Depends-do you get a lot of snow? My old sheds were 10' high at the opening and when it was very snowy, they could bonk their heads on the way in. My new one (different property) is a giant and my horse HATES it. I'd guess the top of the doorway is about 20' high. I made it higher bc I learned the snow lesson and we get more snow here. It is lovely and airy in there, nice footing, good shade, no bugs. He HATES it. Won't even go in there. If I drag him in, he bolts out. WTH? I think maybe because the doorway is so huge it echoes in there and that upsets him. I am about to carve some 'windows' in the back wall to see if that'll help but I fear my answer is to lower the roof! ;-/ So basically, I'm no help except to say don't make it too tall!
Proud and achy member of the Eventing Grannies clique.
Mine are 9' low side with a 3/12 pitch, so nearly 12 in front. I did it 10' low side to start and it looked so ridiculous I had them lower it a foot. That was too tall! It looked silly, higher than it was wide, and my run-in is not small --14 by 36. It is still pretty tall and I have no worries that a horse will hit its head.
The shed off of the side of my barn is 7' high on the low side and maybe 10' on the part next to the barn? I hate it and have seen the horses KICK the ceiling of the shed (well, only my mare by accident while being witchy, but still!). We don't get snow....in fact I would say that the sheds get taller here in the winter because of the mud
My youngster also has two big lumps on his head (an inch or so above the noseband in the pic) that I would put money on being a result of that low side. With that being said, the horses love it in there and prefer hanging out there to just about anywhere else in their field. We built a stand-alone shelter with plans to get rid of the overhang and it's 9' on the low side and 12' on the high side. The horses won't stand under there for anything. So they still live in their short(ish) one. Fortunately my gelding seems to have moved past the destroy-himself-on-anything-and-everything phase (the random puncture wounds all over his lower half and war wounds around his head were really a special part of his 3yo year ). But if I were to rebuild I would definitely aim for higher than 8' and preferably closer to 9'.
__________________________________ Forever exiled in the NW.
Every horse I who has been at my place strongly prefers the too-low shed over the one adjacent to it that is of a normal height. The only difference is height. If my horses, and other horses I've had hear, are representative, you will have to fence them out of the Alpaca shed or those are the ones that they will choose to use.