Flat Roof Extensions – Your Guide to Designing & Building a Flat Roof Addition

Though you won’t achieve a lofty, vaulted ceiling as with a pitched or asymmetric roof, there are still unique options for your interiors. Building a structure with a warm roof, where the insulation is above the roof deck (more detail on this later), means you can expose the rafters in the room below. “This elevates the perceived height of the room by reducing the overall solid element of the roof build-up,” says Nick.

Wallbarn’s M-Tray modular green roof system has been installed around lantern rooflights on this extension, delivering an infinity roof finish that flows into the garden. The modular green roof trays are grown to maturity on Wallbarn’s nursery and delivered to site within days, ensuring they are in optimum health

If you have the budget, a living roof covered with sedum and wildflowers could provide an additional wow-factor element. “The garden is extended and sightlines from the upstairs to ground level are improved, as seamless greening is achieved from the extension roof over the parapet towards the back garden,” says Julian Thurbin, director at Wallbarn.

“It almost becomes an infinity roof garden.” A green roof is also a fantastic way to create a biodiverse environment for bees, butterflies and other insects. Plus, it’ll provide an additional element of insulation to prevent heat escaping from your extension.

Getting the Structure of a Flat Roof Extension Right

Perhaps the most important detail to know about a flat roof is that it’s never actually set at an angle of 0°. “As we all know, Britain is renowned for its weather so all horizontal structures must incorporate a slight slope to allow rainwater to drain off without causing damage,” says Katrina Hughes, an architect at Butcher Bayley Architects. “Any pitched covering less than 10° is considered a flat roof.”

So, one of the most important structural details to get right is the angle on the surface to prevent water accumulating on top. Building Regulations suggest a minimum slope of 1:40, whereby there are 40 units in length for every 1 unit in the height of the fall. This gradient can be achieved in numerous ways – but first, let’s take a closer look at how a flat roof is constructed.

The Douglas fir structure that forms the skeleton of this flat roof side extension has been left exposed, creating a standout design feature. The roof material is elZinc Anthracite with a Bauder green roof on top. Design work for this project was carried out by Merrett Houmoller Architects. Photo: lan williams

Typically, flat roof extensions are comprised of a series of structural timber joists that are laid out with a flat deck on top, usually made of timber boarding. There will also be a suitable membrane to reduce the risk of condensation build-up, plus a layer of insulation to prevent heat from escaping. Your architect or structural engineer will determine the size, type and spacing of the joists to ensure it’s suitable for the unique dimensions of your flat roof extension’s design.

The timbers will be cut to size in order to incorporate any rooflights you decide to install in the roof. One way to achieve the right gradient is to position the joists to the required angle when the roof structure is initially built, or by fitting tapered insulation boards that already have the slope built in. Another solution is to place wedge-shaped chunks of timber on top of the roof joists to establish the desired pitch.

Granit Architects designed this extension to a semi-detached house in Teddington as part of a full remodel of the ground floor. The flat roof means views and light to windows on the upper level of the house are unobstructed. Photo: Andrew Beasley

The drainage system, including guttering, downpipes and fascia, is another important detail to get right. “If the look is sleek and modern, you may not want to install a gutter along the flat roof,” says Richard Morgan, an architectural technologist and connect consultant at Resi. “You may want to hide it instead by using a valley or box gutter which sits within the roof itself.”

The height of the parapet to the edge of the flat roof will depend somewhat on the covering you’re laying. A build-up of 150mm is common, but it depends on the covering. “A parapet of this height is adequate for a flat roof and its flashing or coping stone should be angled to allow water to fall off onto the flat roof, with adequate drip beading,” says Nick.

Yard Architects drew on this property’s 1930s roots to formulate a flat roof extension design. A curved wall to the rear mimics the bay window at the front of the property, and the green tiling references the home’s charming  green window cills. Crisp white render flows seamlessly from the main house to the extension for a cohesive look. Photo: Richard Chivers

“However, if you’re using a single product such as glass reinforced plastic (GRP), it’s possible to have a smaller parapet to as low as 50mm – but you should check with building control.” To ensure each of these details is achieved to a high standard, choose a firm registered with the National Federation of Roofing Contractors.

For flat roof extensions that feature a living roof, one of the most common solutions is to lay the covering onto a concrete roof. “The structure needs to hold at least 100kg per m², plus a snow load,” says Julian from Wallbarn. “Insulation is usually through rigid insulation boards laid either under the membrane as a warm roof design or above the membrane in an inverted waterproofing design.”

Insulation Options for Flat Roof Extensions

A potential argument against the construction of flat roof extensions is their lower thermal efficiency compared with a pitched structure. “The latter can naturally accommodate more insulation, so is typically a more energy efficient option,” says James from Plus Rooms.

However, with the right detailing, a horizontal setup is perfectly capable of beating the U-values (a measure of heat loss, where lower numbers indicate superior thermal performance) stipulated by Building Regulations – 0.18W/m²K or lower.

Standing seam zinc gives this flat roof extension, by George & James Architects, the wow factor. Photo: Megan Taylor

There are two types of system for flat roof extensions and they differ depending on whether the insulation is fitted above or below the structural roof deck. Historically, a cold roof design was the most common solution, whereby the insulation is installed in between the structural joists, with a gap for ventilation above.

