Tarpaulin repair

Try a chandeliers or boat yard.

Why dont you try one patch sewn, one glued with contact adhesive. Nothing much to lose, and you might be able to stop paying in future. I dont have the experience you want.

NT

Sew or weld (ultrasonic), both of which require large-scale non-DIY kit. Gluing (right glue, which is uncommon but available) might work for vinyl but good tarps are polyurethane rather than vinyl and so it doesn't.

I also repaired one of my tarps with pop-riveted aluminium. As it was only used to cover one machine outdoors, and reguarly wore through in the same couple of spots (where it sat on the girders) then the easiest fix was to rivet in metal patches at just that location. I used some cheap bowls in pairs, and the tarp also became self-locating and easier to install.

The sewing cant be done by hand?

NT

Purchase an eyelet making kit. Purchase a pack of good quality heavy duty cable ties. Purchase a few tubes of silicon mastic.

Make holes for the eyelets along each edge of the worst tear approx 1" apart around 2" in from each edge opposite each other across the rip. Use plenty to reduce tear possibilities between the eyelets Overlap the edges to align the eyelets on top of each other. Thread the ties through the eyelets longitudinally after spreading a good film of mastic between the canvas edges. Seal with more mastic through the eyelets and await to dry. If required add waterproof Gaffa tape over the eyelets to give more protection.

Actually, somewhere I've got a sailmaker's palm, but one keeps a SWMBO for sewing tasks, and she has already complained about the tarp being laid on the lawn.

Pop-riveting plates sounds like a useful idea, but for another year, perhaps. This is a boat cover, about 25 feet square and sits on jagged bits of mast, stanchions etc over the winter. We try to protect it by things like bits of old plastic guttering or carpet over the protrusions, but it is on a site where I've frequently been unable to stand in the freezing, gusting wind. The ropes often saw themselves apart in the trailer so everything becomes a bit of a movable fiasco. I can only just lift the tarp, so anything like sewing on the lawn would be a huge job.

Google brings up a few sites selling things like hot air guns for welding this sort of stuff, but I have no idea what the plastic component of it is. It is the usual blue or green heavy stuff they use on lorries.

I was hoping someone would say something like "xxxx glue with a hot air gun is what you need", but I think I'm resigned to my over-wintering gaffer tape this year.

Are they "plastic" I thought they where canvas thus cotton. 25 ft square and plastic I wouldn't expect to be particulary heavy, decent bit of canvas on the other hand would be rather heavy.

Canvas would be repaired using sail making techniques, sewn on patches etc I expect you can fine the stiches and methods to use on the web. I can't see why the same techniques wouldn't work on a plastic tarp provided you took into account the possibly larger weave and the layout of the "threads" making it up.

It's definitely heavy! It is quite a close weave - sort of coated canvassy stuff. The cotton or whatever inside is white, but the blue 'plastic coating covers this up on both sides. One side is fairly shiny, the other shows the pattern of the weave. It's nothing like the 'covers' they sell in Poundland or Halfords. When I've taken it back to the makers to repair, they have stuck patches on in some way - no stitching.

Anyway, time is running out. As soon as a capable son comes home we'll have to dart across the country and heave it on.

Thanks everyone, I'll think about all the suggestions for next year.

You can sew canvas and modern synthetic canvas by hand, with suitably large needles and a palm.

As the OP suggested "welding", then I suspect they have a polyurethane coated fabric. This stuff is, IMHO, unsewable by hand. It's too "grabby" on the needle. YMMV. A triangular needle might do it.

You could try sticking it. The polyurethane glue sold for resoling soles is likely to be your best bet (NB - _NOT_ Evo-stik, that's rubber in naptha) as the chemistry will at least be thinking about compatibility. However I've never found that stuff too good when flexed as much as a tarpaulin.

Probably worth the try though, and better than gaffer tape.