should a lightweight turnout rug be 100% waterproof even in the worst weather?

If it's waterproof, you shouldn't leave it on 24/7. It'll trap her own moisture inside, causing discomfort and even rain rot.

If she was wet just on her quarters and sides, it sounds reasonably waterproof. Rain can blow in from below and through any openings, and soak from there. If she layed down on wet grass, she'll get wet that way.

Just make sure you take it off and let her dry out. Don't put the blanket back on until it's dry, too.

Unless it's quite chilly, it's really better not to rug them up. Mine have excellent field shelters and it's windy and rainy, but they're out happily grazing. They don't mind.

Well are you sure its waterproof? You may have bought a stable blanket by accident...If your horse has a shelter (which is should or you technically arent providing proper care for it : food, water, shelter etc.) A shelter is no biggie to build...get a permit, and build a 3 sided-shelter...thats better than a rug for the rain...and you will want a thick blanket for her for the winter (350grams or more of insulation)

What a awful rug! of course,a turnout rug should be waterproof.i think you can try to use other brands.

i bought a HH rug for my pony,it kept my boy dry and warm.i also like HW but its rugs are a bit expensive.and you can buy HH and HW turnout rugs from ebay.

A turnout should be waterproof... yes.

That being said, depending on where you bought it, I have seen and heard of complaints from people buying generic, possibly homemade turnouts on eBay and they turned out to be not waterproof.

Sif a rug is going to bo wet proof, even a light weight.

Beside your horse might have just sat down or rolled..