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This is a question borne purely of aesthetics. My wife doesn't want anything blocking the view out of the deck.

I'd like to install string lights above the deck. The deck has the house on two sides and nothing on the other two. It measures ~21'x16'.

My thought was to install a pole on the far corner from the house and run rope wire from the house to that pole (pink line in picture). Then, I'd run the string-lights-on-guide-wire from the house to the wire (black zig-zag lines).

My only experience with rope wire is an old laundry line. I know it can be strong, but I'm not sure if it can withstand this sort of stress. The string lights can't weight more than 20lbs, but I'm worried that the stress of a nearly 20' run, combined with the pulling towards the house is going to be too much.

Can anyone walk me through how to figure this out?

House Diagram

Update: Here's a thought for overcoming the pulling force on the end of a long pole...

Brace Idea

  • It's not an aesthetic question (which would be off-topic). It's a structural one. – isherwood May 29 '18 at 15:52
  • FYI, the keyword for lighting swept across areas from strings (e.g. Christmas tree farm) is festoon lighting. – Harper May 29 '18 at 17:00
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Funny, I'm facing this exact project with the same spousal parameters at home. Your issue isn't with wire or rope strength. That's not a concern, though 1/16" steel cable is what I'll probably use to keep the diameter small.

You're only talking a few tens of lbs. of tension. Try it yourself by running out an appropriate length of twine and pulling until you have the sag you're after. A gently-sagging string of lights actually tends to look better--more relaxed--so you might consider not pulling it laser tight.

Your concern is with the post. You'll need it quite tall--probably 8' or higher depending on the aesthetic you want. And it will need to be rigid. Multiple cables pulling the same direction will want to tilt or flex the post, so it needs to be robust and have a very solid footing or mounting.

If you accomplish that, the rest is cake. No physics or trigonometry necessary. (I'm not sure why some folks insist on a pile of math when some good ol' intuition does just fine.) Select a rope or wire that won't rot away in a few months and be happy.

  • Keeping aesthetics in mind, I've been thinking of using a relatively wide black/galvanized pipe, secured in the ground and levered against the deck. No idea of the amount of flex in a pipe at 8', or if the lever against the deck will kink it right out of the gate. Time to experiment. – nathanziarek May 29 '18 at 17:56
  • Could work. 3" is probably adequate. I was going to use a 4x4 cedar post, but I'm not pulling things tight. – isherwood May 29 '18 at 17:58
  • Here's another thought ... run the cable through the support pole, then down at a pretty sharp angle to a terminal point. This would stop the force being applied to "pull" the pole, no? !Brace Idea – nathanziarek May 29 '18 at 20:33
  • The brace idea would only work if the cable was fastened to the pole such that it couldn't slide (making it essential independent from the light support cable). – isherwood Jun 11 '18 at 18:50
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Practical Advice

Let's consider the solution to a more difficult application: hanging a fabric shade sail. The static load is higher due to the weight of the fabric and the wind load is higher due to the sail effect. Furthermore, when a company publishes instructions for hanging this shade they have a strong interest in making sure it doesn't rip off your house. So if you follow their advice for you application you have applied a strong margin of safety.

For a 12' sail the linked instructions specify a 1/8" thick galvanized steel 4" x 4" post mounted 10 degrees from vertical or a 5" x 5" timber post mounted 20 degrees from vertical. Either post to be mounted in a concrete footing measuring 15.5" square and 31" deep. If you anchor the primary wire into the ground you could further stiffen this setup.

For the house anchors they recommend steel supports between the fascia and the rafter/truss. I suspect this is overkill for the four secondary anchors but may be advised for the primary wire if you want it to be rather taut. As @isherwood suggested mock it up and tug hard on the primary wire to get a sense of the forces involved.

