So you’ve settled on a single storey rear extension to create an open plan kitchen/diner. Like many homeowners you may have automatically assumed it will have a sloped roof with standard Velux type roof windows, however this isn’t always the best option in the real word. Let’s weigh up some of the benefits and downsides of a flat or sloped roof extension:

Many homeowners overlook the potential benefits of a flat roof and here’s why; unlike the leaky flat roofs of old, modern flat roofs can be both long lasting and cost effective when properly implemented, making them a great option for many extensions.

While there are sound aesthetic reasons to use a sloped roof, and they are certainly appropriate and highly successful in many residential applications, there are advantages to flat roofs which should be considered. Most people assume that a sloped roof is inherently superior to a flat roof, but that’s an outdated viewpoint which may restrict the potential for bespoke design solutions.

Flat or sloped roof extension comparison

The most common reasons cited by homeowners for preferring a sloped roof are:

  1. Flat roofs leak
  2. Sloped roofs look better
  3. Flat roofs don’t last as long
  4. Flat roofs cost more for structural reasons

Whilst these concerns are valid with ageing flat roofs, modern technology and design techniques are bringing flat roofs back into style:

Do flat roofs leak?

The most common misconception regarding flat roofs is that they leak simply by virtue of their flatness. This may have been true in the past when individual felts were adhered to the deck below with mopped asphalt and/or ballast (a tar-and-gravel roof), but modern membranes and adhesives come with 25-30 year warranties as standard and the membranes themselves are durable, economical, and long lasting.

The reality is that the most vulnerable location for any roof in terms of water ingress is at the connections, i.e. the flashing points, and if that is poorly detailed and executed then the type of roof (flat or sloped) makes no difference. Water will get into a poorly constructed joint regardless.

Which looks better – a sloped or flat roof extension?

Aesthetically a sloped roof is certainly appropriate in many situations, but where space is tight and neighbours close, as in a terrace situation, then a sloped roof can quickly become a dominant feature which limits the flexibility in plan arrangement.

How long do flat roofs last?

In terms of longevity, as with any build up, the system will last only as long as the lifespan of its weakest component. For sloped roofs and flat roofs alike this is generally the flashing.

How much does a flat roof cost?

When it comes to cost, there’s very little difference depending on the prevailing economic climate. Roof tiles couldn’t be had for love nor money just a year or so ago for example, making flat roofs a practical and economical option.

Add flexibility to your extension with a flat roof

The biggest benefit that a flat roof provides over sloped is the added flexibility of plan layout below. The spatial arrangement of the accommodation can be ideally tailored to suit your needs without having to fit the form into a particular mould, thus allowing the plan to be designed from the inside out. Daylight is still provided via roof lanterns (flat for a contemporary feel or hipped for more traditional flavour) and the thermal benefits are not limited by rafter depth.

Other benefits of a flat roofed extension

A great reason to consider a flat roof is the potential to create a green living roof via plants which provide all manner of environmental benefits and create a truly unique feature for your new extension.

Another option and one that is less well known is the potential to create a blue roof, which is where rainwater is held for a short period (24-48 hours) after a storm to provide micro-mitigation and not overwhelm the drainage system. What is in effect a shallow rooftop pond is then drained after the storm has passed. Owing to the minimal additional loads involved there is almost no increase in cost or complexity, and it is something that can be retrofitted later.

Whichever choice of roof form you prefer, the overarching consideration should be which is the most appropriate solution that will work for the overall design, and not restrict yourself to preconceived notions based on outdated received wisdom.

If this article has got you wondering which option might be best for your extension project book a free consultation with one of our home extension experts today!