Cables Shouldn’t Be Too Tight or Too Loose
While the cables in your railing shouldn’t be tight to the point where they bend your rails or posts, they absolutely cannot be too loose. Loose, sagging cables aren’t attractive, but more importantly, they’re not up to cable railing deck code requirements. International residential codes specify that a four-inch diameter sphere shouldn’t be able to pass between vertical or horizontal cables. This is for child safety reasons, to prevent small heads and shoulders from being able to squeeze through the space. Loose cables let children stretch them and widen the space between cables to crawl or stick their head through. Even if no children are in the home, maintaining enough tension to prevent cables from moving or stretching is still a requirement to pass a building inspection. From his work in construction my friend knew this, which is why his first impulse was to massively over tighten.
My friend’s cable railings are vertical cable, and the result was a bent bottom rail on one of the shorter sections of the railing. Over tightening can also result in both rails–or posts, in the case of horizontal cable railings–bending inwards. Since railing codes dictate the height of the top rail this is also a potential code violation, and a safety hazard. Bent steel and aluminum have different, weaker force distribution characteristics than unbent metal, and bending permanently weakens them. Wood, on the other hand, has more flex to it, and an over tightened cable in a wood-framed railing can be released. If the wood hasn’t been damaged, it should return to being straight. However, it is possible to crack the wood by over tightening, and it might not be obvious that this has happened. To tension your cables correctly, you need to know how much force your frame can take before bending or breaking
How Tight Should Cable Railings Be? Consider Frame Strength
Both wood and steel are popular materials for cable railing frames, and they require slightly different tensions. While, as we mentioned earlier, wood has some flexibility to return to its original shape and position if you bend it through over tightening, wood’s strength is also a universal—and pretty measly—335 pounds per square inch. This a force load that you can easily exceed with a tension-er and a wrench, which means that wood can be broken as easily as it is bent. Metals are stronger, with steel tubing able to carry around a 1,000 pounds along a 6- or 8-foot section before yielding. As my friend’s example demonstrates, though, you can exceed this by just being overenthusiastic.
While codes specify that cables should be tight enough that they cannot be stretched apart to allow an object through, there is no single set standard for how tight cable railings should be. There are too many different manufacturers of cables and cable railings, and too many types of frames to definitively say. Estimates of the typical tension of a cable railing range from 200 to 400 pounds of force, but there are even higher estimates out there. Five hundred pounds of force is stated fairly often, and 1,000 pounds has been mentioned as a goal. Since wood’s reliable safety rating is about 335 pounds per square inch, this is cause for some concern, and would suggest proceeding with caution when creating a wood-framed cable railing.
I would suggest that if you’re building your own cable railings, you’ll want to aim for 200 to 300 pounds of force. That’s tight enough to prevent your cables from being deflected without stressing the frames–whatever they’re made of. Tension can be determined through the use of a cable tension meter, which retails for around $100 to $150 and has a handy table for translating the readings to pounds of force. Of course, when setting up your own railings, this is just an educated estimate. Those who want more definitive numbers and certainty that their railing frames will support the force may wish to choose a pre-engineered cable railing system from a manufacturer.
Pre-Engineered Cable Railings Offer Great Assurance
A cable railing system in which a professional has engineered the railings has an advantage in that everything is already calculated to hold up and to meet code. Manufacturers will also include a handy installation guide that provides directions on how to tighten the tensioners and how much they should be tightened to ensure correct installation and safety. Guidelines like these would have helped my friend avoid damaging his rails when he installed his railings, although whether he would have bothered to read the instructions is hard to say.
One thing to keep in mind is that once you’ve figured out what’s a reasonable tension to adjust your cables to, you’re not done. Cable railings generally depend on external, threaded tensioners to keep them tight. External forces–like fidgety board gamers waiting for their turns or even just wear and tear–can loosen them, meaning they’ll need to be re-tightened now and then. If re-tensioning your cables every so often doesn’t appeal to you, look for systems that use internal tensioners that will keep railings tight and reduce the need for maintenance over the life of the product.