Flat Roofs

Problems, Improvements and Insulation


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Flat roofs and flat roof problems

Flat roofs are renowned for problems. Over the years we have come across all sorts of problems from finding major ponding which makes the flat roof look almost swimming pool like, having rainwater coming in to what looks like a perfectly good roof and not being able to find the problem area at all. We even had a few roofs where we seemed to chase around the problem, as soon as we solved one flat roof problem, another seemed to appear.

Flat roof swimming pool


Ponding to flat roof

Ponding to new flat roof.

Lack of supervision has allowed this roofer to become a fiddler on the roof!


Ponding on felt flat roof

Internal gutter that has all rainwater running to it but then it doesn't discharge into downpipe


Are Flat Roofs getting better or are we just getting used to them?

Over the years the good news is that since the 1950's/1960's when flat roofs first started to appear in abundance we have got much better at producing materials for flat roofs however the bad news is that there is increasing pressure on tradesmen to work faster and faster and like many of the building trades, we don't come across roofers who have done proper time served apprenticeships very often. Having said that, we believe that flat roofs are generally becoming less problematic, we no longer expect to find problems when we get on a large flat roof.

Problems that we do come across when surveying flat roofs tend to relate to the following areas:

  1.   Quality of workmanship on flat roofs
  2.   Quality of materials being used on flat roofs
  3.   Quality of detailing on flat roofs
  4.   Thermal efficiency of flat roofs


Original flat roofs

If you wish to know about original flat roofs, which are covered with lead, zinc or possibly copper or indeed any other pliable metal then we suggest that you have a look through the various other articles within the 1stAssociated website. We are looking at here flat roofs of the 1960's onwards, which were covered in felt.

Older style felt with loose chipping covering


You may also find the following articles of interest:

Asphalt Flat Roofs

Leaking Flat Roofs

Changing quick weather causing spring house problems!

Homebuyers Report or Structural Survey


Traditional cold felt flat roofs

Flat roofs from the 1950's/1960's had many different coverings. The main ones that we come across on residential properties are felt with loose chipping covering and on commercial properties an asphalt roof. This type of traditional flat roof consists of ceiling joists onto which a decking is laid and then a roof finish of felt added in a varying number of layers. This type of flat roof has become what is known in more recent times as a cold deck flat roof as it doesn't have any insulation in it or possibly does have some. Please read on.



Modern traditional cold felt flat roof construction

In more recent times with a greater consideration for thermal efficiency and energy saving we started to add insulation into our traditional roofs between the roof joists. This meant that no longer when you heated the property did heat go straight through the flat roof and warm the garden/village/town/city. However as is often the case with one step forward, you also take one step back and with our building surveys we did then come across problems where the insulation had been added without additional ventilation which caused problems with condensation and dampness. As we began to understand this problem that it was causing we started to add vents into the roofs but this was not before a lot of flat roofs were built without any ventilation.

Modern flat roofs or warm roofs

This is also a mineral felt finished flat roof however there is a layer of insulation present.

Other types of modern flat roofs

As time progresses there seems to be more choice with regards to flat roofs. Some of the flat roof materials that we have seen in action are:

1.  Mono-ply system which is a single rubber type roof very popular with developers

Mono-ply roof

Rucking in mono-ply roof

Detailing to mono-ply roof. Yes, this is a wastepipe discharging onto the roof. Yes, it was raining when we did this roof survey!


Mixture of materials

Mono-ply roof with a felt edging! Yes it was snowing when we carried out this roof survey!


2.  Fibreglass which is very popular with some roofing contractors

Ponding on fibreglass flat roof

Pen marks blister on fibreglass roof

Fibreglass repairs by original contractor


3.  Rubber roof, older style

4.  Living roofs/green roofs which literally are meant to have a grass like vegetation growing from the roof.

Living roof


Difficult to view flat roofs

The first problem with flat roofs when we survey them is that they are often difficult to get to. It tends to be something you are not looking at regularly therefore tend to forget about until such time as it is a problem and in this particular case the flat roof was over the bathroom and the bedroom and it wasn't till water actually came into both of these that we realised there was a problem. Below are photos of the property, the first is of the flat roof from the front so you can see how little you can see of it, the second photo is from the rear of the property so you can see how little you can see, other than the edge of the flat roof and the fact that it is a wide flat roof.

Water coming down where chimney was in the bedroom


Photos of difficult to view flat roofs

Flat roof from the front

Flat roof difficult to view to the rear


Finding the leak

Unfortunately on older style flat roofs where they have had a felt and then a shingle covering it is very hard to see where the flat roof leak is when carrying out a survey. In this particular case we had to remove all the shingle before we can even see the flat roof felt and only then are we able to see a split in the flat roof. We had already made the decision to re-roof the entirety of the flat roof area.