Way more then you want or need to consider

Force on a deflected wire

First consider Why Can't a Rope be Pulled Completely Straight. Each wire end will exert a transverse force F_tw = mg / (2 sin θ) where m is a single mass at the center of the wire, g is the acceleration due to gravity (9.8 m/s/s), and θ is the angle of deflection. If you are willing to accept a 12" deflection of the primary wire then F_tw ~ 50 N/kg * m = 16.7 ft * m.

For m on the primary wire use the total mass of the supported wires and lights as well as the primary wire itself.

Dynamic loads

Besides the static load of the wires and lights you have to consider transverse wind and ice loads. Utility companies do this when sizing poles (pdf). In the coldest climates they plan for 4 lbs/sq ft wind load on a wire radius that has increased by 0.5" due to sleet and ice accumulation. In warmer climates there's no increase to the wire radius but the wind load planned is 9 lbs/sq ft.

When doing these calculations it is customary to ignore the bracing effect of the other wires. So when computing the wind load on the primary wire we would ignore the fact that the light wires will resist the wind blowing away from the house.

For your configuration I would consider two orthogonal loads. The primary support wire can push right-to-left. The other wires can pull in-and-out of your figure. In each case add up the sq ft of wire and lights and multiply to calculate a wind load. Your support beam will need to support this load at the anchor point near it's top. Your other anchor points will need to support some fraction of these loads as well.

Force on a cantilevered beam

You need a beam that can support the resulting transverse wire and transverse wind load. It also needs to support it's own vertical weight but this is not normally an issue. Your question doesn't specify the height but the deflection of the pole grows linearly with the height of the mount point. Assuming the pole can handle the loads it must be securely anchored into the ground like a fence post.

Building anchors

The wires need to be anchored to the building. For highest strength you should anchor to the fascia (not the soffit) at a point close to the rafters. Don't forget that these anchor points have to carry the very same loads as the post. There are four secondary anchor points splitting the load from the lights and one primary anchor point which carries the same load as the post.

Safety Factors

When planning an installation it's important to apply safety factors. Overload capacity factors for utility poles are included in the link above and they range from 1.0 to 2.5. The strength of the pole or anchor points are also derated by a factor as low as 0.65. These are values applied by knowledgeable engineers. For a DIY project I would replace brains with brawn and build it at least twice as strong.

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It'll be a pretty simple geometry problem to find out the strain on the support wire.

You will also want to have a concrete footing for the pole and some bracing, either by continuing the wire down to the ground and anchoring it down there or a diagonal support from the inside. (I suggest the latter to avoid tripping hazard).

As for the strain strength required find out the pull on each attachment point and brush up on your trigonometry and the static force diagrams from physics class. From that you will find that the more slack you allow in the support wire (or the more bend you allow), the less strong it needs to be. High strength weather-proof steel wire shouldn't be that hard to find in your hardware store.

  • I'm not sure the suggestions to brush up on trigonometry and physics are really helpful; the rest of your answer was. Can you link to some sources online where that information could be found? – UnhandledExcepSean May 29 '18 at 13:26
  • There are also math and physics stacks here. I agree you'll pull the pole over sideways unless you use a guy or push stick to stabilize it, or just install a very, very, very tough pole. However the guy doesn't need to diagonal straight to the ground, it can continue horizontally to another pole and then diagonal to the ground. – Harper May 29 '18 at 14:59
  • Geometry doesn't concern itself with forces such as tension. – isherwood May 29 '18 at 15:52
  • @isherwood At the attachment points the support wire is bent as the light wire exerts a force and the support wire must counteract that with a force that is equal and opposite. The closer the bend is to 180° the more tension there must be on the support wire to be able to have that supporting force. – ratchet freak May 29 '18 at 16:01
  • I've done my share of math and physics coursework, and I'm not sure what you're saying. The angle of attachment is unrelated to the tension in the span. That said, I have little interest in semantic or mathematical debate. I have loads of practical experience that inform my planning, and in most cases that's more useful. – isherwood May 29 '18 at 16:07

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