Flat roof design questions from a customer answered

Thanks for contacting me with these great questions which I’ll try to answer for you.

Q1: The roof is between 25 and 35cm thick concrete (the falls being provided by varying thickness of the roof), with concrete ceilings underneath the roof. As far as I know, there is no insulation.

Given this thickness of concrete involved, should I be considering adding insulation on top, or is the thickness such that I would get very little extra benefit for the cost?

A1: The concrete offers some insulation properties, and by doing a U-value calculation of the buildup, we may find that you don’t trigger part L1b of the building regulations and therefore you will not have to insulate. However this doesn’t mean you should not insulate, it just means you don’t have to. Personally I would insulate if conditions allow; regulations will get tighter over the next 40 years and the best place to put your insulation is above the concrete and below the new roof covering, thus forming what’s called a warm roof.

Q2: If you were still to recommend insulation for this roof, would it be easy to lay it on top of the felt coverings, or would you strip the coverings off?

What about the integral gully (which would become recessed below the insulation)? Would you have to raise it? Could this be done easily?

A2: If the existing felt is sound, it’s best to lay over it, saves stripping it, and it now becomes the vapour barrier; the best vapour barriers are felt so happy days.

We don’t have to raise the outlet. We stop the insulation around it and dress the roof down to it. However, if possible, I always like to use external outlets, so if conditions allow, discard the internal outlet and replace it with an external one which gives you more room in the property and less possibilities for a internal leak blockage or disaster as time passess.

Q3: As I am currently in the process of purchasing 2 Velux flat roof windows to replace 2 old skylights, I noticed that these new skylights have the optional extra of adding an extension kerb to raise the window by 15cm to account for warm or sedum roofs.
Is this usually necessary in your opinion, or is the clearance normally available in these skylights sufficient for most insulation of this type?

A3: Yes you will have to raise the new Velux flat roof windows so that there is at least 150mm of upstand before you get to the window after the insulation is installed. At this moment assume the depth of the new roof and insulation is going to be 110mm.

Q4: Finally, regardless of whether or not I add insulation, I am interested in eventually replacing the covering of the roof with something that will last a very long time (like 40+ years), instead of messing about with felt or asphalt, even if I have to spend more upfront. I read with interest about (and saw your video on) EPDM, but also read the sections on your site on other coverings, like liquid rubber, kemper, and roofkrete. All seem to say they are the best and most durable…! What are the tradeoffs? Will they all last 40+ years without leaking/cracking?

A4: Guarantees for flat roofing is a real bugbear of mine as they are alway mis-sold. Basically there are:
Manufacturers’ guarantees on materials.
Manufacturers’ insurance-backed guarantees on labour and materials.
Contractors’ guarantees on materials and labour.
Contractors’ insurance-backed guarantees on materials and labour.
Option 1) If the contractor is no good then you are stuck!

Option 2) This is the best although expensive – normally a percentage of the contract amount (around 3% + 5% tax) which will have to be over £5K, or there may be minimum costs for setting up before the insurance company will issue a policy.

Option 3) This is only worth the word of the contractor. If he goes bankrupt then you have nothing.

Option 4) The contractor buys an insurance policy that covers the cost of the labour and materials, this is the best for small contacts up to around £50K.

The best guarantee is making sure the materials match the design of the roof and the contractor knows what he is doing, then the chances are that you will never need an insurance policy. The roofing materials should be easy to lay so there is less to go wrong at the time of installation; the more complicated the roof system, the more chances of something going wrong.

This is why we how stick to:
3m, Triflex and Kemper are all liquid products and are good for detailed flat roofs and large flat roofs. Our professional opinion is that we think sheet EPDM rubber should only be used on low detailed flat roofs. EPDM is good – it’s fantastic, but loses its excellent qualities once you start cutting it up to go around lots of details etc.

3m comes with a 25 year manufacturers’ guarantee and we can give 10 and 20 year insurance-backed guarantees.

Triflex comes with a 20 year manufacturers’ guarantee and we can give 10 and 20 year insurance-backed guarantees.

Kemper System comes with a 10, 15 or 20 year manufacturers’ guarantee and we can give 10 and 20 year insurance-backed guarantees.

EPDM comes with a 20 year manufacturers’ guarantee and we can give 10 and 20 year insurance-backed guarantees.

Although we have promoted RoofKrete, we are now not installing this system, as its too complicated to lay, too expensive, and the company is too small to support its contractors. It’s a lovely system but a bit of a cottage industry.

We now try to stay with the big manufacturers – the 3M system is easy to lay and comes with a 25% manufacturers’ guarantee, and the manufacturer is bigger than most insurance companies. The company backup is good and we feel comfortable laying their system.


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