Dream Homes (and how to avoid a nightmare roof)

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Your Dream Home

Dream homes, Bespoke residential, Designer, Exclusive & Luxury – all terms to describe the creation of a home built around a client’s own personal requirements in terms of space, facilities, appearance and environmental credentials. 

Designing your own home is a way of achieving the space and appearance you desire in the location of your choice if fitting yourself and family into a predetermined building layout among a collection of similar houses on a site is not for you.  And with the benefit of sufficient resources, a designer home can be a genuinely exclusive and bold statement (or an understated one if that’s your preference).

Design Your Dream Home

Converting your vision into a reality is a challenging process that will start with your brief – the size, shape, performance and look of your home – being created in some format which can be communicated to various parties for the next stages which will involve Local Authority Planning and Building Control.  You will also have chosen a route to the actual construction of your dream home.  This may be yourself as a self-builder, you may have appointed an architect or other professional to work up the designs and source suitable builders, or maybe found a specialist building contractor to make the design of your dream home into a reality.  No route to construction is right or wrong, and a quick search of the internet will produce suppliers of ‘dream-home’ house plans, many of which allow you to see imagery of the finished product in 2D and 3D interactive graphics.

However, in the early stages of design any attention which has been paid to the actual methods and materials of the construction will have concentrated on the internal finishes and external wall materials (brick, stone, timer, metal etc) from an appearance point of view.  And if the roof is to be a pitched roof the same consideration will have been given to the roof covering materials.

But if the chosen design style includes a flat or even low-pitch roof this aspect will generally have been left blank in terms of detail.  At some point in the process, the details of the flat roof construction will need to be determined, not just in terms of the external covering, but aspects such as the means of condensation control, thermal insulation, ventilation and drainage falls are vital to the long term weatherproofing and security of the building.  If these aspects are left too late in the development process it can cause other complications when interfacing with other parts of the construction.

Flat Roof Design Options

This is where specialist flat roof contractors can assist in the smooth planning and running of the house, by offering assistance with assessing the different flat roof design options (the same advice will be available if it is just a flat roof extension design which is required).

The first critical decision to take regarding the flat roof is ‘where is the insulation going?’, which in turn will be dictated by the type of roof structure.  With a lightweight roof deck (timber, metals, SIPS panels) the insulation layer can feasibly be placed either above or below the deck – i.e. in either a warm roof or cold roof configuration.  With a more substantial deck of concrete, which may be required to support an intensive green roof covering (see our other blog on green roofing for further information on this), it is unlikely that the insulation would be placed below the deck for technical and logistical reasons so will most likely become a warm roof system.

Cold roof configurations are less common now due to the Building Regulation requirements (based on BS 5250 advice on condensation control) giving the need for cross-flow ventilation under the deck and between all joists, with air inlets/outlets at each end of the voids.  Warm roof systems, with the insulation layer placed on the deck, and with the waterproofing system laid over it, are far more common for practical reasons.

Some roof build-ups, however, are designed around the ‘inverted (or upside-down) roof’, where the insulation layer is laid externally over the waterproofing layer and then overlaid with a ballast of paving or green roof covering.

Different calculation methods will be employed in the design of the insulation in these roofing systems to ensure that the thermal layer complies with Building Regulations, or maybe even with an enhanced thermal performance where the client is interested in a more energy-efficient or even zero-carbon home.

Flat Roof Design

Allied to the type of thermal insulation system will be the criteria for roof drainage falls.  All flat roofs are required to have drainage falls to remove rainwater and these were recently upgraded in the code of practice for flat roofing – BS6299-2018 – to a requirement for minimum 1:40 falls, or 25mm per metre run of roof fall.

The roof falls can be achieved through forming a gradient in the roof deck construction, laying the deck to falls on its supporting members, or by laying the deck completely flat and utilising a tapered insulation scheme to provide the falls.  Specialist insulation suppliers can create insulation boards which have varying thicknesses and will provide a scheme which drains water to the outlet locations.  The scheme will also need to meet the required thermal performance for the roof as a whole.

It is the aspect of roof falls which often causes design & construction issues later in the project if not considered early enough, since one side of the roof can be significantly higher than the opposite side in order to create the correct falls, and this can then encroach on abutment details such as door or window thresholds, cavity trays or roof edge kerbs.  Bear in mind that the fall across a 10m wide roof will be at least 250mm, which can be very significant in terms of the finished levels of the building.

Getting The Right Flat Roof

Having decided on the roof configuration in terms of thermal insulation and drainage, a waterproofing system will then be required.  Numerous types of roof waterproofing systems exist and contractors such as ourselves are often asked which is the best type.  In truth, there is no ‘best’ type, but a quality product with appropriate accreditations which will be installed by a specialist contractor employing operatives trained in the particular product, and all approved by the manufacturer, is the most effective way of achieving a reliable and weathertight roof covering.

At AAC Waterproofing, we have approved installers of several waterproofing systems including single-ply membranes such as Prelasti EPDM, Protan and Sarnafil, and liquid applied waterproofing such as Maris Liquids.

These are all products with 3rd party accreditations from BBA and significant manufacturer training and technical support.

When it comes to the design of the flat or low-pitch roof for your dream home or extension, we suggest that you act early in choosing the right criteria, and we at AAC Waterproofing are in a position to assist with the design and its integration into the finished product.