The non-trafficable roof, and the best-performing element in the set thermally at U 0.21 W/m²K. It is also the lightest against its traditional equivalent: 102 kg/m² against 443 kg/m², a 77% reduction. Joist depth varies with span, which moves the total build-up between 503 and 653 mm.
D.08
Non-trafficable roof
Cubierta no transitable
603 mm
Total thickness · Revision 01 · 2026-08
D.08
Non-trafficable roof
SCALE 1:10
Dashed layers sit inside the joist depth, not in addition to it. Keyboard navigable with the arrow keys; the same data is published as a table in the layer schedule below.
Photographs
1 / 15
Layer schedule
| Layer | Material | Thickness | λ | Function |
|---|---|---|---|---|
| 1 Self-protected bituminous membrane | Bituminous waterproofing membrane | 5 mm | — | Waterproofing |
| 2 Lightweight concrete to falls | Lightweight concrete | 120 mm | — | Forms the falls Laid to falls: 120 mm at the high point, 50 mm at the low point. The 603 mm total is taken at the high point. |
| 3 OSB board | Oriented strand board | 15 mm | — | Substrate for the screed |
| 4 XPS | Extruded polystyrene | 50 mm | — | Thermal insulation |
| 5 OSB board | Oriented strand board | 15 mm | — | Structural decking |
| 6 Void within the joist depth within joist depth | Air | 250 mm | — | Services zone Approximately 250 mm: the 350 mm joist depth less the 100 mm of glass wool. |
| 7 Glass wool within joist depth | Glass mineral wool | 100 mm | — | Thermal and acoustic insulation |
| 8 Steel lattice joist | Prefabricated cold-formed steel lattice joist | 350 mm | — | Structure — the joist depth contains layers 6 and 7 Drawn at 350 mm. Joists are also made at 250, 300 and 400 mm, which moves the total between 503 and 653 mm. |
| 9 Ceiling void | Air | 20 mm | — | Ceiling void Drawn as 20 to 30 mm. The 603 mm total is taken at 20 mm. |
| 10 Knauf ceiling channel | Steel ceiling suspension channel | 15 mm | — | Carries the ceiling |
| 11 Pladur plasterboard ceiling | Gypsum board | 13 mm | — | Ceiling finish |
| Total | 603 mm |
An em dash means the source document does not state that value; it is not a zero. Layers within the joist depth are indented — they sit inside the joist, so the column does not sum to the total.
Calculated performance
| Property | Value | Basis |
|---|---|---|
| U-value, bridging corrected | 0.21 W/m²K | EN ISO 6946, one-dimensional steady state, with a +15% bridging correction for horizontal elements, applied as an engineering estimate. |
| Self-weight | 102 kg/m² | Mass estimate from the layers and dimensions in the source document. |
| Traditional equivalent | 443 kg/m² | Derived from the published self-weight and reduction percentage. |
| Weight reduction | −77% | Self-weight against the traditional equivalent build-up. |
| Fire resistance (EI) | Test programme in progress — contact technical department | Awaiting laboratory test report |
| Airborne sound reduction (R_A) | Test programme in progress — contact technical department | Awaiting laboratory test report |
| Structural span | Test programme in progress — contact technical departmentSpan selection tables in preparation | Awaiting laboratory test report |
| Watertightness | Test programme in progress — contact technical department | Awaiting laboratory test report |
U-values are one-dimensional steady-state calculations to EN ISO 6946. Bridging correction is applied as an engineering estimate: +20% for walls, +15% for horizontal elements. 2D numerical analysis to EN ISO 10211 is in progress; values will be updated on completion.
Not yet stated
- EIFire resistance · in test
- R_AAirborne sound reduction · in test
- spanStructural span · in test
- watertightnessWatertightness · in test
Test programme in progress — contact technical department
Weight against traditional
| Element | Solid System | Traditional | Reduction |
|---|---|---|---|
| D.08 Non-trafficable roof | 102 kg/m² | 443 kg/m² | −77% |
Where to use it
- Flat roofs with no pedestrian access
- Plant and service zones reached only for maintenance
Design notes
- Joist depth is available at 250, 300, 350 and 400 mm, giving a total build-up of 503 to 653 mm.
- Span selection tables are in preparation. No joist depth should be assumed suitable for a given span until they are issued.
- The external INSULTERM 600 of the façade must be continuous across the roof junction.