Flat roofs are quite rightly regarded as remarkably versatile and open up design possibilities unlike any other roof form. There are two benefits to this versatility: first there is the virtually limitless design possibilities which means green roofs can be incorporated as recreational spaces or natural habitats, photovoltaic arrays added for energy production, and then there are very specific and strict requirements in terms of insulation and waterproofing.

The flat roof also offers some very practical advantages: first and foremost, easy access. When a roof area is formed into a terrace or planted to achieve a green roof, the building gains more than simply an attractive exterior. It is also enhanced by the creation of extra living space – whether in a new build or as part of refurbishment.

Not surprisingly, a flat roof is the ideal place for a PV installation as it provides an unobstructed space that is perfectly positioned to catch as much sunlight as possible.

At the same time, a flat roof permits optimum utilisation of the interior space, as there are no sloping ceilings. All in all, a flat roof offers many advantages for everyone.

Flat Roof Design Considerations Overview

Thermal design

Thermal design is concerned with the flow of both heat and water vapour through the roof construction and their subsequent effect on the performance of the roof and the various components in the system. The designer therefore needs to consider the amount of insulation required to control both heat loss and condensation.

In the case of a flat roof, insulation is usually in the form of either a rigid board above the deck, either encapsulated within the waterproofing or atop it, or a fibrous quilt immediately above the ceiling, depending on the type of roof construction.

Warm roof construction – Insulation is encapsulated between the waterproofing layers and an air and vapour control layer separates the deck construction from the insulation to prevent moisture vapour (condensation) being forced into the insulation.

Inverted roof construction - In an inverted roof construction, the principal thermal insulation layer is located not only above the roof structure but also above the waterproofing, resulting in the waterproofing, structural deck and support structure being at a temperature close to that of the interior of the building. No vapour control layer is required.

Cold roof construction - This flat roof construction is where the principal thermal insulation is placed below the deck resulting in the deck and waterproofing layers being close to the external climatic temperature.

It is recommended in BS 6229:2018, Section 4 Design, Section 4.2 Types of Flat Roofs, Sub-section 4.2.3 Cold Roof System that this design be avoided.

Hybrid roof in refurbishment projects - This is where various roof construction types are created as the roof on a current building is refurbished and the existing insulation is left in-situ. There is an increased risk of interstitial condensation, so a risk analysis needs to take place before the design of the upgraded insulation within the replacement waterproofing system is finalised.

SIPS - The insulation is incorporated within the panel and additional insulation added either internally or externally. If poor design or construction occurs this can increase the interstitial condensation risk and further remedial actions will be required.

Insulation

There are many considerations for the correct section and specification of insulation for a flat roof including compatibility with the waterproofing solution, required fire resistance, expected access requirements both during and after construction, and many more to take into account.

Minimum levels of insulation required in flat roof constructions is governed by Building Regulations. These levels are rising on an ever-increasing scale as the Government strives to meet its targets for reducing CO₂emissions.

Condensation

The temperature at which air becomes fully saturated with moisture (100% RH) is called the dew point. When warm moist air meets a cold surface, it is cooled, and if its temperature drops below the dew point it will give up moisture in the form of surface condensation.

In flat roofs, depending on the location of the insulation within the roof construction, condensing moisture, called interstitial condensation, can enter the waterproofing system, which is detrimental to the integrity of the system, if not designed out with appropriate product selection.

Deck types

Materials used for the roof deck should be selected from those listed in BS:6229 2018. It is important to refer to this document for design guidance, but for convenience, the various materials are outlined below.

Reinforced concrete - Includes both in-situ and pre-cast forms, which can be covered with a cementitious screed if required.

Profiled metal - Metal decking constructed to support built-up waterproofing systems with a suitable profile, i.e. featuring crowns that are wider than the respective troughs to achieve a minimum 45% bond area.

