/16-inch and 1/4-inch cable and notes that 1/4-inch cable is not recommended for wood frames. A thicker cable is a little stiffer, but you should still follow the spacing and post rules in the installation guide. Near salt water, 316 stainless is the standard choice, and our article on aluminum vs. steel railing on the coast explains why.

Stairs need angled holes and attention to the triangle.

On stairs, cables run parallel to the stair pitch. The holes in the stair posts must match that angle, and manufacturers use beveled washers or angled fittings so the end hardware bears flat against the post. Feeney also calls for protector sleeves where a cable enters a wood post at an angle, such as at stair transition posts.

Watch the triangle at the bottom of each tread. The cable closest to the stair nosings, together with the tread and riser, forms the opening that must block a 6-inch sphere under IRC R312.1.3. Measure that opening on a real stair, not just the drawing.

Some jurisdictions restrict horizontal infill.

The 2000 IRC once barred guards built with horizontal rails that create a ladder effect. That language was removed in a later supplement and is not in the current IRC or IBC. Some local jurisdictions still prohibit or restrict climbable horizontal infill through their own amendments, especially on multifamily and commercial projects. If your project has a climbability rule, vertical cable or a different infill may be required. Ask your building department before you finalize the design.

Frequently asked questions.

Can I space cables 4 inches apart if I tighten them hard?

No. Even tight cable deflects under a push. Manufacturers such as Feeney test their systems at 3 inches on center or less so the gap stays under 4 inches when loaded.

How far apart can cable railing posts be?

Feeney limits the free span to 36 inches and allows posts up to 6 feet apart with a midspan spacer. Your system’s guide sets the actual limit.

Do intermediate pickets have to be structural?

Usually not. They keep cables aligned and reduce deflection, while the posts and top rail carry the loads.

Is horizontal cable legal everywhere?

The model codes allow it, but some local amendments restrict climbable guards. Confirm with your local building department.

If you are planning a cable project, send us your run lengths, corners, stairs, and rail height through the quote request form, and we will lay out posts, spacers, and cable for the system you choose. You can compare options on our cable railing page. We supply the materials and are building a nationwide network of independent installers.