Lifetime costs: flat roofs | Features

Lifetime Costs - Roofing
Asphalt
Polymer-modified asphalt with third-party assurance, supplied in blocks similar to traditional asphalt, or in molten form in purpose-built transporters. Applied to low-traffic roofs or accessible roofs with additional protection such as tiling or paving. Traditional asphalt application and detailing to BS 8218.	Capital cost £32/m²
	Net present value for 60-year life £65/m²
	Service life 50 years
Asphalt minimum thickness 20 mm applied in two layers horizontally or up to 30° pitch, three layers vertically or over 30° pitch with vapour barrier below. Three-layer work applied to bitumen-primed stainless steel lathing. Asphalt to BS 6925; type R988.	Capital cost £27/m²
	Net present value for 60-year life £60/m²
	Service life 40 years
Single-ply polymer
Single-ply polymer elastomeric membrane with third-party assurance. Minimum 1 mm thick. Installed to manufacturer’s recommendations.	Capital cost £25/m²
	Net present value for 60-year life £33/m²
	Service life 25 years
Single-ply polymer thermoplastic membrane with third-party assurance. Minimum 1 mm thick. Installed to manufacturer’s recommendations.	Capital cost £32/m²
	Net present value for 60-year life £42/m²
	Service life 25 years
Built–up bitumen membrane systems
Modified bitumen cap sheet with third-party assurance. Underlay and bonding to cap sheet manufacturer’s recommendations and third-party assurance requirements.	Capital cost £35/m²
	Net present value for 60-year life £47/m²
	Service life 25 years
Polyester-based cap sheet to BS 747; type 5. Underlay and bonding to cap sheet manufacturer’s recommendations.	Capital cost £32/m²
	Net present value for 60-year life £47/m²
	Service life 20 years
Glass-fibre based cap sheet to BS 747; type 3. Underlay and bonding to cap sheet manufacturer’s recommendations.	Capital cost £23/m²
	Net present value for 60-year life £40/m²
	Service life 15 years
Table notes
Costs are based on a functional unit of 1 m² of flat roofing.
Costs include initial supply and installation; removal and replacement of coverings at the end of their service life.
Costs are discounted to net present values at a discount rate of 6%. This is to be consistent with previous whole-life cost assessments. The recent treasury guidelines suggest a discount rate of 3.5%.
Maintenance activities include recoating solar reflective paint on asphalt every three years.
Service lives represent typical average lives to enable a comparison of the whole-life costs.
The 60-year period reflects a typical design life to BS ISO 15686–1:2000 Buildings and constructed assets: Service life planning – general principles.