Flat Roofs
Problems, Improvements and Insulation

Introduction to Independent Surveyors

We can provide help and advice with regard to building surveys, structural surveys, independent valuations, property surveys, structural reports, engineers reports, specific defects report, home buyers reports or any other property matters. As you can see from this flat roof article we use lots of sketches and photos as our clients have told us that the sketches and photos help them understand the reports. We, of course, like to meet you at the property during the survey and we are more than happy to talk to you about your property report.

Please free phone 0800 298 5424 for a friendly chat.

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 website. We are looking at here flat roofs of the 1960's onwards, which were covered in felt.


Older style felt with loose chipping

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.

Flat roof_warm

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

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


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


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.

Heat loss house and flat roof

Example of insulation added to flat


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 0800 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 included is wrong then please do not hesitate to contact us (we are only human).

The contents of the website 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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