Flat roof covered with shingle which hid this split in the felt beneath

General view of roof when shingle was removed. This type of roof was typical in the 1960's/1970's.


Repairs to Asphalt roofs

We will cover this in a more in-depth Article.

Asphalt with tar repair

Asphalt with solar reflective paint and flash band repair


Other problems with flat roofs

During this survey it became apparent that there were damp walls in the property. In this particular case although it looked like the render was causing the main dampness as it had come away, the biggest problem was where the roof was flat and had started to be absorbed into the wall. It was so flat in fact that moss was growing on that part of the roof.

Leaking gutter and flat roof caused dampness into the wall and the render comes off

Poor flashing not in lead. If you look up you can see the insulation


Felt roof with moss indicating that water is sitting on the roof

Felt roof with some moss on it indicating that water is gathering in the corner of the roof

Felt with moss indicating there is no fall on the roof at all.


You won't believe how many people use flat roofs as storage areas!

General rubbish dumped on a roof

Builders rubbish dumped on a roof rather than taken away.


Flat roofs have improved?

Undoubtedly we believe that flat roofs are improving and manufacturers are getting to understand materials a lot better. As such we have modern high performance felts and also insulation being added to help thermal efficiency

Modern high performance mineral felt roof

Improved detailing. A lead flashing with coping stone and insulation being added underneath the felt.

Flat roofs and insulation

What we really wanted this article to be about was the insulation that you can put into flat roofs to improve their energy efficiency. You can see in the photograph that we have used a new mineral felt which is known as a HP felt which is high performing. It looks like a modern flat roof. However beneath the flat roof is insulation.




Example of insulation added to flat roof

Sterling boards used as decking with Insulation sheets to the back of the photo

Roof complete with insulation added and hidden by felt


Heat loss from flat roofs

Traditional flat roof construction consists of a ceiling joist with a decking on top of it and then the felt being applied to this. You would of course have ceiling plaster below it but effectively it is only about 30mm to 50mm of material between you and the outside world. With the addition of insulation you can have various depths. In this case we were limited to 50mm by the detail with the pitched roof so in this case the insulation is 50mm thick so you can see you are doubling the thickness of material and in this instance you are adding a dense closed cell insulation which should reduce the loss of heat through the roof considerably.




Flat roof insulation is it the way forward?

We would comment that the jury is still out as to how successful flat roofs and insulation are. During our surveys we have come across instances where water has got past the felt and the protective roof covering and has got into the insulation and the insulation has become soft. With an insulated roof you now have the traditional ceiling joists, a decking, the insulation, a second decking and then the felt and of course again, the ceiling plaster.



You may sometimes see a vent in a flat roof or commonly you see vents to the perimeter of a flat roof. This is due to the increased heat that the properties have inside them and interstitial condensation being caused within the roof which will eventually rot it. To minimise this air ventilation is added.

Interstitial condensation defined

Condensation within the structure.


Vent to prevent dampness

Don't forget about the flashings to flat roofs

Our final comment would be that often a good flat roof can be ruined by a poor quality flashing and poor detailing around the chimney or a parapet wall or a flue. We would also recommend lead flashings wherever possible. Unfortunately in the adjoining photo, whilst the felt work is fairly good the flashing is in felt.



Flue on new mineral felt flat roof

Poor felt roofs

Felt roof with ponding and rucking

Can you spot what's wrong with this roof?


Independent Surveyors

If you truly do want an independent expert opinion from a building surveyor we are happy to do valuations, building surveys, structural surveys, structural reports, engineers reports, specific defects reports, home buyers reports or any other property matters. Please contact us on 0 800 298 5424 to have a free of charge friendly chat with one of our surveyors.

We feel our surveys are quite unique, as they are written to your level of knowledge. The surveys include photos and sketches and definitions. The survey will also include an action required section and an estimate of costs in the executive summary. We are more than happy to meet you at the property whilst carrying out the survey to discuss any specific issues you may have or have a general chat about what we have found at the end of the survey.

Commercial Property

If you are looking for commercial property, whether it is freehold or leasehold, we would recommend a survey as this will prevent dilapidations claims in the long run. You may wish to look at our Dilapidations Website at www.DilapsHelp.com and for Disputes go to our Disputes Help site www.DisputesHelp.com , both of which we have been advised are very helpful!

We hope you found the article of use and if you have any experiences that you feel should be added to this article that would benefit others, or you feel that some of the information that we have put is wrong then please do not hesitate to contact us (we are only human).

The contents of the web site are for general information only and is not intended to be relied upon for specific or general decisions. Appropriate independent professional advice should be paid for before making such a decision.

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