I am sure that many Building Surveyors (and others who inspect property on a regular basis) will agree that flat roofs are something that warrant a particularly close inspection and is an element where regular problems are identified. In next week’s article I will focus on some typical defects/problems to look for when inspecting flat roofs however for the remainder of this article I want to focus on different types of flat roofs typically found in the UK. When we refer to different types of flat roofs we can basically consider these in terms of both construction and coverings.
|Figure 1 - Source: Source: http://www.diynot.com/|
In terms of flat roof construction there are two common types used in the UK, cold roof construction and warm roof construction (I will discuss flat roof coverings a little later). Buildingregs4plans.co.uk (online) provide a concise explanation of warm and cold roof construction which is used below;
Warm roof construction - In a warm deck roof the insulation is positioned above the structural deck and no ventilation is required. Throughout the course of the year the roof deck and all below it is kept at a temperature close to that of the inside of the building, therefore the roof structure is protected from extremes of hot and cold, lessening the potential for damage caused by thermal movement.
|Figure 2 - Source: Source: http://www.buildingregs4plans.co.uk/|
A warm deck also provides added protection from the dangers of condensation as the structure is kept warm, at a temperature above dewpoint, by the insulation above it. Therefore water vapour which enters the roof structure from the room below will not have a cold surface on which to condense. NHBC recommend that this type of roof be considered as the standard form of construction. (Figure 2)
There are two forms of warm deck roof, sandwich and inverted. The sandwich warm deck roof is the most common type of flat roof. The insulation is placed below the waterproof covering and is either mechanically fixed or bitumen bonded on to the top of the deck.
|Figure 3 - Source: Source: Chudley R. & Greeno R (2005), Building Construction Handbook.|
The insulation boards in an inverted warm deck are laid over the structural deck and the waterproof covering. The insulation is secured by a layer of ballast or paving slabs to prevent wind uplift. The waterproofing membrane has the added protection of the insulation from foot traffic and degradation caused by exposure to solar radiation. However, it may be a more difficult to locate defects in the membrane (insert image 4)
Cold roof construction - In a cold roof the thermal insulation is laid between the joists below the structural deck. As the insulation is not required to take any loads, quilts and other loose fill materials can be used as well as rigid insulation. Because the structural elements of a cold roof are not protected by from the heat of the sun by a layer of insulation they are liable to suffer the damaging effects of thermal movement. Ventilation is required above the insulation in a cold roof to prevent the build-up of moisture vapour in the roof void. (Figure 4).
|Figure 4 - Source: Source: http://www.buildingregs4plans.co.uk/|
In addition to the construction types described above, flat roofs are also often referred to by the type of covering that is used. In the UK, built up felt, mastic asphalt and single ply are the common types of coverings installed. Some of the typical problems with flat roofs that I will discuss next week are a direct result of the type of covering selected. It is therefore essential that an appropriate covering is considered during design and installation.
Built up Felt Roofing - involves the installation layers of tar impregnated roofing felt which are rolled out onto a roof. Each layer overlaps the previous layer and in between the deck material and each layer of felt a layer of hot tar is applied over the surface.
Mastic Asphalt Roofing - premiermasticasphalt.co.uk (Online) define mastic asphalt as; comprises suitably graded aggregates bound together with an asphaltic cement (primarily refined bitumens) to produce a dense voidless material. It cannot be compacted and is spread rather than rolled. As mastic asphalt is installed as a ‘hot liquid’, when it cures (cools down) it provides a continuous impervious membrane with no joints making it extremely waterproof.
Single Ply Membranes - everybodyneedsaroof.com (online) define Single-ply membranes as; factory-manufactured sheet membranes. which are generally catagorised as thermoplastic or thermoset. Thermoplastic materials can be repeatedly softened when heated and hardened when cooled. Thermoset materials solidify, or "set," irreversibly after heating. Single ply membranes commonly are referred to by their chemical acronyms, such as ethylene propylene diene terpolymer (EPDM). Single-ply membranes can be installed fully adhered, mechanically attached or held down with ballast. Most single-ply roof systems do not receive surfacings.
The information above provides a brief introduction into UK flat roof construction and flat roof coverings and should be used as a reference point for the problems/defects associated with flat roofs that will be discussed next week.
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