Ventilation for flat roofs

We had a call to our technical line earlier today from a customer asking about when or whether to ventilate a flat roof. The answer may be useful to some of you.

Firstly, there are two types of flat roof. A ‘Warm Roof’ and a ‘Cold Roof’.

Warm Roofs

A warm roof is defined as being when the all or the majority of the insulation is above the joists or deck. The roof itself is therefore kept warm. By ‘majority’, the rule of thumb is that in a domestic house (not flats), approximately a third of the total thermal performance of insulation can be placed below the deck with two thirds above the deck (sometimes this is called a ‘hybrid warm roof’). Ventilation is not needed in a warm roof.

Cold Roofs

A cold roof is where all of the insulation is placed either between and under or completely under the joists or deck. No insulation is on top. In this way the roof itself is kept cold. You only need ventilation in a cold roof. Cold roofs, by the way, are not recommended for high humidity areas – see the blog on ‘How to insulate roofs of swimming pools, saunas and high humidity areas’.

BS 5250: 2011 Code of Practice for Control of Condensation in Buildings stipulates that there should be a ventilation gap between the insulation and the bottom of the flat roof deck of at least 50mm. In a concrete deck, for example, this gap could be created by fixing 50x50mm timber battens at 600mm centres and attaching the insulation to underside of these battens. Where the battens abut the wall, there should be a ventilation gap created to allow air from the outside to be drawn into this space.

The amount of ventilation or air change needed can be calculated by using Approved Document C and Approved Document F (in England & Wales) and Section 3 of the Technical Handbook (Scotland). However, it should be noted that cold flat roofs are not encouraged under Scottish Building Regulations because of more extreme weather.

The majority of domestic cold flat roofs use insulation ‘between and under’ the joists. The rule of thumb for foil faced boards (like Thermapitch TP10 or Kooltherm K7) in this scenario is that you can have no more insulation under the joists than you have between the joists. It is preferred to have the bulk of the insulation between the joists to reduce any risk of condensation.

More information:

BR 262 – Thermal Insulation avoiding risks (Pages 17-21 cover warm roofs and pages 22-23 cover cold roofs, giving a summary of the regulatory requirements and general good practice recommendations for VCLs, ventilation etc).


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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “Ventilation for flat roofs
  1. include this in your answer too that warm roofs are mostly used in the humid areas like swimming pools where there is less insulation, so whenever you talk about flat roof you must think that which area of your house is concerned. Then you will be able to use cold or warm roof on flat roof.

  2. Thanks for the useful information. We are finding more of our own customers are asking about ventilation and your guidance will give us a useful rule of thumb to help them make a decision.

  3. I had not known the difference between a warm roof and cold roof, until reading your blog. Thanks for the information though, it is always wiser to consult experts like you before we decide to ventilate flat roofs. Also serves as a good post for the maintenance of roofing in different houses with varying climates.

  4. I am looking at insulating a cold gage roof ,as i can not put a air strip above the felt as it is a continues piece.Can I insulat at the end of the garage by drilling through the brick above the line of insulation.Do you supply a vent that would do this

  5. Can I warm deck a flat roof with say 50mm insulation on top of the deck and 50mm between the joists to achieve the required u value for building regs? If so would the 50mm air gap be at the top or bottom of the joists? My joists are 100mm height.

    Many thanks chris

  6. I own a flat in a block with a 2m high flat roof void which is suffering from interstitial condensation. What means of curing the interstitial condensation can you recommend?

    • Hi Richard,
      There are a number of causes of interstitial condensation in this situation, that it would be best if you had a chat with one of our advisors so we can pin it down for you. Some will depend on whether you have any insulation; where that is located and also if you have adequate ventilation or any ventilation at all.
      Give us a call on 01544 388 601 and ask for the technical team.


    • Hi Barry

      Yes, 50mm minimum cross-ventilation will be required between our insulation (installed between the rafters) and an unbreathable plywood deck with felt finish. We recommend speaking to a ventilation specialist company, such as Glidevale, for any further advise.

      Hope this helps.

  8. I have an outhouse with a flat concrete roof, rough 6-8 inches thick with no insulation above or below. The ceiling is boarded and skimmed. The ceiling low and no room to put an insulated ceiling below. Could I raise the roof above with joists, damp proof, insulate, timber roof on top then felt? At the moment I am getting damp all over ceiling. Please any help would be gratful!! Thanks.

