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Flat roofs - Choosing the covering

• Introduction
• Forms of flat roof construction
• Notes on types of construction
• Notes on materials

Introduction

BS 6229 – The code of practice for flat roofs with continuously supported coverings – defines flat roofs as those having a pitch not greater than 10⁰ to the horizontal. For the purposes of this Knowledge page we have excluded materials, such as long-span metal roof sheeting, suitable for "low-pitch" roofing to concentrate on products suitable for roofs laid at slopes of 1 in 60 or less.

There are a wide range of products suitable for the waterproofing of flat roofs but a quick poll of a small number of experienced architects revealed a remarkable degree of consensus on a very small number of preferred specifications. One technical director of a major UK architectural practice even went as far as saying that just two specifications (Concrete deck, inverted roof, hot-applied polymer-modified bitumen membrane; Metal deck, warm roof, single layer sheet PVC membrane) accounted for 95% of the roofs specified by his practice.

The table below grades some generic flat roof specifications into four categories, and provides notes to explain the grading for each combination.

Forms of flat roof construction

	Heavyweight deck Regular or Limited access		Lightweight deck Limited or Maintenance access
	Warm roof	Inverted roof	Warm roof	Inverted roof
J21 Mastic asphalt roofing/finishes - Read more ››
BS 6925 Type R988 asphalt	Alternative See Note 3 below	Preferred See Note 1 below	Alternative See Note 3 below	Acceptable See Note 5 below
Proprietary polymer-modified asphalts	Alternative See Note 3 below	Preferred See Note 1 below	Alternative See Note 3 below	Acceptable See Note 5 below
J31 Liquid applied waterproof roof coatings - Read more ››
Cold-applied polyurethanes	Caution See Note 6 below	Preferred See Note 1 below	Caution See Note 6 below	Acceptable See Note 5 below
Hot-applied polymer-modified bitumens	Caution See Note 6 below	Preferred See Note 1 below	Caution See Note 6 below	Acceptable See Note 5 below
J41 Bitumen sheet roof coverings - Read more ››
Two/Three layer systems	Alternative See Note 4 below	Caution See Note 7 below	Preferred See Note 2 below	Caution See Note 7 below
Single layer systems	Alternative See Note 4 below	Caution See Note 7 below	Preferred See Note 2 below	Caution See Note 7 below
J42 Single layer polymeric sheet roof coverings - Read more ››
Thermoplastics, e.g. CPE, FPO, PVC, VET	Alternative See Note 4 below	Caution See Note 7 below	Preferred See Note 2 below	Caution See Note 7 below
Elastomerics, e.g. EPDM and PIB	Alternative See Note 4 below	Caution See Note 7 below	Preferred See Note 2 below	Caution See Note 7 below

Notes on types of construction

1 : Asphalt and liquid applied coatings in inverted roof construction on heavyweight decks [Preferred]

• Experienced architect-specifiers are in absolutely no doubt that inverted roofs, comprising a concrete deck overlaid with asphalt or one of the liquid-applied coatings as the waterproof membrane and finished with paving slabs or stone ballast, are the most satisfactory form of flat roof construction.
• This type of roof construction has a long track record of satisfactory use in the UK and is particularly suitable for situations where there is regular pedestrian traffic across the roof. Properly applied liquid coatings, fully-bonded to the underlying deck, eliminate the potential for leakage tracking through interstitial voids in the roof construction.

2 : Sheet roof coverings in warm roof construction on lightweight decks [Preferred]

• Warm roofs, comprising a lightweight deck overlaid with insulation and finished with one of the many proprietary sheet roof coverings, are the preferred form of construction where there is infrequent maintenance-only access to the roof and the structure has not been designed to take the weight of an inverted roof. They are more susceptible to both mechanical and environmental damage than inverted roofs.
• Special insulation, with a higher compressive strength, and specially-reinforced membrane material may be necessary in designated walkway areas (areas of Limited access) to ensure that crushing of the insulation does not lead to a greater risk that the waterproofing membrane will be punctured.

3 : Asphalt in warm roof construction [Alternative]

• There is nothing, essentially, wrong with using asphalt in warm roof construction and this form of construction is commonplace in areas with a long history of asphalt use such as, for example, residential construction in the London area. There is, however, no doubt that protecting asphalt from direct sun and/or excessive temperature variation will prolong its life and inverted roof construction will provide this protection.

4 : Sheet roof coverings in warm roof construction on heavyweight decks [Alternative]

• As with asphalt (see Note 3 above), there is nothing essentially wrong with this form of construction. It is just that many specifiers believe that inverted roofs are preferable to warm roofs on heavyweight decks.

