Cable Railing Systems: Tension, Post Spacing, and Staying Compliant

The sphere rule applies to cable, under tension

The guard rule is the same for cable as for anything else: no opening may allow a 100 mm sphere to pass through. The complication is that a cable is flexible. Push a sphere against two adjacent horizontal cables and they spread apart, so the opening under load is larger than the opening at rest. Cable spacing therefore has to be set tighter than the bare 100 mm dimension would suggest, and the tension has to be high enough that the cables resist spreading when pushed.

• Spacing is measured under load, not at rest, because cable deflects when pushed.
• Tension is what resists spreading, so an undertensioned run can fail a sphere check that a tensioned one passes.
• Stainless cable stretches over its life, so tension drops without intervention.
• Longer spans deflect more for the same tension, which is why post spacing governs the design.

Post spacing is the single most important decision

Every problem in a cable railing traces back to how far apart the posts are. Longer spans mean more cable deflection under a given push, which means the cables have to be tensioned harder to stay compliant, which means a larger inward force pulling the end posts toward each other. That force is real and cumulative: a run with many cables applies a substantial combined tension load to the terminal posts. Underbuilt end posts bow inward over time, which slackens every cable in the run at once. The end posts and their anchors do the structural work in a cable system, and they should be sized and anchored for the full accumulated tension.

Intermediate supports keep long runs honest

Where a run has to be longer than the post spacing allows, intermediate supports carry the cables without terminating them. These are drilled posts or standoff spacers that the cable passes through, breaking the unsupported length into shorter segments so deflection stays within limits. They add cost and they interrupt the clean look slightly, and they are usually the difference between a long cable run that stays compliant and one that does not. Designers who want an uninterrupted forty-foot sightline with no intermediate posts are usually asking for something that will not hold its spacing.

Vertical cable is an alternative worth considering

Running the cables vertically instead of horizontally removes the climbability concern entirely, since there is no foothold, and it changes the tensioning geometry because the cables run between a top and bottom rail rather than between end posts. The trade is that the top and bottom rails now carry the tension and must be stiff enough not to bow. Vertical cable is less common in Metro Vancouver than horizontal but is a reasonable answer where children are present and the client still wants the cable look. Climbability considerations are covered in our guard climbability guide.

Stainless grade: 316 for anything near salt water

Cable and its fittings are stainless steel, and the grade decision matters more here than on most railing hardware because the fittings contain crevices where water sits. For interior work and inland exterior sites, 304 is generally adequate. For waterfront, near-shore, and salt-exposed Metro Vancouver locations, 316 is the specification, and the price difference is small relative to the cost of replacing a corroded run. The 304 versus 316 guide covers where the line falls. Note that stainless is not maintenance-free outdoors: it needs periodic washing to remove salt deposits, or it develops surface staining even at 316.

Hardware families and what they cost you later

Cable systems terminate in one of a few ways, and the choice affects both installation and every future retensioning.

What maintenance actually looks like

A cable guard is not a fit-and-forget product. It needs the tension checked periodically, more often in the first year or two while the initial stretch works itself out, and less often after that. It needs washing on exterior installations to keep salt off the fittings. And it needs the sphere check repeated whenever the tension is adjusted, because that is the reason the tension matters. We tell clients to plan on checking a new exterior cable run within the first year and then on a regular interval after that. An owner who will not do this is better served by pickets.

Where cable is the right answer, and where it is not

Cable is a good fit on a deck or balcony where the view is the reason the space exists and the budget will not stretch to glass, on a long modern interior run where the sightline matters, and on commercial and industrial stairs where the look suits the building. It is a poor fit where nobody will maintain it, where the run is very long with no tolerance for intermediate posts, and on a family home with a high drop where the horizontal geometry is a climbing concern. Cost comparison against picket infill is in our picket vs cable cost guide.

The failure we get called about

The recurring call is a cable deck railing three to six years old where the cables can be pushed apart by hand well past where they should move. Almost always the cause is a combination: post spacing that was too generous, end posts that were not built for the accumulated tension and have bowed inward, and no retensioning since installation. The fix is rarely just retensioning, because the geometry that caused it is still there. Adding intermediate posts, or replacing end posts with a properly anchored detail, is what actually resolves it.