Renovating
In Praise of Flat-roof Extensions
The idea that “you’d have to be mad to build a flat roof” is from another age. These days, they can be both beautiful and reliable
While it’s often the least expensive option, lots of people hate the idea of a flat roof on their extension, partly because the roofs have a reputation for leaks and partly because many of us have memories of a particular style of 1960s and 1970s flat roof that’s no longer to everyone’s taste.
However, flat roofs have come a very long way, both in practical terms and aesthetically, since then, so open your mind to broad – and horizontal – horizons.
However, flat roofs have come a very long way, both in practical terms and aesthetically, since then, so open your mind to broad – and horizontal – horizons.
Be savvy about slopes
The other myth about flat roofs is that they are flat. All flat roofs have a slight slope on them, so rain can drain away easily. Usually, this is about 1:50 or 1.5º, but the general convention is that any roof with a slope of less than 15º is technically a “flat” roof.
This extension to a Victorian house has a zinc-clad flat roof with a pitch of approximately 10º, but, unusually, the slope is upwards to the garden, opening up the view and creating an uplifting sense of space inside.
The other myth about flat roofs is that they are flat. All flat roofs have a slight slope on them, so rain can drain away easily. Usually, this is about 1:50 or 1.5º, but the general convention is that any roof with a slope of less than 15º is technically a “flat” roof.
This extension to a Victorian house has a zinc-clad flat roof with a pitch of approximately 10º, but, unusually, the slope is upwards to the garden, opening up the view and creating an uplifting sense of space inside.
Spill over
One of the main things that makes many of those 1960s flat roofs look, in my opinion, so dreadful is poor detailing of the eaves – where the roof meets the walls it sits on. Simply sitting the edge of the roof on top of the wall with a minimal overhang, then adding a grotty fascia board and a plastic gutter, rarely looks stylish.
One way to make this work in a much more pleasing way is to create a deep overhang, as here, and to detail the overhanging part of the roof so it appears slender and, accordingly, more lightweight. The effect is less of a box and more of an assembly of horizontal and vertical planes.
One of the main things that makes many of those 1960s flat roofs look, in my opinion, so dreadful is poor detailing of the eaves – where the roof meets the walls it sits on. Simply sitting the edge of the roof on top of the wall with a minimal overhang, then adding a grotty fascia board and a plastic gutter, rarely looks stylish.
One way to make this work in a much more pleasing way is to create a deep overhang, as here, and to detail the overhanging part of the roof so it appears slender and, accordingly, more lightweight. The effect is less of a box and more of an assembly of horizontal and vertical planes.
Let it float
This project takes this “assembly of planes” idea even further. Where the height is available, one solution that can really create the illusion of the roof as a “floating plane” is to have a strip of high-level clerestory windows beneath the roof plane, as here.
Again, deep overhangs and slender detailing enhance the effect.
This project takes this “assembly of planes” idea even further. Where the height is available, one solution that can really create the illusion of the roof as a “floating plane” is to have a strip of high-level clerestory windows beneath the roof plane, as here.
Again, deep overhangs and slender detailing enhance the effect.
Propose a parapet
The other very common way of dealing with the eaves is to have a parapet. This simply means that, instead of sitting the roof on top of the wall, you build the wall a little higher and hang the roof inside it, thus concealing the edge from view, as seen here.
Given that parapets are common features of many Georgian and Victorian houses (even though they did not conceal flat roofs) this approach can work particularly well when designing an extension to an older house, as parapets are very much a part of our traditional vernacular architectural language.
The other very common way of dealing with the eaves is to have a parapet. This simply means that, instead of sitting the roof on top of the wall, you build the wall a little higher and hang the roof inside it, thus concealing the edge from view, as seen here.
Given that parapets are common features of many Georgian and Victorian houses (even though they did not conceal flat roofs) this approach can work particularly well when designing an extension to an older house, as parapets are very much a part of our traditional vernacular architectural language.
Let in light with a lantern
Another traditional feature many people like is the orangery-style glazed roof lantern. While the lantern itself has sloping surfaces, it will almost always sit within a flat roof.
