For roof dead-load calculations, polycarbonate sheet mass is derived from solid density rather than nominal overall thickness. The solid density of unfilled polycarbonate is 1.20 g/cm³ under ISO 1183-1:2019 and ASTM D792-20. A solid sheet therefore contributes 1.20 kg/m² for each 1 mm of thickness. Polycarbonate weight per square metre: sizing a roof without overloading it requires that the specified profile, sheet thickness, and support spacing be converted to an areal mass and then combined with snow, wind, and access loads using EN 1990 and EN 1991 instead of relying on the low self-weight alone.

Polycarbonate Weight per Square Metre: Sizing a Roof Without Overloading It

Solid extruded polycarbonate sheet has a direct thickness-to-mass relationship. Using the 1200 kg/m³ density, a 3 mm solid sheet produces 3.6 kg/m², a 6 mm sheet produces 7.2 kg/m², and a 10 mm sheet produces 12.0 kg/m². These values are measured on unconditioned sheet; moisture absorption of polycarbonate is generally below 0.15% at 23 °C and 50% RH per ISO 62:2008, so the service mass shift is negligible for roof calculations.

Multiwall configurations deviate from the solid-thickness formula because internal ribs and air channels remove material. A 10 mm twin-wall panel typically has an areal mass of 1.7–1.8 kg/m²; a 16 mm triple-wall panel is typically 2.7–3.0 kg/m²; and a 25 mm X-structure panel is typically 3.5–4.0 kg/m². The values are not derived from nominal thickness alone and must be taken from manufacturer datasheets prepared under EN 16153:2013+A1:2015.

From the table, a 25 mm X-structure multiwall panel has roughly half the mass of a 6 mm solid sheet but more than twice the overall thickness. This is relevant when roof insulation and light transmission are specified together with low dead load. The manufacturer’s declared mass per unit area under EN 16153:2013+A1:2015 is based on the delivered sheet dimensions and may exclude protective masking, condensation barriers, and sealants. In load calculations, the designer adds 0.10–0.20 kg/m² for fixings and gaskets, and if aluminium glazing bars are present, their mass per metre is converted to an equivalent areal load by dividing by the panel width.

Along the load path from panel to purlin, the self-weight of a 16 mm triple-wall polycarbonate sheet is approximately 0.027 kN/m² after converting 2.7 kg/m² by 9.81 m/s². When the roof has a moderate ground snow load of 1.0 kN/m², the snow load exceeds the panel dead load by a factor greater than 35. This disparity does not mean dead load is irrelevant; it means that EN 1991-1-3 snow shape coefficients and exposure factors, rather than sheet mass, usually control support spacing in snow-bearing climates.

For non-accessible roofs, EN 1991-1-1 imposes a characteristic imposed load of 0.4 kN/m² for maintenance, and 0.6 kN/m² where access is provided. The design combination with EN 1990 using partial factors γ_G = 1.35 and γ_Q = 1.5 gives a dominant variable action contribution of 0.9 kN/m² from access load, compared with 0.036 kN/m² from the sheet dead load. Therefore dead load only becomes controlling where support spans are long, solid sheet is used, and snow or access loads are extremely low.

When Does Dead Load Become the Controlling Failure Mode?

Dead load controls in retrofit situations where an existing glazing bar or timber purlin is already close to its permissible bending stress. If a 6 mm solid polycarbonate sheet at 7.2 kg/m² replaces a 6 mm wired glass pane at approximately 15 kg/m², the dead load decreases, but if the replacement is a 10 mm solid polycarbonate sheet at 12.0 kg/m², the reduction is smaller than expected and the existing fixings may still be inadequate under wind suction. The governing criterion is not the material name but the combined bending stress in the supporting member, commonly limited to design strength according to EN 1995-1-1 for timber or EN 1999-1-1 for aluminium glazing bars.

