People tend to overdo it with their barns. After all, the barn is kind of the centerpiece of any sort of working farm or ranch. If your operation involves interaction with the public, like a boarding stables or training facility, it makes sense to have an impressive structure...
For strictly practical purposes, it's a bit of overkill. People think that their critters are as uncomfortable in chilly weather as we are. But, unless you live way up north where the temperature slips well below zero and the wind chills even worse, most farm animals can tolerate the cold pretty well. (Except for sick, geriatric, or newborn specimens.)
In this part of Dixie, the more common problem is that Winter isn't cold enough. 25F and snowing wouldn't bother any of the livestock. 35F and raining is murder... And Summer heat is inevitably brutal. So all animals really need is a roof to keep off the freezing rain or provide some shade. The walls need not be solid, as free-flowing air is usually tolerable in Winter and absolutely essential in Summer. The best design may be little more than a row of covered corrals.
First thing to decide is where to put your barn. (See the Layout section.) It needs to be on high, solid ground. Animals will turn anything else into a muddy mess that'll suck your boots off. You also need to keep possible future expansion in-mind when selecting a site.
The proper first step to construction is to get a bulldozer and render your building area perfectly level... What you'll more likely do is find the flattest area available, then do the best you can to scrape down bumps and fill in holes with such tools and equipment as you have on hand.
If you have a perfectly flat foundation, you'll want to build professional style, keeping everything as close to true level as you can. This will look neat and tidy.
If you try building true level over an irregular foundation, you may find that you wind-up with low ceilings and head-bumping doorways in some places. It may be better to follow the lay of the land, making each vertical post the correct height relative to the floor. This could create a slight roller-coaster effect to your barn roof, but your head clearance will be more consistent. And looking a little shabby may not be a bad thing. (See Security section.)
A simple row of stalls is usually a good starter barn. It can be built quickly and economically, then easily upgraded and expanded as the need arises. 12' x 12' stalls are big enough for all but the largest of draft horses, and can be subdivided for smaller animals. 8'6" height at the front and 7' at the back provides adequate head room and draining pitch. You don't want to go too high, or rain will blow under even in mild storms. Doors always need to be at the front (high end) of the stall both for head clearance and to avoid the knee-deep muck from animals walking through the runoff-softened ground behind the barn.
Side view of stall row.
It takes five posts to build your first stall, then three more for each additional stall in the row. I've seen utility poles and railroad ties used to good effect. 4" x 4" lumber will do. I used treated 4" x 6" here. Depending on the post length available and nature of the ground you're building upon, it might be a good idea to set the posts in concrete... But I'd suggest waiting until you get some, maybe all, of your horizontal boards into place before doing so. Very difficult to re-position a post after the concrete sets-up!
While sinking your posts, you will be determining the width of your stall doors. I went with 6'. Narrow barn doors can get you into a wreck with bigger critters.
Your walls are basically just stout plank fences. I suggest 12' x 2" x 6" boards. (Rough cut is sturdier, if you can get it.) Spaced a board-width apart. Five planks will give you 5' in wall height, which is usually sufficient. If not, you can always add more planks to add height later. You can also fill-in the gaps between the planks with more boards should you need a solid wall for some reason.
Two stalls. Front view.
An extra plank of 12' x 2" x 6" board at the top of the posts, inside on the front, outside on the rear, will serve as the beams to support the roof. Six 16' x 2" x 4" boards will be placed on-edge, equally spaced across the beams to act as rafters for each stall. (One rafter flush with the each end of the beam, the others on 28.4" centers starting from the center of the end rafter.) I like to secure these with simple, steel hurricane tie straps fastened with wood screws, as it's easier to avoid knocking the rafters off their marks that way, and toe-nailing always seems a little sketchy to me. Have the front of the rafters overhang the beam by exactly two feet. Irregular overhang will mess with you when putting up the battens.
You may note that, where stalls meet, you wind-up with two rafters side-by-side. This will make it easier to nail the ends of the battens. It also keeps the roof framing modular, making the addition of future stalls to the row simpler.
Six 12' x 1" x 4" boards nailed flat-down across the rafters, equally spaced, will suffice for battens. (One flush with the front ends of the the rafters, another flush with the back, the rest on 37.6" centers.)
Posts & beams, then rafters, then battens.
