Pergola Roof Pitch Calculator
Estimate pergola cover pitch, drainage fall, beam or post height difference, rafter length, panel fit, and overhang geometry.
⚙ Pergola presets
📏 Roof inputs
Pitch results
📊 Project spec grid
📘 Cover minimum pitch table
| Cover type | Typical minimum | Preferred pitch | Best use |
|---|---|---|---|
| Open slats or battens | 0:12 | 0.5:12 to 1:12 | Shade only, no water shedding |
| Tensioned shade cloth | 0.5:12 | 1:12 | Shade with light drainage |
| Water-shedding fabric canopy | 1:12 | 1.5:12 to 2:12 | Low slope patio cover |
| Corrugated polycarbonate | 1:12 | 2:12 | Clear or tinted rain cover |
| Twinwall polycarbonate | 1.5:12 | 2:12 to 3:12 | Panel roofs with end channels |
| Ribbed metal panels | 2:12 | 3:12 | Reliable rain runoff |
| Glass or solid acrylic | 2:12 | 3:12 to 4:12 | Cleanable solid glazing |
📐 Pitch conversion table
| Pitch | Fall per foot | Angle | Fall over 10 ft |
|---|---|---|---|
| 0.5:12 | 0.5 in | 2.4° | 5 in |
| 1:12 | 1 in | 4.8° | 10 in |
| 1.5:12 | 1.5 in | 7.1° | 15 in |
| 2:12 | 2 in | 9.5° | 20 in |
| 3:12 | 3 in | 14.0° | 30 in |
| 4:12 | 4 in | 18.4° | 40 in |
🔧 Overhang and rafter reference
| Detail | Common range | Calculator use | Note |
|---|---|---|---|
| Low-side overhang | 6 to 18 in | Adds to roof run | Helps drip clear the beam |
| High-side overhang | 0 to 12 in | Adds to rafter length | Check wall flashing space |
| Rafter spacing | 16 to 24 in | Estimates rafter count | Confirm with cover span tables |
| Gutter drop | 0.25 to 1 in | Lowers fascia line | Useful for drip edge clearance |
| Panel trim allowance | 5% to 10% | Area with allowance | Allows squaring and cuts |
⛈ Drainage guide table
| Exposure | Suggested minimum | When to increase | Result warning |
|---|---|---|---|
| Sheltered | 0.5:12 | Leaves, dust, or short panels | Below cover minimum |
| Normal backyard | 1:12 | Long runs over 12 ft | Low fall rate |
| Heavy rain or leaf load | 2:12 | Tree cover or wide gutters | Ponding risk |
| Solid glazing | 2:12 | Hard-to-clean roofs | Maintenance access |
💡 Practical tips
A pergola can be seen by most homeowners as little more than a decorative framework to hang vines from. However, nature doesn’t like flat things much, so when Mother Nature deliver a rainstorm, that flat roof becomes a shallow bowl. That water puddles up and then dribbles down onto chair. Then it works its way inside at the joints.
That’s not really a function of how good wood is. It’s a function of slope. Pitch is a ratio of vertical rise to horizontal run, where rise is the vertical change and run is the horizontal distance the roof moves sideways. So if I had a 2-in-12 pitched roof, it would drop two inches for every foot of horizontal distance. This sounds like nothing on a 12-foot length, until you realize you’re talking about a two-foot height change between the back post and front.
Why Your Pergola Roof Needs a Slope
The beauty of this calculator is that you don’t have to try to figure out those changes; it does all the math after you enter in the dimensions of the roof, plus which cover you want to use. Just know that what you put into the calculator is what you will see in your own backyard.
How steep your slope must be depends on the material. Rain can flows right through open slatted wood, so it can basicly be flat. It doesn’t try to shed water; it just provides shade. It is similar with tensioned shade cloth, which requires a bit of a break to avoid puddling on top of fabric.
For semisolid or solid covers, the rules shift. A pitch of at least 1-in-12 should works for corrugated polycarbonate panels (though I’d go steeper… More like 2-in-12 (to assure good drainage)). Metal panels are even thirstier and typically wants 2-in-12 or better so no water backs up behind their seams. Going lower than the minimum specified by the manufacturer guarantee leaks, leaky or not.
But many builders forget to account for the overhang. That’s the amount of space between the beam or wall and edge of the roof. And no, it’s not just for looks. It’s structural. With a six to twelve inch overhang, dripping water will fall well away from supporting posts, keeping the wood dry and the concrete footings stable.
The tool considers this when calculating the rafter length, and provides you with a cut list that actualy fits the space. If you don’t adjust for this, you’ll find yourself with rafters that are too short…exposing your beams to elements.
Another hurdle is post height. Generally speaking most of us envision 4 identical posts on a pergola. That is not happening with a pitched roof. It’s either going to be taller on one end or the other. The calculator will tell you how far down to go on the short side to get the pitch you desire. If you don’t account for this difference in height, it can look jarring. You can use landscaping grading or a retaining wall to hide this. Alternatively, you could embrace the lack of symmetry as part of the design. If you do nothing, chances are the roof just won’t fit and will seem like it’s slipping off one end.
The amount of drainage you need depends on exposure. People think it doesn’t matter, but it does. A gradual slope in an out-of-the-way section of the yard may be okay; so will a sharp incline in an exposed spot where rain comes from every direction (or even more if there’s a lot of leaves falling). If your roof faces into the prevailing wind, you want to ensure quick runoff and gutter clearance. That means a sharper slope.
Leaves clog gutters and trap moisture against the roof surface. A steeper slope helps gravity pull away debris that is hard to reach and clean. This is made clear in the reference table on the page (which pairs cover type with pitch preference). Use that as a sanity check for your design.
For example, if you’re putting up metal panels and live in a rainy climate, stick to the high end of the recommended range. Better to go a bit too tall then leaky.
The pergola is a bit of a trade off. More shelter or less? More rain or less? The solution lies in figuring out what works for your specific site conditions and material limitations. Then the rest of construction just fits together. It’s covered, it has some sort of form to it, and if the sky opens up, you’ll remain relatively dry. That is the entire purpose of building the thing in the first place.