Close boarded timber - Timber boarding suitable for roof decks should be a minimum 19 mm nominal thickness, planed and closely clamped together, tongued and grooved or closely butted. This decking is not recommended for hot melt or cold applied liquid systems.

Plywood - Plywood used for roof decks is generally 18 mm thick, but it is possible to specify a minimum 15 mm if the supporting structure is more closely spaced. The plywood should comply with BS EN1995 to be suitable for structural use. Materials suitable for flat roofing would be marked BS EN 636-2 or BS EN 636-3 and must be suitable for exterior use.

Oriented strand board (OSB) - OSB used for flat roof decking should be type OSB/3 or OSB/4, conforming to BS EN 300: Thickness and fixing recommendations are similar to those for plywood.

Wood particle board (chipboard) - This type of deck is no longer recommended for use in flat roof applications due to its long-term structural instability, especially if contaminated by moisture.

Creating falls on a roof for rainwater drainage

It is a requirement of Building Regulation Part H that adequate provision is made for rainwater to be carried from the roof of the building. According to BS 6229 & BS 8217, flat roofs should be designed with minimum falls of 1:40 to ensure a finished fall of 1:80 can be achieved, allowing for any inaccuracies in the construction. This applies to general roof areas along with any internal gutters.

Tapered insulation can create gradients on a roof to allow water to flow towards the installed drainage system. A 1:60 tapered insulation will usually achieve a minimum fall of 1:80 allowing for local deflection, but where large deflection is expected or known to exist, 1:40 tapered insulation may be the solution.

Drainage

Drainage needs to be provided via internal rainwater outlets and downpipes or via external guttering systems or hoppers. Even if a roof is very small, it is recommended there are at least two drainage points in case one becomes blocked.

BS EN 12056-3 and the Building Regulations Approved Document Part H contain relevant design information to enable rainfall calculations to be undertaken and give design principles for gutters and downpipes.

Fire resistance

The performance of a flat roof against external fire is covered by Building Regulations Fire Safety: Approved Document B test TS 1187. The test method classification covers ‘External Fire Penetration’ and ‘Spread of Flame’ for a complete waterproofing system (not individual products comprising that system). The results from testing under TS 1187 with BS EN 13501-5 classification are given as European Class ratings B_ROOF(t4), C_ROOF(t4), D_ROOF(t4), E_ROOF(t4) and F_ROOF(t4).

It is important to embrace all interested parties concerning the building with regard to fire performance, including the building insurer who may have certain criteria for the fire strategy that will need to be met.

Wind load design

The calculation of wind load on a flat roof is essential to confirm the installed system will resist the pressure and uplift forces expected because of the design, height and location of the building.

Wind Load Calculations are calculated in accordance with the UK National Annex to Eurocode 1 - Actions on structures Part 1 - 4: General actions - Wind actions (BS EN 1991 - 1 - 4).

Effects of wind pressure on a building

When wind strikes a building, it is deflected to generate a positive pressure on the windward face. As it accelerates around the side of the building and over the roof it creates a reduced or negative pressure in its trail. The greatest pressures are experienced at the windward corners and edges of the roof, where the negative pressure exerted on the roof can be several times that experienced in the central areas.

Critical Layer

When there is no wind, the air pressure on the upper surface of a roof system is the same as that on the underside. Wind changes this equilibrium by reducing the atmospheric pressure on the surface of the roof system.

The atmospheric pressure acting on the underside of the roof will remain the same or may be increased if windows or doors are open on the windward side of the building. The result is a net upward push acting on the underside of the roof. This upward thrust will be exerted on the lowest air impermeable layer in the roof construction, which will be required to stop air flowing further into the system. In most roof constructions there is one layer that provides the dominant barrier against the upward thrusting flow of air and this is referred to as the critical layer.

Inverted Roofs

With an inverted roof specification, the insulation is laid loose and its security will be provided by a loading coat, typically gravel ballast or paving slabs.