Now comes the "tin"... Actually galvanized steel panels. I prefer corrugated (continuous ripple pattern) over 5V (flat with a ridge in the middle and two at each edge) because of its versatility. More options for nail placement and overlap.
The wide-spaced rafters and battens don't make for a structure you should be walking around on. But they do make it possible to work from an A-frame ladder, popping up through the framework to nail the tin to the battens.
Start at a corner of the barn and put a panel into place on the low end of the roof. (Remember, lower panels must go on first, so that the panels uphill won't drain UNDER them!) Position it so that the tin overhangs the frame evenly by a few of inches at the edges of the barn.
Nailing roofing tin is a little challenging. You have to figure out where the batten is. (X-ray vision would be a boon!) And you have to drive the nail into the metal at the peak of a ripple, not down in the valley. Water is going to be flowing down there when it rains. You need your nails punching through on the 'high ground' where they won't make leaks. You may want to hold the nail in-place with pliers, because you kinda' have to hit hard to get the point through the tin. Roofing nails for this application usually have rubber or lead washers to act as gaskets. You want to drive the nail just until there is solid pressure on the washer, NOT far enough to crush the tin flat into the batten.
If you somehow got a bit out of square anywhere along the line, and the tin doesn't want to go on perfectly straight, don't sweat it too much. Just try to keep your sawtooth effect consistent. Pretend you did it on purpose!
What I have detailed here is the economical roofing I've seen serving well on countless barns in the South and Mid-Atlantic, and what has worked on our barn for twenty years and counting. You can, of course, upgrade if you feel your circumstances warrant it. Heavier lumber, more rafters and battens closer-together. Plywood rather than battens. Shingles instead of corrugated. So long as it's you doing all that ladder work and not me!
Back on the ground, be sure to hing your stall doors so that they can swing 180 degrees and latch open, flat against the wall. Running (or being slammed) into an open door edgewise is hard on your bones.
Ideally, stall floors should be reinforced concrete with heavy rubber mats. But that can get expensive, so most folks go with the natural ground with any holes that develop packed with pit gravel. This usually works acceptably well for horses, goats, etc. if you do regular upkeep and the weather isn't godawful. Cows, however, will turn any unpaved stall or corral into a septic swamp. They're gifted that way.
One great thing about barns is that you don't have to go too big at the start. With foresight, you can begin with a modest structure and expand it later. The obvious way to do so with a simple barn is to add more stalls onto either end.
Another way to expand is to build a second mirror-image stall row parallel to the first, creating an aisle between them. You can take this up a notch by extending the roofs of the stall rows until they meet at a peak over the aisle, giving you a nice, covered area that comes in handy for all sorts of things and eliminated rain blowing into the fronts of the stalls.
Lofts are a common addition to barns. Traditionally used for hay storage.
Lofts are good for reducing hay loss to weather, rot, and rodents. But I've become rather uncomfortable with storing tons of highly flammable material in the same building with valuable animals, tack, tools, and equipment. I think it's better to store the hay away from the main barn. But a loft can be useful for other things, including living quarters.
The loft-over-aisle design became popular in the 20th century, with the transition to baled hay. It's relatively easy to construct and, I think, aesthetically pleasing. But it does tend to leave you with a relatively low ceiling over the aisle, and does not easily lend itself to discreet access from a floor-level tack room as the other loft styles do.
The loft-over-all design created the maximum loft space. This was very popular in the old days when hay was stored loose. If your place already has an old barn of this style, use a bit of caution with the loft floor... It may not have been built to withstand concentrated loads like a full loft of baled hay.
A few closing notes on the subject of the barn...
If you install electric lighting, make sure that it is impervious to the animals or out of their reach. It's really safer to have portable lights that you can take out with you rather than permanent fixtures in the stalls. I like to have all the power come through one heavy extension cord to an external outlet on the house so that I can completely disconnect the barn when unattended, and easily plug into an alternate source when the need arises.
Many farm animals are easily capable of breaking down stalls. Strategically placed hotwire is trashy-looking, but usually discourages barn wrecking.
Some barns use steel tube corral panels instead of board walls. We're experimenting with this as well. So far, our draft-cross fillies are beating the panels up pretty good. Something tougher may be in order.
Install good latches and hinges. Then back them up with safety chains. Some animals can channel Houdini!