How to save money when installing a flat roof construction

11 June 2021 Kingspan Insulation UK

Keeping budgets under control is a key concern on any flat roof project. With Building Regulations and Standards around energy performance becoming increasingly demanding, roofers not only need to ensure the surface is weathertight and drains efficiently, but also that it provides sufficient insulation. In a warm deck flat roof construction, the cost of the above deck build-up alone can easily exceed £200 per m2. 

To avoid additional costs, it is important to carefully consider the method used to achieve the fall (pitch) on the roof required for drainage. Research from Currie & Brown has shown that by fitting a tapered roof system, it is possible to save as much as 30% on the installed cost above a new concrete deck when compared with  other systems.

How can you provide a fall on a flat roof?

When constructing or refurbishing a flat roof, there are three common approaches to providing a drainage fall:

•    screed laid to falls;
•    timber firrings – angled strips of wood which are laid beneath plywood boards; and
•    tapered roof insulation (such as Kingspan Thermataper) – which feature tapered insulation boards and hip and valley boards, along with standard flat insulation boards, providing both drainage and insulation in a single system. 

Unlike screed and timber firrings, tapered roof insulation systems are typically designed bespoke to each application. For example, our tapered design service team can use architectural plans to design the layout and thicknesses for the Kingspan Thermataper system, ensuring it provides correct drainage and meets the required U-value. They can also provide a number of additional services such as onsite surveys and checks where required.

Learn more about our tapered roofing design service.

How can you save money on a flat roof construction?

We asked Currie & Brown to look at how these different approaches could impact project costs. The research looks at four new build case studies with concrete decks ranging in size and complexity. For each case study, it compares three warm deck flat roof solutions designed to achieve a U-value of 0.16 W/m2K. The three constructions considered are:

•    screed laid to falls - standard cement: sand screed insulated with Kingspan Thermaroof TR27;
•    timber firrings - timber firrings beneath a plywood deck insulated with Kingspan Thermaroof TR27; and
•    tapered roof insulation - Kingspan Thermataper TT47 

In addition to the price of the materials used within each construction, the Currie & Brown analysis also considers practical costs including labour in laying and waste disposal. The results are shown below:
Case Study Fall Method Above the Deck & Other Cost Impacts (£/m2)
Hospital extension with roof area 192 m2 Tapered roof insulation £140
Screed laid to falls £194
Timber firrings £172
Rebuilt office block with roof area 338 m2 Tapered roof insulation £163
Screed laid to falls £206
Timber firrings £178
New build office block with roof area 3,073 m2 Tapered roof insulation £136
Screed laid to falls £194
Timber firrings £162
New build office block with roof area 1,512 m2 Tapered roof insulation £136
Screed laid to falls £182
Timber firrings £165

In each case, the tapered roof construction is the most cost-effective option. 

When compared with the screed laid to falls, it provides savings of between £43 and £58 per m2 (up to 30%). In practice, these savings could be even greater as the additional weight and drying time of the screed approach would likely lead to extra structural and operational costs not considered as part of the research. 

Discover how tapered roof insulation can save time on a flat roof installation.
Learn more about how tapered roof insulation can reduce the weight of a flat roof construction.

The tapered roof constructions also provides significant savings of between £15 - £32 per m2 (up to 19%) when compared with the timber firrings. In addition, in the roof plans used for this study, tapered roof insulation systems are also likely to provide savings over timber firrings on timber or metal deck structures.

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