Nominal width, coverage width, and the ten percent nobody counts
A corrugated sheet is quoted by its overall width. It is installed with one or more corrugations sitting on top of the sheet next to it, so the width it adds to the roof is smaller. The arithmetic is simple once it is written down: coverage equals the number of corrugations minus the number lapped, times the corrugation pitch.
Take a common profile at 2.67 inches crest to crest with ten corrugations. Overall width is 26.7 inches. Lap one corrugation and coverage is nine times 2.67, or 24.03 inches, which is 10.0 percent of the sheet gone. Lap two, as a shallow slope or an exposed site may call for, and coverage drops to 21.36 inches — 20 percent. Over a 20 ft wide patio cover that is the difference between ten sheets and twelve.
| Corrugations lapped | Coverage of a 26.7 in sheet | Lost to the lap | Sheets for a 20 ft width |
|---|---|---|---|
| None (butted, not a real option) | 26.70 in | 0% | 9 |
| One | 24.03 in | 10.0% | 10 |
| One and a half | 22.70 in | 15.0% | 11 |
| Two | 21.36 in | 20.0% | 12 |
The side lap is not a free choice. How many corrugations a profile needs lapped depends on the profile, on the slope, and on how exposed the site is, and the panel maker publishes it. What this page does is show you what each option costs in sheets, so the conversation with the supplier is about a number rather than a shrug.
Slope decides the end lap, and the end lap is the leak
If the sheets reach from eave to high side in one piece there is no horizontal joint and nothing to go wrong. The moment a run needs two sheets, water arriving at the joint has to be persuaded to go over the lower sheet rather than under the upper one, and on a shallow patio cover slope there is very little energy pushing it along. That is why end laps get longer as the slope gets flatter, why sealant appears in the specification at low slopes, and why cut-to-length sheets are worth paying for on any cover that a stock length nearly reaches.
On the default cover — 12 ft of run at 1 in 12 — the slope length is 12.042 ft and the sheets need 12.292 ft with a three inch overhang. A 12 ft stock sheet does not reach, so the run becomes two sheets with an 8 inch lap, and every sheet count doubles. Ordering 12 ft 6 in sheets instead is one phone call and removes the whole problem, including the ten horizontal joints that would otherwise exist on a 20 ft wide roof.
Purlins are a table lookup, not a rule of thumb
How far a panel spans between purlins depends on the profile, the thickness, whether it is polycarbonate or PVC or metal, and the load it has to carry, and the load is set by whatever code your jurisdiction adopted. Every panel maker publishes a span table for exactly this and it is the only defensible source. This calculator takes the maximum spacing you give it, works out how many rows that needs over the slope length, and reports the actual even spacing that results — which is always tighter than the maximum, because the rows have to land on the eave and the high side.
On the default numbers, a 12.042 ft slope at a 24 inch maximum needs 8 rows, and the actual spacing works out at 20.64 inches. That gap between the maximum and the actual is normal and is not waste; it is what happens when a run does not divide evenly.
The part that is not arithmetic
Plastic roof panels do not carry a person, and they do not warn you first. Falls through plastic roof sheets and rooflights are one of the most consistent causes of serious injury in roof work anywhere. Nothing about the smoothness of a lay-in or the fact that the purlin is right there changes it. The second thing worth knowing is thermal movement: plastic sheet expands and contracts far more than the frame under it, which is why the holes are oversized and the fasteners have sealing washers and are not run down hard. The thermal expansion calculator will put a number on it for a given material and temperature range.
For the structure underneath, the patio cover headroom calculator works out where a lean-to finishes, and rafter length and roof pitch handle the sloped geometry. Where the roof is a trapezoidal or standing-seam metal panel with a coverage width you already know, the metal roof panel calculator covers panels, trim pieces and screws per square. Water leaving the cover is gutter and downspout sizing, and for a polygon roof the faces come off the gazebo roof calculator.
Questions people ask
How do I work out how many corrugated sheets I need?
Divide the roof width by the coverage width, not by the sheet width, and round up. Coverage is the corrugation pitch times the number of corrugations left after the side lap. A ten-corrugation sheet at 2.67 inches is 26.7 inches overall but covers 24.03 with one corrugation lapped, so a 20 ft roof takes ten sheets and not the nine that dividing by the overall width suggests. Then multiply by the number of sheets it takes to get up the slope, which is one if you can buy them long enough.
How many corrugations should the side lap be?
Whatever the panel maker specifies for that profile at your slope and exposure, which is commonly one and sometimes two. It is not a preference. A single corrugation on a shallow slope in a windy spot is how water gets driven back up the lap, and the answer to that is more lap, not more sealant. What is worth knowing before you ask is the cost: going from one corrugation to two takes coverage from 24.03 inches to 21.36 on a typical profile, which is two more sheets on a twenty foot roof.
Can I walk on a polycarbonate patio roof?
No, and this is the one item on the page that is not about quantities. Plastic roof panels and rooflights give way under a person without warning and falls through them cause deaths and life-changing injuries every year. It makes no difference that a purlin is nearby or that the panel felt solid underfoot for the first step. Access to a plastic roof is planned around not standing on it, using boards that span between the structural members, and that planning belongs to whoever is running the job rather than to a calculator.
Why are the fastener holes drilled oversize?
Because plastic sheet moves with temperature far more than the metal or timber purlin under it, and a fastener through a tight hole turns that movement into a crack at every fixing. Oversized holes plus a sealing washer let the sheet slide while staying watertight, and the fastener is set snug rather than driven down hard. It is also why a long panel run in full sun is more demanding than a short one. The thermal expansion calculator on this site will give the actual movement for a material and temperature swing if you want the number rather than the principle.
Do I need closure strips?
At the top and bottom of the run they close the corrugations against wind-driven rain, insects and birds, and most makers include them in the installation instructions rather than treating them as optional. Foam closures and moulded ones behave differently and are sold in fixed lengths, so the take-off is a lineal-feet-divided-by-piece-length problem, which is what the calculator does with whatever piece length you enter. Leave the length at zero and they are simply left out of the list.