The overhang that keeps rain off the wall drops it on the ground you stand on
A four foot overhang past the open side of a run-in does its job: it keeps weather out of the opening and gives a dry strip under the eave. What it also does is put the drip line four feet out from the wall, and the splash off that goes further — call it another eighteen inches on a hard surface, more off metal, more again in wind.
So water is landing on ground five and a half feet from the wall. Pour a four foot apron and the drip line falls a foot and a half past the edge of it, onto whatever is there. That strip is where the mud forms, it forms exactly where the animals walk in and out, and it forms in a line the full width of the shed.
| Overhang | Water lands at | 4 ft apron | Apron to cover it | Extra concrete, 24 ft wide at 4 in |
|---|---|---|---|---|
| 2 ft | 3.5 ft | 0.5 ft spare | not needed | none |
| 4 ft | 5.5 ft | 1.5 ft short | 5.5 ft | 0.44 cu yd |
| 6 ft | 7.5 ft | 3.5 ft short | 7.5 ft | 1.04 cu yd |
| 8 ft | 9.5 ft | 5.5 ft short | 9.5 ft | 1.63 cu yd |
Under half a yard of concrete closes the gap on the default numbers, which is less than most people expect and considerably less than the cost of dealing with the mud strip for the next fifteen years. The other route is a gutter on the eave, which moves the water somewhere you choose instead of somewhere the roof chooses. The calculator prints the gap and the extra volume so the comparison is a number rather than a feeling. For the concrete itself, the concrete slab calculator handles bags against ready-mix and the short-load fee that decides which is cheaper at this size.
Facing is a bearing comparison and not a wind model
Enter the direction the opening faces and the direction the prevailing wind comes from and the page gives you the angle between them and the cosine of that angle, which is the share of the opening face the wind is square-on to. An opening facing south at 180 degrees with wind from the north-west at 315 is 135 degrees off, and the cosine is negative, so nothing of the opening face is presented to it. Facing the opening south-east instead, at 135 degrees, puts it 180 degrees off — directly away.
That is geometry, and it is worth doing because it takes five seconds and it catches the shed drawn facing the wrong way on a site plan. It is not an assessment of shelter. On a real site the wind that matters is the one that gets past the tree line, over the bank and around the barn next to it, and none of that is in a compass bearing. Prevailing direction is also a seasonal average, and the storm that empties the shed is not the prevailing wind.
The per-head figure is yours, and it stays yours
The calculator will divide the footprint by the number of animals and it will multiply your per-head figure by that same count so the two sit side by side. It will not tell you what the per-head figure should be, it will not say whether the shed is big enough, and there is no combination of inputs that makes it say so. That number depends on species, size, age, whether they are horned, whether they are fed in there, the climate and the group dynamics, and it comes from your veterinarian and from the cooperative extension service in your state.
The same applies to the opening. How wide the opening should be, whether a wider one reduces the chance of an animal being trapped, and how much ventilation the building needs are all questions with real answers and none of them are here. What the page does with the opening is arithmetic: what fraction of the open side it is, and what bearing it faces.
What sits next to this
The building itself, if it is post-frame, comes out of the post-frame bay layout calculator — a 24 by 12 shed at 8 ft bays is four post lines a side, eight posts once the corners are shared, and whatever the endwall spacing adds between them. The cladding is the wall panel and wainscot take-off. For a fully enclosed barn with stalls and an aisle rather than an open shed, the stall block and aisle footprint calculator works from stall count instead of from a footprint, and the manure storage pad calculator sizes the pad for what comes out of it. Fencing around the shed is the fence post calculator, and if it is on a slope, the hardscape slope and drainage fall calculator works out which way the apron should fall.
Questions people ask
How many square feet per animal should a run-in shed have?
This page does not answer that and will not, whatever numbers you put in it. Space per head depends on species, body size, whether the animals are horned, whether they are fed inside, the climate, and how well the group gets on, and getting it wrong has consequences that a calculator has no business guessing at. The figure comes from your veterinarian and from the cooperative extension service for your state, both of whom can see your animals and your site. What this page does is take your figure, multiply it by your head count, and put the result beside the footprint you entered so the arithmetic is done for you.
Which way should the opening face?
Also not a question this page answers. What it does is compare two bearings you enter — the one the opening faces and the one the prevailing wind comes from — and report the angle between them and the fraction of the opening face square-on to that direction. That catches a shed drawn facing the wrong way, which is a real and common error. It is not shelter advice: on any real site the trees, the terrain and the neighbouring buildings do more than the compass, sun in winter pulls in the opposite direction from wind, and the prevailing bearing is a seasonal average that says nothing about storms.
Why is the drip line just the overhang distance?
Because that is the simple case and the calculator is honest about being the simple case. Water leaving the edge of a roof has horizontal velocity from running down the slope, so it lands further out than the eave, and how much further depends on the pitch, the roof surface, the rainfall rate and the wind. Metal throws further than shingles. Rather than invent a throw model, the page treats the eave edge as the drip line and gives you a splash allowance field to cover everything beyond it — set that figure from what you can see happening at a building you already have.
Is the apron worth pouring at all?
That is a judgement about your site and your budget, and the page gives you the two numbers the judgement needs: how far out the water actually lands, and how much material it takes to reach that far. On the default 24 ft shed with a 4 ft overhang, extending the apron from 4 ft to 5.5 ft is 0.44 cubic yards, which is under a yard and firmly in bag territory rather than truck territory. Whether compacted stone does the job instead of concrete is the other half of the question, and the calculator prices either — stone by the ton at the density your supplier quotes, concrete by the cubic yard.
Does the roofed area include the overhang on the sides and the back?
No. The overhang field applies to the open side only, because that is the one that governs both the drip line onto the apron and the shelter over the opening, and it is the one most often drawn different from the others. If your roof overhangs all four sides, work the roofed area out separately as width plus two overhangs by depth plus two overhangs. The footprint figure — the one the per-head arithmetic uses — is the inside floor and does not include any overhang at all, which is the right basis for that comparison.