Under uplift, a heavier sheet is beneficial because it reduces net suction. The net pressure on a roof element is calculated as wind pressure less favourable dead load, with partial factors γ_G = 1.0 for destabilising dead load and γ_Q = 1.5 for wind. A 3 mm solid sheet at 3.6 kg/m² contributes 0.035 kN/m² resistance, which is small compared with wind suction in exposed zones that can exceed 2.0 kN/m² under EN 1991-1-4. The designer therefore cannot reduce fastener spacing based on the additional mass of polycarbonate alone.

Because the flexural modulus of unfilled polycarbonate is approximately 2350 MPa per ISO 527-2:2012, panel stiffness rather than dead load generally controls. A single-pane glass of equivalent thickness has a modulus of elasticity around 70,000 MPa, roughly 30 times higher, so the polycarbonate sheet must be thicker or more closely supported to limit service deflection. The benefit is the low areal mass, not a one-to-one replacement of glass stiffness.

Under sustained dead load, polycarbonate exhibits creep. Long-term deflection calculations require a creep modulus, typically obtained under ISO 899-2:2003, because the short-term flexural modulus is not valid for a load duration of 50 years. A single long-term modulus is not sufficient because creep is stress-dependent; ISO 899-2:2003 tests at multiple stress levels are required to establish a deformation curve for the specific sheet thickness and temperature. Published data for non-standard profiles is limited and must be obtained from the manufacturer.

Multiwall Rib Geometry, Deflection Limits, and Fastener Pull-Out Data

The mass per square metre of a multiwall polycarbonate panel is governed by the upper and lower face thickness, rib thickness, and rib spacing. A 16 mm triple-wall panel with X-structure has more vertical walls per linear metre than a 10 mm twin-wall panel, raising the areal mass from approximately 1.7 kg/m² to 2.7 kg/m². The same geometric change increases section modulus for short-term bending, allowing wider purlin spacing at the same snow load, but it also increases dead load and material cost. Published span tables from polycarbonate sheet manufacturers convert these geometry-dependent properties into maximum support spacing for specified deflection limits.

Where project specifications adopt serviceability deflection limits of L/25 for roof glazing, a 16 mm triple-wall panel on 700 mm continuous support spacing under 0.8 kN/m² load is a common datasheet configuration. Published data for alternative profiles may show support spacing up to 1200 mm, but the specific configuration requires manufacturer’s test data because rib buckling occurs before the face sheet reaches tensile yield. Quoting a span without the corresponding deflection limit is not technically valid.

Fastener pull-out and pull-over strength are often more restrictive than panel bending. Polycarbonate expands thermally at approximately 0.065 mm/m/°C when tested by ISO 11359-2. A 3.0 m sheet subjected to a 50 °C temperature rise requires approximately 9.75 mm of expansion movement. If the sheet is drilled and fixed without oversized holes or slotted channels, the resulting thermal stress can initiate cracking at the hole boundary. Designers should size the hole diameter to permit movement and use fasteners with load-spreading washers to limit bearing stress.

With an increase in ground snow load from 0.5 kN/m² to 3.0 kN/m², support spacing for a given polycarbonate panel can fall by roughly 40–50%, while the panel self-weight remains unchanged at 2.7 kg/m². This is why selecting a heavier polycarbonate profile rarely solves an overloading problem; the variable load still controls the bending moment. For roofs in cold regions, the snow load shape coefficient for a monopitch roof may exceed 1.0 under EN 1991-1-3, and drifting at obstructions can raise local loads to 2.0 kN/m² or more. The panel areal mass remains below 0.05 kN/m² in most multiwall configurations, so roof failure prevention depends on span tables, fixing pull-out values, and load combinations, not on substituting a slightly thicker multiwall panel.

Handling weight is a separate operational limit. A standard 2.05 m × 3.05 m sheet of 3 mm solid polycarbonate has an area of 6.25 m² and a mass of approximately 22.5 kg; a 16 mm triple-wall panel of the same area is approximately 16.9 kg. Manual handling standards such as ISO 11228-1 may restrict lifting frequency and posture, but the weight per square metre of polycarbonate remains low relative to glass. For large roof installations, panel dimensions are often selected to keep individual sheet mass below 25 kg to avoid two-person lift requirements on site.