A flat roof is a common choice for a low-rise first floor extension, perhaps for a kitchen space or garage. Very often you will see them on a temporary building, or perhaps a more modern square box home construction.
But a flat roof is never level. Code states that there must be a minimum slope of a ¼ inch in 12 for all but specialist coal tar roofing, which means that in every foot the roof height must decrease by ¼ inch. So, when is it a good idea to choose a flat roof construction?
Advantages of Flat Roofs
Let’s start by looking at the main advantages of flat roofs.
Relatively Low Cost
The most obvious advantage of building a flat roof is the cost. Building a flat roof will use between 30% and 60% less timber material than either a mono or dual pitched alternative.
It seems obvious, but a flat roof will mean you, or your contractor, will need less scaffolding, as the build will not be going as high as it might be with a pitched roof. So, there are incidental hidden savings that will all impact the most important item, the bottom-line cost of a project. Not spending money here will mean more wiggle room in other areas of the project.
Repairing a flat roof is also less costly due to the lower cost of materials and the ease of access to a flat roof structure.
A flat roof is the faster option. From open skies to a dry inside space takes less time, which also means savings with regard to labor costs too.
Potential to Be Utilized for a Garden or for Solar Panels
A flat roof can be used as an alternative, or additional outside space. An elevated garden or roof terrace can help make the most of any view, whether city or country.
That said, while roof gardens are a great, romantic idea, trailing root systems and pests, attracted by the plants, soil nutrients and water, could cause problems. Invasive roots can find their way into the roof structure if left unchecked, developing leaks that are entryways for water and insects.
As such, if your flat roof is exposed to the sun, let’s say it is south facing, then you might consider it to an ideal site for positioning solar panels instead. On a flat roof they will be simpler to install and maintain than on a pitched roof.
Ease of Adding an Extra Floor in the Future
If you are considering a single storey extension, building a flat roof would make a future second storey extension simpler as the roof structure could be re-used as the floor for the new level.
There might not be the money in the pot right now, or the need for an upper storey extension, but putting in a flat roof now makes a potential second-phase, second-storey extension more cost effective.
Access to Light
If you are in a built-up area in a city the access to light is a serious issue for planning authorities and there are some districts that have adopted strict codes to protect this amenity.
A flat roof extension could deliver the additional space you require without impeding the access to light enjoyed and demanded by your neighbors.
But it’s not all about your neighbors either. A flat roof will not impinge on any existing second floor windows in your own home, meaning fewer alterations and the views enjoyed from these upper floor rooms will also not be blocked.
Disadvantages of Flat Roofs
While they come with some important advantages, there are also some negatives to consider.
Ponding Problems and Lower Durability
A flat roof can suffer from ponding over time, this is where outlets that get blocked are not cleared as quickly or as regularly as they should be. Standing water is a problem on its own but when it goes through cycles of freezing and thawing it can damage a felt or mono-skin polymer roofing material.
To counter problems with ponding, allow for regular maintenance inspections so that problems and potential issues do not go ignored.
Flat roofing materials such as felt and single ply polymer systems don’t last as long as slates or tiles and will require renewing at more regular intervals. While some systems give a 20 year guarantee, others will need to be replaced within ten to fifteen years.
A flat roof is not a good idea in areas that receive regular heavy snowfall. Snow is fun and light to throw around but can become heavy at relatively low depths. Over time this will stress the flat roof structure.
In the summer, a flat roof exposed to the full force of the sun all day will need to be properly insulated to ensure that the room below does not turn into an oven!
Access to Services
If you are planning on siting HVAC or solar panel systems on a flat roof then you will have to ensure ease of access. This might mean forming a new door from an upper floor room, or installing a galvanized drop-down ladder, which will add to the expense of the project.
Fixing HVAC or solar panel systems securely on a flat roof will also make access to repairs on a flat roof more difficult. While a bank of solar panels might save money on your energy bills and afford the roof surface a little more protection, the cost of uplifting the panels to give access to future repairs and renewal of a roofing membrane, should not be discounted.
Another issue with choosing a flat roof is that penetrations for such things as chimneys, cable and ducts, no matter how well they have been flashed and waterproofed, will remain possible entryways for water and pests.
Access might be easier, however, foot traffic can contribute to membrane damage, and ultimately make leaks more likely.
Sometimes it can be almost impossible to discover the source of a water leak on a flat roof, which would mean the uplift and renewal of the entire roof. An expensive prospect.
In some parts of the world flat roofs are the usual way a house is built, but in the US, where there is a wide range of climate zones, there is also a wider range of architectural styles. If you are concerned about the budget for your extension or new build project, then a flat roof option will save money and time upfront, without looking out of place in the neighborhood.
All home repairs cost more than we would like, so be prepared to allow for regular monitoring. This could be something as simple as remembering to look out of an upstairs window once a month, or after notable weather events.
Model building codes are frequently updated and your own local code may have specific requirements, so before you decide on the way forward it is always advisable to check. You should also check our comparison of pitched and flat roofs.
Lastly, it’s good to note also that the technical term for a flat roof is in fact a ‘low slope’ roof and is oftentimes used in official documentation.