Whole life costing: Single ply membranes

Single ply membrane options

As a flat roof covering option, single ply membranes offer many whole-life performance benefits: including quick installation, the potential to prefabricate, ease of repair and good service performance.

Unfortunately, there is no specific performance standard relating to single-ply membranes as EN 13956, the European standard for flexible waterproofing sheets, is currently a draft.

Products with third-party certification provide a guarantee of meeting relevant test criteria and usually include an indication of the expected service life based on accelerated ageing tests and performance in use. The ability of membranes to withstand solar heating is one of the key properties for durable performance.

A substantial range of single-ply membrane material options are difficult to compare directly as the testing parameters do not allow this. Below we consider some of the more common options.

Polyvinylchloride (PVC)

The problems of PVC membranes becoming brittle in the 1960s have been overcome, and current systems having an expected life of 20 to 30 years. Manufacturers quote examples that have been on buildings for more than 35 years. PVC sheets have good resistance to pollutants, bacteria and extreme weather conditions. Damaged areas can be readily repaired.

Thermoplastic polyolefin (TPO)

A more recent option, also known as flexible polyolefin (FPO) is not as flexible as PVC and so may have increased installation labour costs. Unlike PVC, TPOs do not contain chlorinated ingredients and have excellent weathering properties. TPOs are commonly used for inverted or green roofs, with an expected life of 20 years.

Ethylene propylene diene terpolymer (EPDM)

EPDM has excellent resistance to weathering, ultraviolet radiation, ozone thermal shock and cyclic fatigue. Use adhesive tape and talc-free types to minimise joint problems. Expected service life 20 – 30 years. EPDM systems in USA with 40 years service life in practice are reported.

Design issues

The British Standard code of practice for flat roofs BS 6229 covers design issues for use of single-ply membranes. The Single-Ply Roofing Association provides more specific advice in its single-ply roofing design guide. Design issues which can improve the long term performance and reduce whole life costs include:

• The provision of additional insulation to reduce heating costs and reduce risk of cold bridging.

• Use light-coloured membranes, ideally with reflective surfaces to reduce solar heating.

• Design roof falls to be twice the minimum, - at least 1 in 40 to ensure efficient drainage of water.

• Consider green roofs or inverted roofs so the waterproofing membrane is protected from direct weathering and radiation. However, if an inverted roof leaks, the repair costs may be significantly higher than a traditional system.

Installation issues

Use specialist contractors trained in the installation of the roof specified.
Laying the membranes flat is essential as creases, ripples, voids and folds form weak points.

Joints and flashings are critical to ensure the watertightness of the roofing system. Solvent Welded seams should be tested shortly after forming and any discrepancies should be immediately repaired to prevent future failures. Sheets may be preassembled in the factory to overcome site jointing problems.

Loose laid systems offer installation saving but require adequate ballast to hold down the membrane.
Use compatible materials for forming details; membrane manufacturers supply purpose-made detailing pieces or materials for details including corners, edges, penetrations and movement joints in the substrate.

In–use issues

Aim to minimise access to the roof. Where heavier traffic is expected, additional protection such as purpose-made walkways should be provided, for example to service roof-mounted equipment.

Good maintenance is essential. BS 6229 gives detailed recommendations, including:

• Routine inspection every spring and autumn. More frequent inspection may be required where roofs are close to trees or areas of high pollution or in extreme climatic zones.

• Maintenance should allow for removal of debris and organic material lying on the roof. Minor repairs should be carried out as soon as a defect is noticed.

• If a protective layer, such as ballast is used, ensure that the cover is even and has not been disturbed by wind scour.

Specification options

Table notes

• Life expectancies as defined by third party assessments. Longer service lives are feasible and would alter the net present value outcomes. Expected service life criteria include a detailed understanding of the molecular chemistry of the plastics, additives such as plasticizers and production methods.

• Life cycle costs are based on a concrete warm roof construction with simple rectangular plan. Costs include: single ply membranes 1.2 – 1.5mm thick, perimeter upstands, gutter, outlets and protrusions, insulation and vapour control layer. No allowance for walkways or access. In use costs include inspections, minor repairs and occasional cleaning.

• Costs used are averages. Best value should be determined from a whole life assessment of roof covering options which satisfy project specific criteria, as much of the cost lies in the detailing and relative costs between options is marginal.

• A discount rate of 3.5% is used to calculate net present values.

First published in Building 2005

Further information

BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450.

• Pitched roofing materials compared