Source: http://www.angleseyroofing.co.uk/gallery/ |
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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