Four separate things take height, and only one of them is the slope
People plan a patio cover by picking a height at the house and a slope, and then discover the far end is low. The reason is that the slope is only the first of four deductions, and on a shallow roof it is not even the largest one.
Work the default numbers. The roof surface starts 108 inches above the slab at the wall. A 1-in-12 slope over a 12 ft projection takes 12 inches. The 2x8 rafter takes another 7-1/4. The doubled 2x10 beam the rafters sit on takes 9-1/4 more. That is 28-1/2 inches gone, leaving 79-1/2 inches — and only 12 of those 28 came from the slope. Then the slab gives a little back: a patio falling a quarter inch per foot away from the house has dropped 3 inches by the outer beam, so the clear height there is 82-1/2 inches, or 6 ft 10-1/2 in.
| Line | Inches | Running height above the slab at the wall |
|---|---|---|
| Top of roof at the wall | — | 108 |
| Fall, 1 in/ft over 12 ft | −12 | 96 |
| Rafter depth, 2x8 | −7.25 | 88.75 |
| Beam depth, 2x10 | −9.25 | 79.5 |
| Slab falling 0.25 in/ft | +3 | 82.5 clear under the beam |
The overhang keeps falling after the beam, so the lowest thing on the structure is the drip edge and not the beam. A 12 inch tail at this slope takes another inch off the roof, and the slab has only dropped a quarter inch further, so the tail underside is 91 inches up — higher than the beam, because the beam depth is not in that line at all. That reversal surprises people. The clearance you actually walk under at the edge of a cover is the beam, and the clearance a ladder or a wheelbarrow needs at the drip edge is a different, usually friendlier number.
Hanging the rafters instead of stacking them
If the rafters hang inside the beam on hangers rather than sitting on top of it, the beam depth comes out of the calculation entirely. On the numbers above that is 9-1/4 inches straight back into the headroom, taking 82-1/2 to 91-3/4 without moving anything else. It is not free — flush framing puts the connection in shear on hangers instead of in bearing, and the outer beam becomes a piece you look at rather than a piece you look under. Whether it is available to you is a question for whoever is sizing the frame. But it is the largest single lever on this page, and it is worth asking about before the slope gets steepened to compensate.
The same arithmetic run backwards is the more useful direction if the projection is not fixed yet. Wanting 84 inches clear at the beam, from 108 at the wall, with a 2x8 rafter and a 2x10 beam, leaves 7-1/2 inches of height to spend. The roof takes an inch per foot and the slab hands a quarter inch per foot back, so the net cost is three quarters of an inch per foot of projection, and 7.5 divided by 0.75 is 10 ft exactly. Anything past that and the beam is below the target — which is why the default twelve foot cover comes out two feet too deep for a seven foot ceiling. The slope is the other knob: at a fixed 12 ft projection the same numbers allow up to 0.875 in per ft before 84 inches is lost.
What the wall will accept is usually the real constraint
The height at the house is rarely a free choice. On a single storey it is set by the soffit and the fascia; on a two storey it is set by whatever window sits above, and by the fact that the flashing has to go up behind something. Then the head of the door you are covering sets a floor under it, because a ledger that crosses the top of a patio door is not a detail, it is a redesign. The calculator reports where the bottom of the ledger lands against the head of the opening so that conflict shows up while it is still a drawing.
When the wall will not give the height, the freestanding option is the honest answer rather than a compromise. A second beam on its own posts a few inches off the wall gives you a roof that is not attached to the house at all: no ledger, no flashing into the wall, no load hung on the rim board, and a height you choose. It costs two more posts and two more footings. For the same reason it is often the route people are steered to by an inspector on an older house whose rim board nobody wants to open up.
For the members themselves, rafter length and roof pitch handle the sloped geometry, post height on sloping ground handles the case where the ground is not a flat slab, header and beam load is where the loads go, and the covering itself is worked out on the corrugated panel calculator or the metal roof panel calculator. If the cover is a slatted one rather than a solid roof, the pergola shade calculator is the page for how much sun actually gets through.
Questions people ask
What slope does a patio cover need?
Enough to drain, and how much that is belongs to the roof covering rather than to a general rule. Corrugated panel, standing seam, membrane and shingle all have different published minimum slopes, and using a covering below its stated minimum is usually what voids the warranty on it. What this page can tell you is the cost of the slope you pick: every extra quarter inch per foot takes three more inches off the far end of a twelve foot cover. That trade — drainage against headroom — is the whole design conversation on a lean-to, and it is why so many of them end up at one inch per foot.
Why is my cover lower at the beam than at the drip edge?
Because the beam depth is in the beam line and not in the overhang line. Under the beam you are standing beneath the rafter and the beam together. Out at the drip edge there is no beam, only the rafter tail, so even though the roof has fallen a bit further the underside is higher. On the default numbers the beam gives 82-1/2 inches and the tail a foot further out gives 91. Anything that has to pass under the edge of the cover — a ladder, a truck mirror, a wheelbarrow with a handle up — meets the beam first.
Does the slab slope really matter?
On a long projection it is worth several inches, which on a marginal cover is the difference between comfortable and ducking. A patio built to drain away from the house at a quarter inch per foot has fallen three inches by the time it is twelve feet out, and every one of those inches is headroom you get for nothing. Measure it rather than assume it, with a level on a long straightedge or a string and a tape. Some patios fall towards the house, which is a drainage problem of its own and also takes headroom away instead of giving it.
Can I just attach the ledger higher up the wall?
Sometimes, and it is the first thing to try, but the wall decides. Above the soffit there is often nothing structural to attach to without opening up the eave; below a second-floor window there is a limit set by the flashing needing somewhere to go. On a single-storey house with a low soffit the height at the wall is frequently fixed within an inch or two, which is exactly why the freestanding version exists. What may be attached to, and how, is not something this page will tell you — it is a structural connection into an existing wall, and it belongs to an engineer and the building department.
How long do I cut the posts?
The calculator gives the cut length as the distance from the top of the post base to the underside of the beam, which is what you actually measure and cut to. It assumes the post base holds the post the distance you entered above the slab and that the slab under the outer beam has fallen by the amount you entered. Cut one, stand it, and check the beam line with a level and a string before cutting the rest, because a quarter inch of error in the slab reading multiplies across a long beam and the last post is where it shows up.