Probably the one thing that people may have heard about natural fibre ropes is shrinkage. Here I’ll tell you just what effect that has and how to avoid problems plus an idea for how much to allow so your ropes drape nicely between fixings.
Well how long is, or should be, a piece of string? No seriously you really don’t want to get to the end of a rope project and find you haven’t quite got enough. If you are laying a patio and run out of slabs or planting a border and haven’t quite got enough plants it's not the end of the world, you can pop out and buy some more. However, if you are even 25cm short of the last post on a 5m rope and post fence you are probably in big trouble. I don’t really want you to buy too much rope and waste it but I’d hate you to end up just a bit short when a quick chat could have suggested you were heading for trouble. Anyway, these are some of the things you need to consider when working out how much rope you will need.
This is the biggest thing to remember when installing natural fibre rope in a wet or damp environment. (You don’t usually need to allow for this when using ropes indoors) When ropes like manila and sisal get wet they will shrink in length and at the same time they will get fatter.
There really isn’t anything you can do to stop this and its important you take it into account because the pull the rope can exert is substantial. I know of a Garden Designer who installed some 18mm Manila taut between 3” posts around a deck and the first time it rained the shrinkage of the rope tore one of the posts off its fixing. I have also heard this was used as an ancient method of torture – that’s the tightening of a rope around someone not taking the mickey out of an unfortunate Garden designer!
The biggest change occurs the first time the rope becomes wet. After that it will be much more stable but you will still see a difference in the length of a rope between a soaking wet winter’s day and the end of a long, dry spell. A customer from Scotland once told me about the effect on a swing made with Manila rope on which he had played as a child in India. When he played on the swing after a damp, humid night his feet would be well clear of the ground - but if it was a hot, dry day by the end of it he would have to lift his feet up at the bottom of every swoop.
I have measured first time shrinkage on both 8mm and 32mm Manila rope and came up with a figure of 8%. I suggest you allow 10%, but you can usually round things down a little to make the maths easier.
You may have heard about pre-soaking. Basically this is where you take a natural fibre rope, like manila, and put it into a dustbin or other large container of water for a few hours, depending on how thick it is. Then you take it out and allow the rope to dry. (If you want to really see how much it changes then measure the rope when its dry and again when its wet). After you have done this the rope will be much more stable in length. However, it can take some time for the rope to fully dry, maybe a couple of weeks for something like 36mm Manila when its not very warm. You don’t have to wait for it to dry but as the rope is fatter and stiffer when its wet you’ll find threading it through post holes and tying knots will be easier with dry rope.
In many cases where you are installing the rope in your own garden and in such a way that you can adjust it easily it might be worth putting the rope up dry with lots of slack and letting it rain. Then once the rope has shrunk you can make all your adjustments.
If you are a professional gardener or landscaper installing the rope in a customers garden then popping back to make adjustments when it has rained may be impractical. In this case it might be best to soak the rope and install it whilst its still wet, putting up with the handling difficulties. The benefit is that you know the rope should not become any shorter. If you do this don’t forget you may need larger holes in any posts to accommodate the fatter rope and any knots you tie may slacken off slightly as the rope dries.
Sag between Posts
In most applications, whether it be Rose swags or a rope and post fence it is more pleasing to the eye to have some sag or drape to the rope. Customers can get quite anxious about working out exactly the right amount of extra rope to allow for this. Please don’t. Forget the very long-winded mathematical way to get an answer and just use a tape measure or any old piece of rope. Stretch it taut over roughly the span you want, note how long it is and then let some slack through until it looks about right. When you see how much the length has changed you will almost certainly be surprised, it is usually a tiny amount. Over a 1.5 to 2m span of rope you might only need an extra 5-6cm.