Edge Detailing

Damage caused to flat roofs during severe gales usually starts at exposed windward corners and edges. It is therefore important to make sure that facias, cappings, trims, and drips are adequately fixed.

Locating rooftop equipment

From an integrity point of view, it is far better to site equipment directly onto a finished waterproofing system, as the less interruptions or penetrations to the membranes, the less likely the system is to allow water ingress. Plant is typically supported by paving slabs with a suitable protection layer beneath.

If the plant is either too heavy for the waterproofing to support, or requires fixing down to the structure, then it will be necessary to introduce a support system that can be easily waterproofed.

Foot traffic and defined walkways

Whenever there is a requirement for regular foot traffic across a roof, for example to provide maintenance to rooftop plant or a fire escape route, it will be necessary to provide protection to the surface of the waterproofing.

It is normally sufficient to provide an extra layer of capping sheet/waterproofing in a contrasting colour to denote the walkway and provide sufficient protection. In cases of extreme traffic, promenade tiles can be used on a suitably robust waterproofing system.

Safe access

All flat roofs will have a requirement for a certain amount of pedestrian access, whether this is for maintenance of the roof and any rooftop plant or as a fire escape route. It is therefore necessary to design any flat roof to allow for safe access.

Careful design can eliminate hazards and make the risks associated with roof access easier to manage. Under the CDM Regulations, designers have a duty to ensure that their designs give adequate regard to health and safety. Foreseeable risks should be avoided.

Rooflights

Flat roof, windows, skylights, tunnels or glazing give more light to rooms or spaces in the building below come in a plethora of designs and glazing finishes. The product specified will need to comply with various Building Regulations depending on the type and function of the rooflight.

Considerations include:

Structure
Weatherproofing and detailing
Glazing
Energy conservation
Ventilation
Weight loading
Wind loading

Edge protection

Edge protection is designed into a flat roof construction to protect people that are accessing the roof from falling from height. There are different options that can be incorporated.

Designed parapet.
Guard rail at the roof edge.
Permanent protected walkway for access to plant on the roof.
Preformed sockets to support temporary edge protection guard rails.
Fall restraint or fall arrest systems designed, installed and tested to the relevant standards.

Assessing requirements of a refurbishment flat roof

An appraisal of the existing roof should always be the first step to assess the extent of any failing areas and what the remedial action should be.

Condition and nature of the existing deck - The first thing to examine when considering a refurbishment of a flat roof is the condition of the deck and supporting structure. If this is at all questionable, a structural survey should be commissioned.

Some deck types should always be replaced as their lifespan is limited and any water ingress that may have occurred will have reduced their load bearing strength.

Condition and nature of the existing waterproofing - It is important to diagnose the extent and location of the failing waterproofing. There are three main methods of diagnostics available:

Core samples – Physical investigative method assesses key areas of a waterproofing construction and the condition of the substrate.

Moisture mapping - Non-destructive and accurate method of testing and pinpointing trapped water, and the extent of ingress, within a flat roof build up.

Thermographic imaging - Non-destructive testing method that identifies defective roof areas currently allowing water ingress.

Overlaying existing waterproofing of a refurbishment roof

Fundamentally, in order to overlay a roof system, the existing waterproofing should be generally sound, dry, free from water damage and suitable to take an overlay system.

Overlay materials must be compatible with existing materials and consideration should be given to comply with the latest Building Regulations to improve the thermal performance of existing buildings, and the solution must be able to withstand the additional weight imposed on the structure.

Upgrading insulation on a refurbishment roof

Increasing the thermal insulation within a roof makes perfect sense and has many benefits to the building, its reduction in energy usage for heating or air conditioning as well as supporting the environment.

Many current flat roofs will require the addition of insulation to meet current Building Regulations, and this is added ideally as the waterproofing is renewed. It is recommended that confirmation of the planned solution is gained from Local Authority Building Control (LABC) before submitting the specification for tender.