UKC

What the title says. I've always worn my (reasonable quality Decathlon) waterproof jacket underneath my (ditto) running backpack, but it occurs to me that that may make it less waterproof and also of course slower to put on (I was very conscious of that when struggling to waterproof up at the start of the thunderstorm in the Lakeland Trails 55km last month). What do others do? If on top of backpack, do you go one size bigger to accommodate the bulge?

Harrier do a running waterproof jacket with a pleated back that goes over a race vest, but never used it in anger. 

Most people I’ve seen go under vest, some pros go over. 
mine fits over but is a bit of a faff to get it on and off if carrying a lot in the vest, I usually stop and put it on underneath though (good excuse for a rest) but over does make more sense as you can just grab it from the dump pocket and put it on quickly without stopping 

I seem to recall reading that Salomon do ( or at least used to do ) a jacket that was designed to go over a race vest, and also that was designed so that you left it done at the waist and took it off the arms and upper part of your body and it could be rolled down into a big sort of waist sleeve. The idea was if runing in changeable conditions you could change from wearing it just around your waist to a full on jacket over your pack, without needing to stop and faff about

Haven't done it with a waterproof but I bought a windproof a size bigger to do just this and it's been a bit of a revelation, seem way less faff and you need to take the vest off a lot less often. Not sure if I'd do it with a waterproof or not. I guess the inner surface of a waterproof might be less robust than the outer one?

I have an (now old and not-on-sale-anymore) Salomon running vest. My waterproof always goes over and I think it's totally(*) the way to go:
- keeps the vest drier,
- it's a bit more waterproof because no straps / backpack are squashing water through the fabric,
- if it's just drizzly and you open the front it feels better ventilated (less muggy),
- if it's windy and freezing it keeps the drink pouches warmer and the nozzles don't freeze up on longer winter days.  

Of course the waterproof has to be big enough (possibly slightly bigger than otherwise) to accommodate this. For me it worked with Ronhill waterproofs, OMM Kamleika and now a Salomon one(**).

The only downside is that to get at anything in the vest one has to either roll up the waterproof from the bottom or open the front ... but let's be honest - if it's bucketing it down you're probably not that dry anyway and if the rain is not so heavy / intermittent than it's ok.

(*) at least for my Scottish hills running. Also on my vest I can take the waterproof out without taking the vest off, so then it's very natural to just put it over the vest. 
(**) I have a Salomon waterproof that in fact has seemingly spare fabric on the back for the vest to fit in. But otherwise I wouldn't recommend the waterproof because it doesn't have any means for adjusting the hood and hence the hood flies off in Scottish winds wind annoying regularity. 

Correct, its the Bonatti Pro.

They do, but it only fits over the smaller vest and a fully loaded 12L is uncomfortable.

Thank you all - very helpful!

Lizzie