Reinforcing MEMBRANE silpoly for ultralight tarps and shelters

24 May, 2016 9 comments Leave a comment

As we continue to push limits on weight and waterproofness with tarp/shelter materials, it's inevitable that some design tweaks will need to be made to keep designs reliable in the field.

About a month ago, I was contacted by a customer seeing some stitch elongation in a rectangular tarp he made with our Membrane Silpoly. We worked through the problem and now have what I think is a good path forward for reliable design, so I wanted to share some of that here.

To give a bit of background, here's a pic of the original problem. This was occurring at both ridgeline tie-outs, but not the ground tie-outs:

stitch elongation

Essentially what's happening here is that the reinforcement patch is too thick with not enough stretch to be compatible with the ultralight Membrane silpoly. In other words, under tension the tarp stretches and the reinforcement patch doesn't. With something this thin, that's not good. You need a different approach. 

Enter the radial reinforcement. After some back and forth and discussion with others, radial reinforcements using the same material emerged as a potential solution. This does two things:

1. Matches the stretch as much as possible between the reinforcement and the tarp material.

2. Spreads the force out radially from the concentrated stress of the tie-out area. This gives a more even distribution than the standard triangular patch.

This turned out to work very well as a retrofit to the rectangular tarp in question. Radial reinforcements were simply bonded with silnet over top of the original triangular reinforcement patches made from HyperD 300. I've kept in contact with the customer over the past month and the report has been excellent. This method of reinforcement appears to have fixed the problem.

Based on this success, I went off and did some testing of my own. For the reinforcement, I used a 4″ radial patch of Membrane Silpoly (same material used for tarp) bonded with sil-net and unsewn around the perimeter. Here are some pics:

Ultralight radial tarp template

Membrane ultralight tarp reinforcement

Ultralight tarp reinforcement panel tracing

Ultralight tarp reinforcement panels curing

I drew up a little template in lieu of using the coffee can, which makes it simple to cut out patches regardless of the tie-out shape. Applied a thin layer of sil-net to the patch, put down, then let cure overnight with some weight on top. Should be noted that I also cut the reinforcement patches in the approximate same direction as the intended force on the base tarp material to match the stretch (pic 3 above).

For testing, I vertically stitched on the 1/2″ grosgrain tie-out and then tested with 50 lbs. For extra stress, I lifted and dropped the whole thing ~2 ft. Inspected everything after and saw no evidence of stitch elongation at the grosgrain stitching or around the tie-out. Repeated a couple more times with the same result.

I then made a horizontal stitch on the grosgrain tie-out and retested. Still looked good. Some pics of the test setup and close-ups of the tie-out after testing:

Ultralight tarp tie-out strength testing

ultralight silpoly tarp radial reinforcement

Ultralight silpoly tarp radial reinforcement close-up

I also tested without reinforcements (just 1/2″ gg) under the same conditions. No catastrophic failure, but definitely some stitch elongation near the outermost stitch line on the grosgrain. No bueno. Here’s a pic of that:

Ultralight silpoly tarp no reinforcement close-up

Overall the silnet bonded radial reinforcement approach seemed to work very well - both for the finished tarp and in my personal tests. I don’t know if there’s an accurate way to convert wind speed to ~force on a tie-out, but I would think the wind would need to be pretty strong to produce a force that would cause this tie-out to snap.

Of course there’s also the force of tension due to the pitch and that’s going to vary depending on the design or shelter type, but hopefully this gives a good point of reference.

I’m going to rig up a way to add more weight and test to failure so that we have those numbers. I'm also planning to test the strength when pulling in the weft and warp directions (ridgeline/side tie-outs) instead of the bias. I’d like to do this same thing with Membrane silpoly PU4000.

In conclusion, my recommendation is that anyone making a tarp/shelter out of Membrane Silpoly use the same method of reinforcement I've detailed here. If you've already completed your tarp/shelter, just keep an eye on the wear around the tie-outs, especially at the ridgeline. You can always retrofit if need be.

To make this process easier for new builds, we're also going to spin off separate UL tarp kits that include 1/2" hardware, the radial template I used in the testing, and full instructions on how to modify the reinforcement areas.

If you have any questions, please don't hesitate. Thanks!

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  1. William Hanson October 12, 2016

    I used this method on a diy megatarp knock off but I sewed the perimeter for insurance and it did well so far.

  2. Beau Fabry August 22, 2016

    Great write up. When you get the rig for testing to breaking set up, I’d be interested to see what (if any) difference different thread/needle size combinations make as well.

  3. Ryan July 07, 2016

    Will this work okay on SilNylon?

  4. Todd Madsen July 06, 2016

    If I remember right from stage rigging classes wind can exert 12 pounds per square foot at 60mph, 6 pounds at 30mph. Wind gusts hitting one side of a 12×10 tarp can equal 720 pounds of force.

  5. Jeremy Franz July 05, 2016

    Would you recommend this approach to the normal (1.1 oz) Silpoly fabric also? Thanks!

  6. Kyle Baker June 06, 2016

    @shawn There’s less bias stretch with the PU4000, so I don’t think the chance of an issue is quite as high. That being said, it wouldn’t hurt to use this method and you would definitely save a bit of weight compared to using the more bulky HD300.

  7. Shawn June 01, 2016

    Thanks for the article. I recently purchased a tarp kit with sillily PU4000. Do you think that a double layer of this fabric would be needed as it is with the UL silpoly? Do you think that I can save some weight by using this method instead of the hyper D? Thanks again,

  8. Kyle Baker May 25, 2016

    Glad we could help John. I agree it’s a bit counter intuitive, but after you think about it things make sense. The extremes tend to illustrate the point and in this case the problem of mismatch doesn’t seem to crop up until you start working with these very thin, ultralight coated fabrics.

    Let us know if you need anything before your build. Thanks!

  9. John Gibby May 25, 2016

    Nice write up and good information. I don’t think I’m going to be able to afford making my own tarp until next year, but will be bookmarking this page to refer back to. I can see how using a tougher material than the tarp actually could end up hurting the situation, but I admit I’ve thought that way for a while also. It seems logical that a stronger, less stretchy material would “reinforce” the tie-outs. But I see now that under stress, those areas could still have problems because something has to give. I like the idea of bonding to the tie-out, a reinforcement material to give a stronger bond.

    Kudos! I can’t wait to build my own tarp.

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