If you’re a headteacher or school business manager for a school that wasn’t built relatively recently, you’ll have flat roofs. But why do some flat roofs leak, and can be done to prevent it? This article looks at the most common causes of flat roof failure so that you can look out for them and advises on what you can do to prevent failure in all your flat roofs.

1. Age – the most common cause of flat roof leaks

All roofing materials have a limited lifespan. However, as technology has advanced over recent decades, newer systems were introduced which have a greater longevity than systems of the past. Your flat roof may simply be leaking because it has reached the end of its natural life.

  • Many 1960s built flat roofs were built with a “pour and roll” 1e1b system, consisting simply of rag fibres covered in bitumen;
  • In the 1970s and 80 technology moved on towards a polyester based HT ranges of membranes known as 5e’s and 5b’s – many of which would have had a 20 year lifespan;
  • Then in the 1990s it became commonplace to use elastomeric membranes. These now have a life expectancy of 30 years or more – over three times longer than the 1960s membranes.

Why flat roofs leak - age

This aged roof had been repaired using additional pieces of bituminous cap sheet, and with liquid treatments. These repairs had failed, partly due to the already perilous state of the existing waterproofing.

If your flat roofs were built in the 1960s or even in the 1980s, there is a very good chance that the membranes will be well past their sell by date. Hundreds of primary and secondary schools will have been built with these membranes and many will have been repaired in various ways.

  • Why flat roofs leak: They have reached the end of their natural life.

2. Temperature Changes and Solar Gain

why flat roofs leak - cracking

The existing asphalt waterproofing here appears to have had its working life significantly shortened by lack of care and poor design. The asphalt is age hardened and brittle, and as a result thermal and structural movement have led to splits and cracks, allowing water penetration.

During the latter part of the 20th century, insulation values in flat roofing have increased faster than the membrane technology, as attempts are made to reduce energy loss from buildings. As a result, simple oxidised bitumen was being used to stick membranes onto relatively highly insulated roofs.

When dried, oxidised bitumen becomes rigid and is not able to expand and contract with temperature changes. Solar gain can cause the membrane to heat up, and insulation underneath can prevent heat from conducting away from the membrane. As a result oxidised membrane can crack, blister and fail, causing leaks.

Newer membranes such as our SIGnature range are engineered to cope with these temperature changes in a way that traditional bituminous systems cannot.

  • Why flat roofs leak: Traditional systems cannot cope with temperature changes like modern systems can.

3. Movement in the Structure

All buildings expand and contract due to heat, and good design will take this into account. Many traditional membrane systems were not developed to take the degree of movement that can be experienced on a flat roof. In particular if your flat roof was designed without expansion joints this could lead to the early failure of the membranes.

Modern flat roofs are designed to take movement into account, but your flat roofs may not have had the benefit of decades of flat roofing development.

  • Why flat roofs leak: Lack of expansion joints and design for movement.

4. Blisters

Why flat roofs leak - blisters

Large blisters appeared in this waterproofing: evidence of trapped air and moisture. There were also large areas of ponding water on this roof; in some locations there were blisters in the ponds!

Blisters are generally a sign of water inside the system. If water has entered the roof structure, the higher internal temperature can turn it into water vapour which needs to escape upwards, producing pressure which causes blisters.

There are a number of ways that water can get into your roof:

  • If the insulation or timber is damp when installed;
  • If the contractors haven’t dried the roof correctly after a period of rain during the installation;
  • Failures in the membrane, particularly at details and laps (where sections of membrane are joined);
  • Punctures and penetrations if these are not correctly detailed and installed; and
  • Interstitial condensation.

A blister produces a weak point in the roof membrane, and the membrane may fail because it isn’t design to expand and stretch that much. The blister may also pull on nearby laps (joins) and cause them to fail. 

  • Why flat roofs leak: Blisters due to moisture trapped in the system

5. Delamination

why flat roofs leak - delamination

If condensation forms between the membrane layers they will start to delaminate

Delamination is where the membrane layers separate and there is no longer a structural connection between them (they aren’t stuck together). If there isn’t a structural connection between the layers of a bituminous membrane roof the roof can lift and fail.

Delamination in a bituminous membrane generally happens either

  • when the membranes haven’t been heated enough when installed, or
  • through water getting between the layers of membrane.

Interstitial condensation can produce a ‘dew point’ between membrane layers (a point where the temperature drops sufficiently so that any water vapour will condense into liquid). Then the liquid water cannot escape and builds up causing the layers to separate.

  • Why flat roofs leak: Delamination of the membrane.

6. Poor Design and Detailing

In addition to age, poor choice of materials and issues with water, poor design and poor detailing of flat roofs are major reasons why flat roofs leak. Here are some examples of poor design and detailing.

  • Overspanning of decks – where the roof deck is spanning between rafters (beams) which are too far apart. An overspanned deck is not structurally sound so it will move more. There is also a safety issue – people will be at risk if they go on roof which cannot take their weight;
  • Inadequate falls – if the roof pitch is too shallow this can create excessive ponding of water;
  • Failure to calculate condensation risks – A major cause of Interstitial Condensation is the failure to calculate whether it is a risk in the first place;
  • Shallow upstands – upstand heights should be 150mm, otherwise during a heavy rain shower water can overflow the upstand and get into the roof, and
  • Poor detailing around penetrations such as rooflights and soil vent pipes. These are weak points in a roof and need to be robustly detailed.

Make sure your roof is designed by an experience roofing specialist that has their own Professional Indemnity Insurance to cover the design. 

  • Why flat roofs leak: Poor design and detailing.

7. Poor Workmanship

Finally, it is essential to remember that roof installation is a professional skill. Any system can be designed expertly; but if not correctly installed it will fail.

Using qualified accredited contractors is absolutely necessary to prevent flat roof failure. Cheap labour with poor skills will make mistakes. A qualified and accredited contractor will be experienced and will also be willing to discuss any issues that with the project manager and designer rather than be tempted to find a workaround on their own.

It is also important to ensure that corners are not cut when installing a roof, either for expediency or cost savings. An accredited professional contractor will not cut corners.

  • Why flat roofs leak: Poor workmanship.

Why Flat Roofs Leak: In Summary

Flat Roofs Leak for a number of reasons, and here are some of the most common.

  1. They have reached the end of their natural life;
  2. Temperature changes in traditional systems;
  3. Lack of expansion joints or design for movement;
  4. Blisters due to moisture trapped in the system;
  5. Delamination of the membrane.
  6. Poor design and detailing;
  7. Poor workmanship.

Many flat roofs designed and constructed towards of the end of the 20th Century will have reached the end of their natural life. It is essential that if a replacement roof is not to fail, it should be designed by an experienced roofing specialist using modern engineered materials and installed by an accredited contractor.

If you’d like to discuss your school flat roofs with us, please get in touch.

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About the author – Ian Dryden

Ian is an experienced specification manager with ten years working in flat roofing. He works with our technical team of surveyors, technicians and roofing specialists at SIG Design and Technology to deliver better flat roofs on schools across the country.