Kieran McCarthy: What natural light options do I have for my attic?

There are pros and cons to choosing a rooflight over a dormer window, writes Kieran McCarthy

Mon, 19 Jun, 2023 - 14:00

Kieran McCarty

Hi Kieran, I am considering converting my attic but before I speak to a builder or carpenter I’d like to get your views on what is involved in adding a dormer window.

I believe standard skylights are fine but my partner thinks it would be better to add a larger dormer window to give more space and light. Is there a significant additional cost and does it require planning permission?

Thank you,

Sheila, Croom, Co Limerick

Hello Sheila, Thank you for your question. So you are thinking of converting your attic and seeking to add light by installing a dormer window. Great idea but you need to get the detailing right.

I might start briefly with the attic conversion element, if I may. There are countless attic conversions carried out every day, many many without full consideration of an efficient spatial layout or, even more importantly, integrating the vast array of building regulations that apply.

In many cases, there is much room lost on the existing first floor to get the stairs in place and then the quality of the space achieved in the attic is not a vast improvement on the space lost on the first floor, which naturally comes with a substantial investment. So best to ensure that you get this attic conversion properly designed out and engineered before embarking on this venture.

The next step, now that we are confident that our attic conversion offers value in terms of space, and will meet all the necessary building regulations, is to look at installing a window to add light and an element of ventilation. As you suggest, the two principal ways to achieve this are to add a simple skylight or to go a significant step further and add a dormer window, so let’s look at what is involved in these two approaches and outline the pros and cons.

A rooflight is undoubtedly the simplest solution in this regard. A rooflight can be purchased from your local building provider. A roofing carpenter can cut back the ceiling plasterboard (if present) internally and remove some slates or tiles externally. and ‘trim’ your roof support timber to the rectangular shape of the outline of your rooflight. They can install the rooflight and reinstate the internal and external finishes.

Externally they will also need to install the flashing to seal to the slates or tiles (these come with the rooflight). Internally they may need to insulate and install airtightness tape and possibly membrane before reinstalling the plasterboard ready for a plasterer to finish out.

The main advantage is this is a quick and relatively cost-effective installation. The disadvantage is that it only really offers some additional light and purge ventilation, it doesn’t really improve the quality of the internal space as you are still somewhat confined due to the presence of the sloped ceilings of the underside of your sloped roof space.

A dormer window however is a totally different ballgame. To install you need a team of roofers to cut back a much larger opening in your roof. This may involve some horizontal supports or purlins to maintain the strength integrity of your roof with an alteration of this size (so check with an engineer).

Next, you need to frame out the new dormer. This involves the side and front walls and a roof structure. The walls will need to be lined with a breather membrane and lightweight cladding on the outside, insulation between the timber studs, and an airtightness layer and insulated plasterboard on the inside.

The roof structure will need to have a roof covering on the outside (depending on what will work in this small area), some level of ventilation, and a layer of insulation and airtightness membrane above your ceiling plasterboard. Next, you will need to install a window (supplied by a window manufacturer) and airtightness tape around the edges. The plasterer can then finally plaster all the internal finishes. Your roofing team will need to create their own flashings externally using lead which they can cut and bend to the necessary shapes to seal all the junctions.

The benefits of the dormer window is they really transform an attic as they raise the ceiling locally and provide a really usable space in terms of functionality and value. The window too is fully vertical so better views can be enjoyed from this space compared with a roof light. The downside of this solution is of course the cost and the complexity. A dormer is probably four times the cost of an off-the-shelf roof light, because of the quality of material and the labour involved.

When it comes to planning permission, rooflights on the rear of a building do not require planning permission but they likely would on a front elevation. A dormer window, even on the rear of a building, will likely require planning permission and in particular you will need to be cognisant of separation distance from your neighbours’ properties and so on.

You will also need to ensure that a dormer window will not detract from the aesthetic of your elevation (an architect can check this for you), though a rooflight does not substantially affect your elevation in any meaningful way.

So, to sum up, a dormer costs way more and is much trickier to get to work visually, but in the long run, if you are using your attic regularly, particularly for a home office, it could save your sanity to enjoy the headroom and distant views whilst stuck on the log rambling phone call at 5.50 in the evening.

Kieran McCarthy is a building engineer and director of KMC Homes bespoke A-Rated new home builder, serving Cork and Limerick. He is also co-presenter of the RTÉ property show Cheap Irish Homes.