PC Sheet Installation: Rabbet Depth and Gasket Selection

The short version

The assembly details of PC sheet glazing compress into “two numbers and one table”:

• Rabbet depth: minimum 20 mm on both sides — the figure that covers thermal movement and load-induced “virtual shortening” in one go;
• Holes: 6 mm larger than the screw diameter (sheet length up to 2 m), plus 3 mm for every additional metre;
• Gasket compatibility table: EPDM, polychloroprene, PE, PTFE and silicone are compatible; PVC usually is not — and gaskets must be mechanically secured, because adhesion is inadequate.

The thermal-expansion coefficient and the 3 mm-per-metre clearance calculation are covered in our earlier article, PC & acrylic distortion control; this one is about the hardware.

1. Rabbet depth: why 20 mm

The reasoning chain behind the 20 mm figure is worth following in full:

1. Sheet and frame profiles expand at very different rates, so the rabbet must first accommodate that differential movement;
2. PC is a plastic: under external load it flexes slightly, which shows up as a “virtual shortening” of the sheet — it measures a little shorter than its cut dimensions;
3. The engineering shortcut: as long as the rabbet depth on both sides is calculated for the expected temperature variation, with a minimum of 20 mm, and the sheet dimensions are determined with the 3 mm allowance per metre, no further calculation is needed for the virtual shortening.

In other words, 20 mm is the combined lower bound for “thermal movement + load deflection”, not a rule-of-thumb guess.

2. Profiles and clamping pressure

• Recommended method: clamp all four sheet edges, exactly as you would glass panels; the profiles must let the sheet move with temperature while remaining air- and watertight;
• Aluminium profiles with EPDM rubbers are the most common pairing with polycarbonate sheet; anything from single profiles to fully modular systems works, as long as PC’s specific characteristics are respected;
• For optimum load characteristics, all edges must be properly clamped with a rabbet depth of at least 20 mm;
• Contact pressure must be regulated and uniform: the pressure from the glazing bead or clamping plate reaches the sheet through the gasket. The gasket is the softest part of the system — excessive pressure makes it bulge or even displaces it from the glazing, and the seal is lost. Use a distance ring, or a profile system whose clamping pressure is preset by the design, so over-tightening is designed out rather than trained out;
• Do not stress the sheet during construction, and use only gaskets chemically compatible with PC (next section).

3. Gaskets and seals: check the compatibility table first

A gasket must be resilient, accommodate the sheet’s predicted expansion and contraction, and be the right shape and material. The compatibility conclusions (verify before incorporation):

Two hard rules:

1. Gaskets must be mechanically secured to the glazing system. The sheet moves with temperature all its life, and “securing by adhesion is not adequate” — glued-in gaskets eventually work loose under continuous shear;
2. PVC gaskets are cheap and common, but they count as usually incompatible. The industry consensus attributes this to PVC plasticisers migrating into PC and initiating stress cracking — see our article on chemical resistance and stress cracking of PC.

4. Supplement: quantified rules for holes and fixings

Where bolts must pass through the sheet (e.g. intermediate supports), the concrete numbers are:

• Oversize the hole 6 mm beyond the screw diameter for sheet lengths up to 2 m, plus an additional 3 mm per metre thereafter; very large sheets may even need oblong holes;
• Use washers or profiled rubber/foam strips of the correct material to distribute local pressure and avoid point damage;
• Intermediate supports must work in both force directions — snow presses down, wind can suction upward — so a single profile placed under the sheet is not enough.

Also, remove the PE masking film immediately after installation (weathering makes it brittle or over-tacky — see our masking film article); day-to-day care is covered in the PC sheet cleaning and maintenance guide.

FAQ

Q: Does the 20 mm rabbet depth also apply to acrylic sheet?

The data here applies to PC sheet. Acrylic’s expansion coefficient is close to (slightly above) PC, and the design logic — allow movement, avoid point pressure — is identical, but the actual figures should come from the acrylic manufacturer’s documentation rather than being copied over.

Q: Isn’t gluing the gasket to the frame an even better seal?

The opposite. The sheet expands and contracts continuously with temperature, and an adhesive layer cannot survive that long-term cyclic shear — the gasket is gradually pulled loose. Only mechanical fixing (grooves, clamping bars) is reliable.

Q: Tighter clamping bars mean better sealing, right?

No. The gasket is the softest element in the system: over-compression makes it bulge, displace or extrude out of the glazing, destroying the seal. Use a distance ring or a profile system with design-preset clamping pressure and assemble to that pressure.

Closing

For hybrid parts such as equipment guards and sight windows — a PC sheet in a metal frame — material selection sets the ceiling and assembly discipline sets the floor. Rabbet depth, gasket material and clamping pressure each get a vote. Wuxi Cremage Technology custom-builds equipment guards and sight windows with precision PC/acrylic machining, and supports rabbet and sealing design at the engineering stage — send us your drawings.

References

• [1] Exolon® Technical Manual Solid (Makrolon® sheet, now Exolon®), Chapter 9 (Installation): https://www.exolongroup.com/index.php/fuseaction/download/lrn_file/mf0261_e_210502.pdf