I often find that the choice of flat roof construction
is usually one of convenience and cost, however, this really is a false economy
as the costs of maintenance, repairs and replacement of a flat roof, from a
life cycle cost perspective, usually work out more expensive than constructing
a pitched roof in the first place
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| Source: http://www.angleseyroofing.co.uk/gallery/ |
Flat roofs are commonplace throughout many parts of the World and are
regularly used throughout the UK. One of the primary functions of any roof is to keep
the internal environment dry and it is essential that water is prevented from
entering the internal environment.
Whereas pitched roofs readily allow rainwater to drain into gutters and
downpipes due to their design, flat roofs tend to be more problematic
particularly if not designed and constructed appropriately. The use of flat roof construction in warmer
parts of the World which experience limited rainfall is less problematic than
in a varied climate such as that encountered in the UK, where the potential for
extensive prolonged rainfall is highly likely.
I often find that the choice of flat roof construction is usually one of
convenience and cost, however, this really is a false economy as the costs of
maintenance, repairs and replacement of a flat roof, from a life cycle cost
perspective, usually work out more expensive than constructing a pitched roof
in the first place. The problem we seem to have in the UK is that property
owners and occupiers do not think long term and most will not reap the benefits
of longer term investment because people tend to move on a regular basis.
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.
A flat roof is defined in BS 6229 as having a pitch (gradient) of 10 degrees
or less. Therefore a flat roof is not
actually completely flat and should be constructed with a subtle gradient which
will allow any rainwater to find its way to gutters or outlets. Even a small gradient of a few degrees will
allow rainwater to become mobile by gravity, although clearly the greater the
gradient the better. There are a number
of ways of achieving this subtle gradient on a flat roof such as the use of
timber firing strips onto of joists (as seen in figure 1), however I have stumbled
upon many examples when these have been incorrectly installed and in numerous
cases completely omitted. In fact the vast majority of problems I encounter
with flat roofs were a result of either poor workmanship during installation or
lack of general maintenance.
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.
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.
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| 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).
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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