Polyethylene (HDPE/LDPE)
Types and Durability
| Description
Polyethylene (HDPE/ LDPE) |
Expected Lifespan
(Years) |
Adjustment Factors |
| Multi-laminate roof tile underlay of low and/ or high density polyethylene membranes and net reinforcement, incorporating anti-condensation fleece. Low to medium vapour resistance and medium to high vapour permeability (similar to minimum quoted in BS 5250 for traditional sarking felt of 50 MNs/g). Complying with the performance requirements of ASTM D6757-02. | 30 | Test certificate from approved local testing authority : +5 years |
| Multi-laminate roof tile underlay of low density polyethylene (LDPE) reinforced with a net or ribs of high density polyethylene (HDPE). Non-perforated, with high water vapour resistance and low vapour permeability. Complying with the performance requirements of ASTM D6757-02. | 30 | |
| Multi-laminate roof tile underlay of low density polyethylene (LDPE) reinforced with a net or ribs of high density polyethylene (HDPE), or cross laminated wiht linear low density polyethylene (LDPE). High initial water vapour permeability which may be significantly reduced following shrinkage, blocking of perforations with dust etc. Complying with the performance requirements of ASTM D6757-02. | 30 | |
| Poluethylene roof tile underlay with or without reinforcement and/ or perforations. Method of fabrication nor specified. Vapour resistance/ permeability not specified | <10 | None |
Assumption: All underlays should be fixed with large head felt nails of copper, aluminium alloy or galvanized steel in accordance with BS 5534 Part 1.
Note: Due to low water vapour permeability of polyethylene, control of water vapour transfer into the roof voids as describe in BS 5250 Clause 9.4.2 may be necessary to avoid condensation nuisance. Extra ventilation (equivalent to a further 10mm per meter as the eaves) in addition to the Building Regulation requirements may also be required as indicated in BS 5534 Part 1 Clause 21.5.1.
Some polyethylene underlays may give rise to noise nuisance in areas of high wind uplift, and may incorporate a layer of aluminium foil intended to increase the insulating effect of the umderlay.
Manufacturer’s guidance should be sought on means of avoiding such problems through design and/ or fixing detailing.