In this example, seen from inside the roof level, the roof’s surface has been decked out with timber and planting – another thing that couldn’t be done on a pitched roof.
Another traditional feature many people like is the orangery-style glazed roof lantern. While the lantern itself has sloping surfaces, it will almost always sit within a flat roof.
In this example, seen from inside the roof level, the roof’s surface has been decked out with timber and planting – another thing that couldn’t be done on a pitched roof.
Go big on glass
Glazing can be used within flat roofs in a number of ways and can even form much of the roof surface itself. To create this large expanse of glass roof, even the support beams have been made with laminated glass, meaning there’s minimum visual block.
Great care needs to be taken with the detailing and specification of such designs to ensure the space won’t become overheated in the sun or freezing cold in winter.
Glazing can be used within flat roofs in a number of ways and can even form much of the roof surface itself. To create this large expanse of glass roof, even the support beams have been made with laminated glass, meaning there’s minimum visual block.
Great care needs to be taken with the detailing and specification of such designs to ensure the space won’t become overheated in the sun or freezing cold in winter.
Make good use of it
Unlike pitched roofs, which cannot really be used for much more than perhaps housing solar panels, flat roofs can be used in a range of different ways. They can become balconies or roof terraces; as already seen, they can incorporate glazing in the form of roof lanterns or flush-glaze roof windows, and they can be planted.
Get helping finding a windows and glazing professional.
A common choice for planted roofs is sedum, which is part of the cactus family. It’s tremendously practical and needs much less soil and irrigation that most other options.
Read a beginner’s guide to green roofs
Unlike pitched roofs, which cannot really be used for much more than perhaps housing solar panels, flat roofs can be used in a range of different ways. They can become balconies or roof terraces; as already seen, they can incorporate glazing in the form of roof lanterns or flush-glaze roof windows, and they can be planted.
Get helping finding a windows and glazing professional.
A common choice for planted roofs is sedum, which is part of the cactus family. It’s tremendously practical and needs much less soil and irrigation that most other options.
Read a beginner’s guide to green roofs
Grow a garden on it
For real gardening enthusiasts, or for those with very limited garden space, a flat roof can – where Planning Permission will allow – be the perfect solution. What a wonderful and beautiful way to put an otherwise wasted surface to work.
Clearly, the amount of soil and absorbed water a garden needs means the roof will have to be constructed to accommodate a heavy load. However, as long as this need for an additional load-bearing structure is known when an extension is being designed, it shouldn’t add horrendously to the cost.
For real gardening enthusiasts, or for those with very limited garden space, a flat roof can – where Planning Permission will allow – be the perfect solution. What a wonderful and beautiful way to put an otherwise wasted surface to work.
Clearly, the amount of soil and absorbed water a garden needs means the roof will have to be constructed to accommodate a heavy load. However, as long as this need for an additional load-bearing structure is known when an extension is being designed, it shouldn’t add horrendously to the cost.
Be budget-friendly (and stylish)
If you’ve still not decided, remember these facts:
What kind of roof did you choose for your extension and why? Share your tips in the Comments below.
If you’ve still not decided, remember these facts:
- Flat roofs are generally the most inexpensive way to build a roof over an extension.
- Leaks should not be an issue.
- With a little imagination and some sensitive detailing, flat roofs can be very cool indeed.
What kind of roof did you choose for your extension and why? Share your tips in the Comments below.
First of all, let’s get the old chestnut that flat roofs are prone to leaks out of the way. It’s true that in the 1960s and 1970s, flat roofs with three-layer felt and no ventilation gaps caused no end of problems, but that’s history. Most commercial buildings in the land have flat roofs and businesses would not invest in a patently unreliable choice.
With modern, single ply EPDM (synthetic rubber) membranes and GRP fibreglass systems, there’s no reason – other than poor workmanship, which applies to any roof – that a flat roof will leak. Also, these days we understand condensation, the cause of damp and rot in so many old flat roofs. Today’s ‘warm deck’ construction avoids such problems.
Get inspiration about materials other than brick from which you can build an extension