This means that the structural deck is outside of the flat roof extension’s thermal envelope hence creating a cold roof. Adequate ventilation is required via eaves vents to minimise the risk of condensation, which could lead to issues such as damp. “A better-performing solution is a warm roof,” says Robert from Simply Extend. “This is where the insulation is not put in between the timbers but covers the whole of the internal new roof’s ceiling (above the roof deck).”

Mulroy Architects designed this kitchen-diner extension to complement a 1920s house in North London. The flat roof was finished with dark grey Sarnafil, a PVCu-based single ply membrane. Photo: Dan Glasser

A polythene vapour barrier membrane is placed on top of the deck, on the warm side of the insulation, to prevent moist air in the rooms below getting trapped in the wooden roof structure. Options for insulating materials include extruded polystyrene boards or rigid wood fibre panels. You could even use structural insulated panels (SIPs) for the roof construction if you want a solution with a thermally protective layer already built in.

Flat Roof Extension Roof Covering Options

Flat roof extensions invite the a variety of roofing materials to choose from. The following represent the main options:

• A reinforced bituminous membrane is the most common, cost-effective solution for flat roof extensions. This setup has come on a long way from the old school felt membranes of the ’60s. Robust, modern systems are applied in layers before a large blow torch is used to apply heat to the underside of the felt, melting it so it bonds with the structure below. Expect a service life of 20+ years.
• Fibreglass GRP comprises a liquid polyester resin, applied to large sheets of glass fibre to create a durable laminate. A wide choice of colours is available to suit the a whole host of flat roof extensions. Most roof guarantees for this type of covering last for 20 or 25 years.
• Single ply/EPDM rubberised roofing is a modern solution growing in popularity for its simplicity and longevity. Large sheets of thin rubberised material are rolled onto the flat roof extension and chemically welded at the seams, which works to create a waterproof finish.
• Metals such as lead, copper or zinc traditionally offered the most durable type of covering. Today, stainless steel interlocking sheets can be used to mimic the appearance of zinc or lead. Bear in mind that metal solutions typically come with a much bigger price tag.
• A green flat roof that incorporates sedums, grasses and wildflowers can provide a stunning feature for your home. A stronger deck is required beneath the foliage to support the weight of the living layer above.

FAQs

What challenges should I be aware of when building a flat roof extension?

There are practical considerations that may seem simple, but could hold up a project significantly if not carefully thought out. “Drainage is a constraint that can catch people out, particularly if you have a main drain running along the back of your house where you plan to put the extension, or a manhole,” says Caroline Shortt, director of RIBA-chartered practice Barc Architects.

If you do have a drain where your extension is, find out if it’s shared with other houses and whether it’s privately owned or by the local water authority. “Do your research early on to find whether you’ll need to relocate the manhole or if you can potentially build over it. Make sure you carry out a decent initial survey to highlight potential issues.”

When do I need home extension insurance & what does this cover?

Many homeowners mistakenly believe that their builder’s insurance or standard home insurance policy will cover the risks of a house extension. Unfortunately, this is often not the case. A specialised house extension insurance policy is specifically designed to cover the unique risks you will face when carrying out a project.

• The Existing property and the extension work will be covered up to the reinstatement value stated in your policy. Employers’ and public liability insurance is included to protect anyone working on site, including family and friends, and their property. It covers third-party injuries or property damage.
• Cover for unoccupied homes isn’t usually included under your standard home insurance, but a tailored extension policy provides protection both before and during the construction phase.
• Legal expenses should be covered and your provider may also offer advice and support in the case of disputes – helping you to avoid costly legal battles.

Do I need a structural warranty for my flat roof extension project?

While it’s not a legal requirement, a structural warranty is a valuable addition to a house extension project – offering long term protection against potential construction defects. This kind of cover can be particularly useful for large extensions, providing added peace of mind for sizeable investments. Issues with the foundations, walls, roofing, or other structural elements that could compromise the integrity of the extension would be covered.

“As the developer of the works, you remain liable under the Defective Premises Act for those works and a structural warranty helps risk manage that exposure,” says Simon from Protek Group. It can also be important if you plan to finance the project or sell the property in the future, as lenders often require this type of coverage before approving loans. Additionally, warranty providers will inspect the works before providing cover to ensure they are being built to standard, which offers another layer of assurance.

How easy is it to get consent for an extension to a listed building?

Any extension to a designated property will require listed building consent (LBC). “LBC covers an entire building, its interior and its surroundings, so most work on a listed building requires listed building consent,” says Simon Graham, director at Yard Architects. “For homes that aren’t listed, you don’t need planning permission to change the internal layout or add a bathroom. However, with a listed building, even moving an internal door requires consent. Repairs don’t usually require consent, as long as they are completed using the same materials as the existing feature.” he says.

“The application process for LBC is similar to a regular planning application, but it can require a lot more information to be submitted, included very detailed drawings. We find some conservation officers are more open to changes to a listed building than others, so it can be a bit of a lottery in terms of how many changes you can actually make,” says Simon.

Can my neighbours reject my plans for a flat roof addition?

“Neighbours can’t reject your extension plans, only the planning officer can do that, but they can object if they think they are going to lose daylight or privacy, or they don’t like the extension’s design, for example. It’s wise to speak to your neighbours before you make a planning application for an extension. Show them your extension plans, design ideas, listen to any concerns they may have and try and address these before you make a submission,” says Simon.