In new flat roofs it’s becoming more and more common to undergo warm flat roof construction to abide by building regulations and save lots of money on your insulation costs. If you do not abide by the current building regulations, then you will not receive a building regulations compliance certificate which could affect the sale of your house should you wish to sell it.

So what is warm flat roof construction?

A warm flat roof is formed by placing a layer of insulation over the top of the roof structure. Due to the insulation being over the top of the roof, the timber structure beneath is warm, which is why the term ‘Warm Roof’ is formed.

Steve from London Flat Roofing explains how warm flat roof construction works in this excellent short video:

Building Regulations

When 50% or more of the roof’s waterproofing layer is removed and replaced this is called refurbishment. Part L of the Building Regulations states that the thermal efficiency of a refurbished roof be brought up to current standard if it doesn’t meet a minimum thermal performance. If your roof insulation doesn’t meet the minimum standards, it has to be upgraded! However, this does not apply if you are repairing your flat roof by overlaying with a new waterproof layer.

These factors can trigger part L of the building regulations document:

1. If you strip the covering of an old flat roof, you automatically trigger part L of the building regulations document.

2. If your current insulation is poor, then you trigger part L of the building regulations document.

3. If you are undergoing a domestic or a commercial rebuild, it must comply with part L of the building regulations document.

The aim of the document was to ensure buildings were constructed or modified to provide better energy efficiency and tackle the problem. As climate change research continued, Part L was further reviewed for England and Wales, to take effect from April 2006. The new goals of the document were to reduce CO2 emissions by 20% on new build dwellings and 27% on buildings other than dwellings, these reductions were taken from the baseline of 1990 and are thought to be achievable by 2010.

You can find a link to the building regulations document here.

What else?

In a warm flat roof construction, the insulation is positioned above the structural deck and no ventilation is required.

Throughout the course of the year the roof deck and all below it is kept at a temperature close to that of the inside of the building, therefore the roof structure is protected from extremes of hot and cold, lessening the potential for damage caused by thermal movement.

A warm deck also provides added protection from the dangers of condensation as the structure is kept warm, at a temperature above dewpoint, by the insulation above it. Therefore water vapour which enters the roof structure from the room below will not have a cold surface on which to condense. NHBC recommend that this type of roof be considered as the standard form of construction.

I hope this clears things up for you.

The R&D Team

2 Comments. Leave new

Martin Normanton
October 1, 2015 10:08 pm

hi Steve. In an earlier video you showed how to seal and insulate behind the fascia of a warm roof, so that you avoid condensation on the fascia, and even more importantly avoid cold air circulating between a warm roof and the ceiling below, which can completely defeat the aim of a warm roof. So far, so good. In my case I have a cold roof which needs re-roofing, and also several boards need replacing.One side has a fascia, one meets the house wall, and the other two have parapet walls in 9″ solid brick. So if I make the new roof a warm roof, insulation behind the fascia is straightforward, if fiddly, I “just” need to persuade the roofer to close and insulate the gaps between joists by taking off the old fascia and inserting pieces of rigid insulation between joists and use expanding foam to seal the insulation to the joists, roof board and wall (including the ceiling plasterboard to the thickness of the IWI in the room below). MY QUESTION therefore concerns the detail at the solid parapet walls. Presumably these also should be insulated in the same manner, as otherwise they form a classic cold bridge in a ring all around two sides of the roof. In this case I have no access from the outer side, unlike the fascia side of the roof, so the way I envisage doing it is to remove about 300mm of roof board along both walls, and work from above. Fit a 150mm strip of new roofing board all along, so that the expanding foam has something to seal to, then insert pieces of 100mm foil faced Celotex in each joist space and seal with expanding foam. And finally fit new roof board to close the remaining strip of removed board. A lot of fiddly work, SO DO YOU HAVE ANY ALTERNATIVE IDEAS? I don’t want to remove part of the ceiling and work from below, as the room is in use as our kitchen-dining room, and also sealing with expanding foam is more difficult working from below.

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Martin Normanton
October 1, 2015 10:16 pm

Sorry, I mean hi Danny, in the video by Steve of London Roofing, he shows…

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