    • Hi Rob

      You have the right idea about how to stop the condensation on your ceiling – by creating a warm roof. However, if your roof is completely flat a fall should be created to avoid ponding of rain water. This can be done by a screed, by adding timber firings with a layer or plywood on top, or by using a tapered insulation. Take a look at the Thermaroof TR27 product literature for guidance on how to create a flat roof with a felt finish.

      Hope this helps.

  9. We are developing a warm/planted flat roof where it’s overall buildup is to be kept to a minimum.

    Within our buildup, we are considering the following arrangement (from top to bottom):

    – Green Roof Buildup
    – Water Proof Layer
    – Tapered Insulation (lowpoint 100mm to highpoint 200mm)
    – 18mm OSB Board Deck
    – 150mm insulation between the roof joists/beams
    – Vapour Barrier

    Is it a problem (interstitial condensation) to have the OSB between the layers of insulation?
    Is the Vapour Barrier required?

    Many thanks!

    • Hi Jon

      The vapour barrier should be on top of the OSB roof deck, lapped and sealed to the waterproofing membrane, rather than below the joists. With the vapour barrier in the correct location, the tapered roof construction will have an interstitial condensation risk on humidity class 3 or higher between the insulation layers.

      We suggest calling our technical department on 01544 387 382 who can run a calculation to specify minimum and effective insulation thicknesses to achieve a particular U-value, without an interstitial condensation risk. Alternatively, simply remove the insulation between the joists.

  10. Hi
    I have a 10 degree sloping roof that was refelted last year (hot torch). Since then we have had a lot of condensation and have had to take down ceilings and remove saturated rock wool. I am looking to increase the insulation and put Kingspan between the joists leaving a minimum air gap of 50mm.
    Should I put a vapour barrier between the new plasterboard and the joists/Kingspan?

    • Hi Steven

      Yes – a 50mm fully ventilated void between the insulation and the roof deck, with a vapour barrier between the plasterboard and the Kingspan insulation, should prevent any further issues with condensation.

  11. Hi
    I have a flat roof which I want to put insulation underneath as above I don’t want raise the floor to much because of patio doors in bedrooms
    On the underside what thickness insulation boards do I need to use , what gap between the insulation and bottom of the flat roof do I leave , do I have to then drill holes from one end of the roof to the other for air to flow through, and what size hole would it need to be all the way across
    Nick parsons

    • Hi Nick,

      For Cold Flat Roof Constructions we would recommend either installing Kooltherm K107 or Thermapitch TP10. Please be advised in this type of construction, where the space above the structural deck is unheated and the space below is heated, a minimum of 50mm of ventilation is required between the insulation and structural deck. Guidance should be sought from a ventilation specialist (such as Glidevale) as to how this cavity should be ventilated. With regards to an appropriate thickness of insulation this will depend on your U Value Target and construction build-up, please see our online U Value Calculator for further assistance.

      Hope that helps,

  12. Hi I have a slight ‘lean to roof’ at the rear of my flat (with no room above it) and I have just had a new GRP roof put on it and they only used 75mm deep trusses. They have then fully insulated the depth of the trusses which I am then expected to just plasterboard on the underside. I am not sure of the exact insulation they have used but it’s not boards, it’s more of a wool. I have the same concern as a lot of other above I.e does this roof need a ventilation void between the GRP roofing and the topside of the ventilation? Any advise would be very much appreciated.

    • Hi Robert,

      Our general guidance for cold flat roof constructions would be that a minimum of 50mm ventilation would be required between the structural deck and the insulation. We recommend that you contact the insulation manufacturer to confirm their specific requirements as to the roof build-up in conjunction with their wool insulation.

      Hope that helps,


  13. hello we have been asked if a cold roof needs extra ventilation. We have an asphalt roof laid on ply with 100mm kingspan between joists with 50mm gap does it need to be cross vented.


    • Hi William,

      The 50mm gap between the insulation and the plywood should be fully ventilated, with constant air flow from one side of the roof to the other, to avoid a condensation risk.

      Hope that helps,

  14. Would a 50mm gap still be required for the above where the trusses are only at 75mm? If so how are you to achieve necessary uvalues if you only have 25mm to play with? THis is something that had bothered me for some time.

    • Good morning

      Ventilation would be a requirement in a cold flat roof regardless of the joist depth, as specified in BR 262 of the Building Regulations. In this case where you have limited space between the joists for insulation, we would recommend either extending the joists using additional timber battens, or fixing the insulation in a continuous layer to the underside of the joists. Using a continuous layer of insulation would result in a thinner build-up overall.

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