CPD 2 2014: Selecting flat roof systems

Introduction

Although there are only three main options when designing a flat roof - cold, warm or inverted - there are a number of considerations that specifiers must be aware of. This CPD module will introduce the roof types, weighing up the advantages and disadvantages of each, before examining key factors in roof design and product selection, such as falls, type of membrane and sustainability credentials. It will also indicate the key standards and guidelines that specifiers should use to help them to assess products.

Types of roof

Cold roof

In a cold roof, the principle insulation is below the structural deck. This option is typically adopted for small or domestic roof areas, where it is possible to provide adequate cross-ventilation above the insulation. Building Regulations Approved Document F states that, for flat roofs, roof spaces must have ventilation openings on two opposite sides. Without ventilation there is a risk that condensation will form on the underside of the roof deck. (The Scottish Building Standard 3.15 does not advocate cold roofs at all.)

One feature of a cold roof is that the membrane is supported directly on the deck, which means it is accessible and easily replaced without disturbing the insulation. However, cold roofs have significant disadvantages: effective ventilation and vapour retarders at ceiling level can be difficult to achieve and upgrading insulation at a later stage is complicated.

Warm roof

This is the most common form of flat roof, where the principle thermal insulation is placed above the structural deck but below the waterproof membrane. Ventilation is not required but a vapour retarder is usually placed beneath the insulation to control condensation.

Roofs should be designed to avoid condensation risks in accordance with British Standard (BS) 5250 : Code of Practice for Control of Condensation in Buildings. Extra care should be taken to seal vapour barriers around penetrations such as roof lights and rainwater outlets.

Warm roofs have several significant advantages over cold roofs:

• The roof structure is protected from extremes of temperature by the insulation.
• There is no need to ventilate the roof space.
• Fire barriers can be accommodated.
• It is straightforward to upgrade insulation.
• Different insulations are available, such as acoustic and high-density options, depending on the application.

Inverted roof

This is also known as an upside-down or protected-membrane roof. It is a warm roof, but with the insulation above the waterproof membrane and structural deck. It does not require a vapour retarder.

Inverted roofs often have a heavy layer of either concrete paving slabs or stone ballast, which holds the rainwater cooling barrier, insulation and membrane down. This type of construction is suitable for terraces and other areas subject to pedestrian traffic.

However, inverted roofs have a number of potential disadvantages:

• They must be able bear the dead load of the system.
• They require a very deep roof zone.
• Waterproofing is vulnerable to following trades before insulation and ballast is installed.
• It is very difficult and expensive to locate leaks.

One way of overcoming the last two issues is to use a warm ballasted roof, which is essentially a warm roof with paving or ballast on top.

Falls, drainage and ponding

Ponding water should be avoided for a number of reasons. In the event of damage, water ingress will be increased. It may also cause progressive deflection of the deck: 15mm ponding on a 500m2 roof has a dead load of 7.5 tonnes.

Pools of water also draw in dirt and leaves, which can be unsightly and may obstruct outlets and become a slip hazard. This is exacerbated if they freeze over.

Moreover, ponding water can lead to thermal stresses, algae, moss, mould and other vegetative growth, which may reduce the life expectancy of roof finishes and may contravene the terms of the warranty for the waterproofing membrane.

It is therefore important that all flat roofs include a positive fall - that is, no backfalls. BS 6229 , which applies to flat roofs with continuously supported coverings, states: “The minimum finished fall, of single-ply membranes, at any point should be 1:80. In the absence of a detailed analysis, a fall of twice the finished fall should be assumed for design purposes.” This would give a design fall of 1:40, which is 25mm over 1m. It is accepted good practice to design to a finished fall of 1:60 (17mm/m of fall).

Types of membrane

Although there are many variations, the three main generic types of membrane are polyvinyl chloride (PVC), flexible polyolefin (FPO) and ethylene propylene diene monomer (EPDM).

• PVC has been in use as a roofing membrane since the early 1950s and is the most commonly used option. It can have a BBA-certified life expectancy of up to 40 years, depending on thickness and maintenance. PVC comes in a variety of colours and can be heat or solvent welded to create a homogenous joint. Some membranes also have a lacquered finish, which further increases their lifespan.
• FPOs have been in general use since the early 1990s and have a typical life expectancy of in excess of 25 years. They come in a limited range of colours and do not have a lacquered finish. They are heat welded.
• EPDM is a synthetic rubber membrane typically with a BBA-certified life expectancy of in excess of 20 years. It tends to come in one colour (black) and an adhesive is used to bond the seams together.

Sustainability

When specifying roofing products, a number of questions should be asked of the manufacturer:

• Check that the production facility is ISO 14001 accredited.
• Ask to see evidence that the manufacturer recycles its materials.
• Check not only whether the products contain recyclate, but also whether this affects product quality - a product with a high percentage of recyclate is not necessarily a long-lasting product.
• Check that the materials are free from
• heavy metals.
• Check the level of embodied energy. For example, single-ply membranes typically contain less than 70MJ/kg while the relevant figure for aluminium is in excess of 1,500MJ/kg.
• Check the BBA certificate to see if the product demonstrates a long service life in the UK climate.

There are also a number of guides that specifiers can use to help them ascertain the sustainability credentials of a product:

• EcoPoint ratings measure the impact of a product compared with the average European person, who has a 100-point annual environmental impact. In the case of single-ply membranes the EcoPoint rating is between 11 and 14 points depending on generic type.
• The Green Guide to Specification provides an environmental impact assessment for an element of a building, such as a floor, wall or roof. This takes into account all elements such as the deck, vapour control layer, insulation and waterproofing material. A rating is given from A+ (the highest)to E. All single-ply membrane types - PVC, FPO and EPDM - can receive an A+ rating, depending upon the deck and insulation used.
• To help specifiers achieve Part L requirements, some manufacturers have updated, or are looking to update, their standard AutoCAD details to provide thermal and vapour transmittance figures.

Other specification considerations

Specifiers should also be aware of these other key factors influencing roof design, and the standards that govern them.

• Wind uplift Protection against wind forces is one of the fundamental principles of good roofing design. Every roof should have a wind uplift calculation to BS EN 1991-1-4:2005+A1:2010 to determine peak wind loads.
• Fire Building Regulations require all buildings to comply with BS 476-3:2004 and DD CEN/TS 1187:2012. In essence, this is concerned with the preservation of life - the loss of the building and its contents is the concern of insurers, some of which have adopted approved roof constructions that exceed the statutory requirements.
• Condensation The correct vapour control specification is essential. Calculations should be carried out in accordance with BS 5250 and
• BS 6229. BS 5250 sets humidity classifications for different building types, depending on expected temperatures and relative humidity.