Picking out the right screws for polycarbonate roofing is usually the last thing people think about, but it's actually the most important part of the whole build. If you use the wrong fixings, you're basically asking for leaks, cracks, and a whole lot of noise every time the temperature changes. Polycarbonate isn't like metal or wood; it's a bit of a "living" material in the sense that it expands and contracts a lot more than you'd expect.
If you've ever walked under a plastic pergola roof on a hot day and heard those weird popping or clicking sounds, that's the sheets moving. If those sheets are pinned down too tightly with standard screws, they've got nowhere to go. Eventually, they'll buckle or crack right at the screw hole. That's why you can't just grab a box of leftovers from your last fence project and hope for the best.
Why you can't just use regular screws
You might be tempted to just use standard wood or metal screws because they're cheaper and you might already have a bucket of them in the garage. Don't do it. Regular screws have a small head and a tiny washer, if they have one at all. When the sun hits your polycarbonate roof, the plastic heats up and expands. Because the hole you drilled is likely just big enough for a standard screw, the expanding sheet pushes against the rigid screw shank.
Since the screw isn't moving, the plastic has to give. This leads to "stress crazing"—those tiny little spiderweb cracks around the fixings. Give it a season or two of winter and summer cycles, and those cracks will turn into actual holes. Then, the next time it rains, you'll have water dripping right onto your patio furniture. Specialized screws for polycarbonate roofing are designed specifically to stop this from happening by allowing the sheet to "breathe."
The magic of the EPDM washer
One of the biggest differences you'll notice with proper polycarbonate fixings is the size of the washer. Most of these screws come with a large, domed washer made of EPDM rubber. It's not just there for decoration.
First off, the EPDM material is UV-resistant. Cheap rubber washers will dry out, turn brittle, and crumble away within a year under the sun. Once the washer is gone, you've basically got a hole in your roof. EPDM stays flexible for years.
Secondly, these washers are usually much wider than standard ones. This serves two purposes: it creates a much better seal against water, and it distributes the pressure of the screw over a larger surface area. This helps prevent the screw head from "pulling through" the sheet during a heavy windstorm.
Understanding "Winged" or "Cutter" screws
If you're looking at high-quality screws for polycarbonate roofing, you might notice some of them have little "wings" or a wider cutting head near the top of the shank. These are absolute lifesavers.
The biggest rule with polycarbonate is that the hole in the sheet needs to be slightly larger than the screw itself—usually about 10mm. This gives the sheet room to shift as it heats up. In the old days, you'd have to pre-drill every single hole with a masonry bit or a special plastic drill bit, then go back and drive the screw in. It took forever.
Modern "one-hit" screws have a built-in cutter. As you drive the screw in, the cutter creates that oversized 10mm hole in the polycarbonate, but then the screw itself continues into the timber or metal purlin underneath. It saves you an incredible amount of time and ensures every single hole is the perfect size for thermal expansion.
Choosing between timber and metal fixings
Before you head to the store, you need to know what your roof frame is made of. The screws for polycarbonate roofing designed for timber won't work on a steel frame, and vice versa.
- Timber Fixings: These usually have a coarser thread that grips into the wood. They're pretty forgiving, but you want to make sure they're long enough to get a good "bite" into the rafter—usually at least 50mm or 65mm depending on the profile of your sheeting.
- Metal Fixings: These look more like a bolt with a finer thread and a self-drilling "teks" point. They're designed to pierce through the metal purlin without you needing to drill a pilot hole first.
If you're working with a hybrid frame or something unusual, always default to the screw type that matches the supporting structure, not the roof sheet itself.
How to actually install them without ruining the sheets
Even with the best screws for polycarbonate roofing, you can still mess up the job if you're too aggressive with the drill. The most common mistake is over-tightening.
You aren't trying to crush the sheet into the wood. You want to drive the screw in just until the EPDM washer touches the surface and forms a seal. If you see the polycarbonate start to "dimple" or dip down around the screw, you've gone way too far. Back it off a half-turn. The sheet should be held firmly, but it shouldn't be under extreme tension.
Another tip: always try to drive the screws in vertically. If you drive them in at an angle, the large washer won't sit flush against the sheet. This creates a little gap on one side where water can sneak in, and it also puts uneven pressure on the plastic, which can lead to those cracks we talked about earlier.
Spacing and positioning
Where you put the screws matters just as much as what screws you use. Generally, you want to fix through the "crests" (the high points) of the ripples, not the valleys. If you screw through the valleys, water is going to run directly over the screw hole every time it rains, which is just begging for a leak. Fixing through the crests keeps the hole out of the main path of the water.
As for spacing, a good rule of thumb is to place a screw on every second or third crest at the ends of the sheets, and maybe every fourth crest on the internal support rails (purlins). In high-wind areas, you might want to be a bit more generous, but don't overdo it. Every hole you drill is a potential failure point, so you want to find that balance between "secure" and "Swiss cheese."
Don't forget the material of the screw itself
While we've talked a lot about the rubber and the cutters, the metal of the screw matters too. Most screws for polycarbonate roofing are galvanized, which is fine for most inland projects. However, if you live near the coast, those galvanized screws will start to show rust faster than you'd think.
The salt air is brutal on hardware. If you're within a few miles of the ocean, it's worth spending the extra bit of cash on stainless steel fixings. They'll last as long as the polycarbonate itself, whereas cheap galvanized ones might rust out and stain your nice clear roof with ugly orange streaks within a few years.
Final thoughts on the "Cheap vs. Quality" debate
It's tempting to try and save twenty or thirty bucks by getting generic fixings, but when you consider the cost of the polycarbonate sheets themselves—which aren't exactly cheap these days—it's a bad trade-off.
Using dedicated screws for polycarbonate roofing ensures that your roof doesn't just look good on day one, but stays waterproof and crack-free for a decade or more. It's one of those rare DIY scenarios where the specialized tool (or in this case, the fastener) really does make all the difference in the world. Just take your time, don't over-tighten them, and make sure you've got those EPDM washers sitting flat. Your future self—the one not climbing a ladder in a rainstorm to go poke some silicone into a leak—will definitely thank you.