Spotlights seem to be the lighting solution of choice for garage conversions, small, effective, and unobtrusive. However, they are not ideal for garages with cold deck flat roof construction. This post explores and explains the relationship between the roof construction and spotlights highlighting the potential issues for your conversion project.

But don’t worry we don’t leave it there; we also suggest some great alternative solutions. Solutions that could save you thousands of pounds and the heartache of pulling apart your garage conversion.

What's a cold deck anyways?

Most garage roofs will consist of timber beams known as rafters which span from the house to the exterior garage wall with a deck on top of them. The deck is typically a sheet timber such as ply or 6 inch boards secured close together. A weather proofing element such as mineral felt is adhered to the top (outside surface) for protection. This type of roof is perfectly good for a garage but as part of becoming a habitable and heated room it requires insulating. And this is where the problems and difficulties begin.

Spotlights can turn your perfectly good garage roof and brand new ceiling into the roofing equivalent of a soggy beermat with the structural properties to match.

Diagram of Spotlights in cold and warm deck roofs

Interstitial condensation - explained via beer

Now while I couldn’t pronounce Interstitial condensation with a beer in me, I can use a beer analogy to explain the science and how it relates to our garage roof. When a cold glass of beer on a warm summer’s day sits on a table it doesn’t take long for water beads to form on the outside of the glass. What starts as a rather attractive mist quickly becomes a small stream making its way to your beermat which overtime becomes a soggy mess.

The science here is warm air which holds moisture as a vapour is condensing and turning back into water due to the cold surface temperature of the glass. The worst that can happen in this instance is a few droplets end up on your trousers as the glass makes its way from mat to mouth. However, in the case of our garage roof the moisture can cause thousands of £'s in damages.

The reason for this is warm air from your newly converted garage makes its way up past the insulation and reaches the underside of your garage roof. This part of the roof is cold like the beer glass, and quickly turns moist air to water which forms on the surface rotting the wood and dripping onto the insulation and backside of your plasterboard ceiling. Of course, this all goes on unnoticed until its often too late and major repairs are needed.

And the spotlights?

The spotlights while not entirely the cause of the issue can certainly exacerbate the process. Spotlights mounted in your ceiling require holes to be drilled. Big 65-75mm holes for the lamp unit to clip into, with large areas of insulation cut away to facilitate their placement. And herein lies the problem, what was planned as a lovely vapour tight insulated build has football size holes cut into it. Holes that draw the warm air up and into contact with the freezing underside of the roof deck. Hopefully by now you realise why this is a bad idea, read on to learn about the options available and what I recommend.

I still want spotlights!

Ok I understand, they are attractive and can solve the low headroom issues that occur in some garage conversions. If you're going to do it, it needs to be done right. Expect to pay the builders a little extra and make sure you explain this upfront.

Fitting spotlights to a cold deck garage conversion requires two things, excellent airtightness around the insulation and ventilation to the cavity between the insulation and roof deck. 


Let's tackle the ventilation first while considering the insulation we are going to fit next. Adequate ventilation will be required across the roof structure. To ensure the air flows across we need to think about where it will enter the roof space and where it will exit. Typically, we use fascia vents or telescopic air vents for the inlet and mushroom vents or abutment vents for the outlet.

Holes drilled through the joists will allow air to flow through the chambers which will be created by the insulation. The drilling and location of these holes should be confirmed with building control or a suitable professional. There are specifics around the size and location which we may cover in a future article.


Create a tight seal, firstly take care with the insulation, which is more than likely going to be rigid PIR to achieve the U value required by building regulations. Make sure the insulation is cut to fit tightly against the rafters and be sure to tape all the joints. This forms a vapour control layer stopping warm air from reaching the cold space. Around the lights themselves can be trick especially if the light unit is bigger than the depth of the insulation. These spotlight covers will help or consider constructing a sealed area above the lamps as shown. You can make the tack easier by buying some of these low profile spotlights which may be able to fit without breaching the insulation.

Changing the roof structure

Insulating a cold deck roof is an essential but difficult part of your garage conversion. An alternative design solution which eliminates potential condensation issues is to completely change the roof structure to a warm deck. A balanced view of the costs and benefits to this decision needs to be taken but generally older roofs with limited remaining lifespan are generally worth changing during your garage conversion project. For a more detailed view on warm deck roofs subscribe to our newsletter and look out for a future article.

Warm Deck Structure

A warm deck roof places the insulation on a deck above the joists on the external side of our room. This means the internal surface is protected from the low external temperatures and remains warm. This warm surface should not cause moist air to condense and therefore prevents the issue of interstitial condensation. See diagram for the arrangement of the insulation in the roof build up and how it compares with a cold deck.

Spotlight Alternatives

Surface mounted downlights

Very similar in design to regular spotlights except these units come in a housing which has a decorative quality and are designed to be mounted on the underside of the plasterboard. This limits the hole in the insulation to around 10mm for the supply cable. 


Pendants are a great alternative to spotlights and a single pendent can provide adequate lighting for a single garage conversion. A pendent requires a single small hole in the insulation layer for the cable which should be no more than 10mm in diameter. 

Wall Lights

Wall lights require no holes in the roofs insulation layer which makes them one of my preferred options. However, on their own they may struggle to provide enough general light to the room and are better suited to task light or accent light duties. As a side note wall lights are great for home office conversions where it is not ideal to have light from above. 

Surface mounted Panel Lights

Like pendants these units have the benefit of a small single hole in the insulation layer. Unlike pendants they don’t hang from the ceiling and therefore are well suited to applications where the floor to ceiling height is on the low side.


Fitting spotlights in a cold deck flat roof is a risk that needs to be understood and detailed properly by your designer and builder. You can remove the risk by upgrading the entire roof system which may make sense if the lifespan of the existing roof is limited. Alternatively choose from a range of lighting solutions such as surface mounted downlights that achieve similar results without the condensation risk.

About the Author Michael NEGC

Michael is the Architectural Designer and Surveyor at North East Garage Conversions.
Michael has a HNC in Civil Engineering and a HND in Construction Management. Previous experience includes multiple renovations, a self build plus working as an Architectural Technician, Designer and Project Manager.

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