New flat roof design problems

This email below was sent to me by a follower asking come very good questions and over the next post I’m going to answer these question for him and for everybody.

It must be noted that I am not trained in building design, my training is only in laying flat roof coverings all other information is information I gain from study and talking to the various engineers, surveyors  architects I work with.

Hi Steven,

I am new to SIG Roofing and have been asked to look at insulation as part of our flat roofing offering. I have followed your videos several times and you are the most knowledgeable person I have come across in this area.

I have a couple of questions that I would really appreciate some help with and your best advice. I hope this will give us a chance to put forward an innovative offer to you and the market if I can overcome them. Image below.

Where new insulation is placed on top of the exiting roof structure, if the existing wall is not built up to the underside of the roof (as a minimum standard) then heat from the room below can bypass the new roof insulation and cause a thermal bridge. If this is how the work is done then surface condensation is very likely to occur on the inside of the ceiling/wall junction leading to potential mould growth. It is essential that the eaves is sealed to at least the same standard (in terms of U-value) as the external wall to prevent the thermal bridging/condensation from occurring. In practical terms, because the roof height is being raised, a new fascia will need to be provided, and this will allow access for the wall to be built up without the need to resort to an internal intervention.

Warm flat roof with roofing insulation on top of the timber decking
Warm flat roof

The likelihood of building up the wall is slim due to the cost and time for a flat roofing job. I would appreciate your comments on the above either why it would not need to be built up in your opinion or an alternative approach to doing block work.

The second question

A cavity tray should be provided at the abutment of a cavity wall and a flat roof, this shown in red below. This should be provided even where one did not exist before (please confirm), however it is an absolute requirement where one does exist. A cavity tray prevents water (which enters the cavity wall above the flat roof) draining down the cavity into the dwelling below. Where an existing cavity tray exists, this will be below the level of the raised roof surface, thus will direct any water into the roof void.

To install the cavity tray, portions of the existing brickwork above the flashing is removed and a single section of cavity tray is inserted. This is then pieced/pointed in and the next section of brickwork is removed etc.

I would have thought it is unlikely that the additional work of raising a cavity tray would be done? And it would either be a straight overlay or thinner insulation which would not interfere with the tray installed. Thinner insulation being something that would not meet Part L for replacement of a flat roof.

In a practical world what do you think?

The easiest way for me to anser this question was to produce a video of the problem and I have posted it below


One comment on “New flat roof design problems

  1. Dave says:

    Hi Steve,
    Thanks for the info regarding the two different types of flat roof. Could you explain how i would through ventilate with vents at soffit and roof abutting existing house? What would you use to vent at the existing house side?

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