Terraces are flat roof areas that are accessible to pedestrian foot traffic.
These areas are usually intended for recreational purposes, and the insulation may be covered with decorative paving bedded in a screed or overlaid with paving slabs on spacers and possibly with vegetation in planters and even incorporate shading structures.
Inverted flat roof insulation provides mechanical protection for the waterproofing layer not only during the construction work, but also in the course of intensive use of the roof as a terrace.
The range of insulation thicknesses allows thermal performance to be matched to project requirements.
The rigidity of the insulation boards provide a firm base for paving layer and shiplapped edges ensure a good interlock between boards, which helps to prevent thermal bridging.
The extrusion production process used during the manufacture of extruded polystyrene (XPS) results in a homogeneous, closed-cell material structure with a smooth surface (called extrusion skin), which provides a number of favourable material characteristics:
- consistently high thermal insulation level
- water and frost resistance
- high compressive strength
- high resistance to vapour diffusion
These characteristics make the XPS thermal insulation material highly suitable for terrace roofs.
The strength of Ravago's XPS insulation used as thermal insulation in the construction of an inverted roof terrace, offers designers special flexibility and freedom. Walkable and green surfaces, heavy planters, legs of shading structures, bases of railings, or street furniture, can be easily installed anywhere without breaking the waterproofing membrane or creating thermal bridges.
Later on, if the building needs renovating, the roof terrace can be rearranged so that work does not endanger the operational integrity of the building and the waterproofing layer will not be damaged in the process.
ROOF TERRACE PAVING SLABS
The wearing surfaces of roof terraces are laid directly onto a vapour-permeable membrane placed onto over the XPS thermal insulation.
The wearing surface of terraces is generally made with:
- concrete paving slabs laid supported at the corners on special proprietary plastic spacer pads
- pavers laid on screed
The thermal insulation boards must be laid such that they flat and fully supported by the waterproofing layer without rocking. If multilayer bituminous felt waterproofing is used, a levelling layer may be needed at the overlaps of the waterproofing layers to ensure an even bed.
The use of separating layers between the waterproof layer and insulation in inverted roof construction is recommended as follows:
- mastic asphalt: BS 8218 requires a loose-laid nonwoven polyester fleece 130 - 140g/m2 lapped 200 - 300mm
- bituminous felts: separating layer not normally required
- single ply polymeric membranes: a loose laid nonwoven polyester fleece is normally recommended
- pPVC membranes - consult the membrane supplier
- PU based systems - consult supplier.
A vapour-permeable membrane (as above) is placed over the inverted roof insulation to prevent dust and debris from being washed through the insulation joints where they could potentially damage the waterproof membrane.
ROOF FALLS AND DRAINAGE
The efficient and quick removal of water is vital to the long-term performance of a flat roof. The roof deck should be without deflections or depressions, so that ponding on the waterproofing layer is prevented.
The insulation boards should not be totally or permanently submerged in water during the product’s lifetime on a roof. If the roof is regarded as ‘zero pitch’, then particular attention must be given to the provision of roof drainage to prevent ponding.
Specify rainwater outlets which will accept run-off from all levels of the build-up from the top of the insulation to the surface of the waterproofing and safety overflow must also be considered.