Getting roofs right
E roof can make or break how a house looks, and what it’s like to live in. Architect Julian Owen shares his insider secrets to help you get the best design for your project
Architect Julian Owen’s top tips for this key design feature
The roof is a fantastically versatile design element of a house. It can be used to create complex and interesting shapes that add character and wow factor, so it’s a crying shame it sometimes seems to be plonked onto a new home almost as an afterthought.
Any house designer worth their salt should be able to visualise a project in three dimensions as the layout develops (a skill that’s particularly important when pitched roofs are involved). So there’s no need to refine the floorplan first and work out how to cover it later – this should all be part of the same coherent process.
The first decision you and your designer will need to make is whether to go flat, pitched or a combination of the two. This choice will help to define the style of the building. So here I’m looking how to choose between them, how to maximise the architectural opportunities with this aspect of your project and what materials might work best.
Flat roofs
The term flat roof technically refers to any roof that has an angle of less than 10°. This style can give either a flavour of cool modernism or a whiff of naff 1970s vibe, depending on your point of view and how well they integrate into the design. Generally speaking, a flat top to a house that has traditional brick detailing will look unfinished. It will fit perfectly, however, over a building that is unashamedly contemporary in style and materials.
Although they’re seen as a 20th century innovation in the UK, flat roofs are an ancient tradition – particularly in hot, dry climates where problems with damp and leaks are rare. Lead versions have been happily incorporated into large public buildings for hundreds of years. The much cheaper standard flat roof construction used on domestic houses, however, once had a reputation as a maintenance liability. The culprit was old fashioned roofing felt, which moves as the outdoor temperature fluctuates, especially in summer, and so quickly develops cracks which in turn leak.
Modern flat roofs use a completely different material, in the form of flexible single sheets stretched between the edges of the building. This allows for movement without the risk of their waterproofing ability failing. These newer types of construction have none of the limitations of bituminous felt, therefore, and some manufacturers will guarantee them for 20 years or more.
For those who just don’t like the grey finish of a flat roof, the eco-friendly alternative is a natural green (or living) roof covering. You can plant a meadow if you wish, although this requires regular maintenance, so access to it has to be considered at the design stage. A compromise is to use succulents such as sedum, which gives a similar affect but will mostly look after itself. It also places less weight on the structure supporting it.
Pitched roofs
A sound knowledge of basic geometry and the properties of materials is needed to make the most of pitched roofs and to decide the most suitable degree of slope.
Anything less than 35° makes it very difficult to use traditional tiles because they will be at risk of being blown off in high winds. Slates are more forgiving of shallow pitches and can go down to 20°, but are easier to install above around 28°. So, if you want to use either of these materials, it will fundamentally affect the floor plan. If the design means the span is very wide, the roof will become disproportionately high compared to the walls. This could look ungainly and may concern the planning department, particularly if it is a lot taller than neighbouring buildings.
One way to create a pitched roof that looks neat and well-designed is to use the same angle throughout the building. It is possible to get away with some minor differences, particularly if the roofs are in separate blocks. But as an example, two very different pitches around a hipped roof immediately will look wrong, because the courses of tiles will be mismatched at the corners.
Sometimes it is possible to get around this rule if the view of the roof is restricted. I recently surveyed the loft of a house with one side at 35° and the other at 40°, but you couldn’t tell from the ground because it was in a row of terraces and the two elevations couldn’t be seen together. Architects wanting to break the mould will intentionally create asymmetric roofs as a design feature, with one side noticeably shallower than the other, but do be aware that this requires a good eye to get it looking balanced.
If the roof pitch has to be shallow, the cheapest covering to go for is concrete interlocking tiles. Unlike plain clay tiles, which rely on their own weight and overlapping each other to keep in place during high winds, these modern alternatives literally lock together and rely on secure fixings to resist any updraft. They are lighter and several
times larger than their traditional cousins, so it takes less skill and time to secure them in place. There are downsides, however. Roof coverings such as clay tiles and slates can be neatly shaped around features such as dormers and bay windows, contributing to their attractive appearance. But interlocking tiles tend to look crude and ungainly when they are chopped up to fit over relatively small-scale variations in the form of the roof. Concrete products also tend to fade over time, whereas clay tiles and slates are through-coloured.
Visualisation & buildability
When you are finalising the roof design of your house, it pays to be able to see it in three dimensions if possible – perhaps in the form of a computer generated or a card model. Two dimensional drawings are little more than diagrams and often fail to give a true impression of how the finished house will look. Pitched roofs that slope away from the eaves appear more dominant on a 2D elevation drawing, for instance, and all sorts of inconsistencies may not be obvious.
I regularly check the planning applications in my area and often come across the same basic design errors. The council only asks for plans and elevations to be submitted and at first sight these drawings may look fine, but on closer inspection the roofs don’t work in 3D. Sometimes the pitches are different to each other where they meet, the junctions between pitches are impossible to construct, or a roof plan doesn’t tie up with its section.
A very common failing that doesn’t tend to show up on elevations is where a flat roof is used to slice off the top
of a hipped roof that would otherwise be too high. This is often the case where the plan has been finalised without bothering to consider a section through the building at the same time. These inconsistencies tend to be overlooked by planners and clients, presumably leaving it to the roofer to bodge it up on site as the timberwork is constructed and the problems become apparent.
The moral, as with any aspect of your project, is to work with your team to lock down a buildable design well before you move on to the construction phase.