5 : Asphalt and liquid applied coatings in inverted roof construction on lightweight decks [Acceptable]

• The only significant reason for these materials being in this category is that lightweight decks are rarely designed to take the load of inverted roof construction. Laying asphalt on plywood sheeting over a metal deck is accepted industry practice and Hydrotech (see below) has an Agrement Certificate for use over plywood sheeting in inverted roofs.
• Some specifiers believe that, as the performance of these materials is better than many types of sheet roof covering, increasing the load capacity of the deck to accommodate the additional weight of an inverted roof is preferable to accepting the use of a sheet covering.

6 : Liquid applied coatings in warm roof construction [Caution!]

• Some of these products are marketed as suitable for warm roof construction, and at least product has an Agrement Certificate for such use, but many specifiers remain sceptical of applying a liquid coating over insulation.

7 : Sheet roof coverings in inverted roof construction [Caution!]

• Many specifiers are sceptical about the use of sheet roof coverings in inverted roofs even though some products may be promoted, by their manufacturers, as suitable for this form of construction. The main issue is the possibility (certainty?) of mechanical damage after installation. Some manufacturers specifically exclude inverted roofs in their trade literature. Alumasc, for example, do not advertise their Derbigum Single Layer product as suitable for inverted roofs.

Notes on materials

J21 Mastic asphalt roofing/finishes

• BS 6925 Type R988 asphalt : Generally laid 20mm thick in two coats on a separating layer.
• Proprietary polymer-modified asphalts : Generally laid 20mm thick in two coats on a separating layer. Some products, e.g. both those named below, may be laid fully-bonded to the underlying substrate in inverted roof construction. Example products:
• Permaphalt by Permanite [www.permanite-asphalt.co.uk]
• Flexiphalte by Rock Asphalte [www.wearerock.co.uk]

J31 Liquid applied waterproof roof coatings

• Generally : The European Liquid Waterproofing Association (ELWA) lists seven generic types of liquid applied roofing systems. The two with the longest track record in the UK are cold-applied polyurethanes and hot-applied polymer-modified bitumens. Polyurethanes are generally laid 2/3mm thick, applied in multiple coats (often as many as five); polymer-modified bitumens are generally laid 5/6mm thick, in one or two coats; both products are fully-bonded to the underlying substrate. Most products are intended primarily for inverted roof constructions.
• Cold-applied polyurethanes : Example products:
• Decothane by Liquid Plastics [www.liquidplastics.co.uk]
• Hot-applied polymer-modified bitumens : Example products:
• Hydrotech 6125 by Alumasc Exterior Building Products [www.alumascwaterproofing.co.uk]
• Permaquik PQ 6100 by Radmat Building Products [www.radmat.com]
• PermaTEC by Permanite [www.permanite.com]

J41 Bitumen sheet roof coverings

• Generally : Modern proprietary bitumen sheet materials are generally 3/4mm thick, making them more puncture-resistant than single layer polymeric sheets. They are, however, much stiffer and the formation of details at, for example, upstands and penetrations is more difficult than for BS 8747 products or single layer polymeric sheets.
• Two/Three layer systems : These may be generic products, generally to BS 8747, or proprietary products. A wide range of manufacturers offer proprietary products, generally referring to them as "high performance" roofing felts. Example products:
• Bauder Total Roof System by Bauder Ltd [www.bauder.co.uk]
• Single layer systems : These are proprietary products combining bitumen sheet technology with either bitumen or polymeric sheet fixing methods. Example products:
• Derbigum Single Layer by Alumasc Exterior Building Products [www.alumascwaterproofing.co.uk]
• System 5000 by Bailey Roofing Systems [www.baileyeaves.com]

J42 Single layer polymeric sheet roof coverings

• Generally : A basic introduction to single layer systems is available from the website of the Single Ply Roofing Association at [http://spra.co.uk/]. The Design Guide for Single Ply Roofing (available from the Downloads section of the SPRA website) provides comprehensive general guidance but nothing to help specifiers choose between the various membrane materials. We have also noticed a lot of misinformation associated with these products: the SPRA link to manufacturers of EPDM membranes includes a manufacturer of TPE membranes; in NBS Building Section J42, we believe that some of the suggested clause inserts in the "Manufacturers" section are technically incorrect; and similar.
• Thermoplastics, e.g. CPE, FPO, PVC, TPE, VET : Example products:
• Sarnafil by Sarnfil Ltd [www.sarnafil.co.uk] (PVC)
• Sika-Trocal by Sika Ltd [www.sika.co.uk] (PVC)
• Elastomerics, e.g. EPDM, PIB : Example products:
• RubberGard by Firestone Building Products [www.firestonebpe.com] (EPDM)

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