5 Flat Roof Disasters and What We Would Have Done Differently

We recently updated our roofing price list (which you can see here) and our flat roof repair troubleshooting guide (located here) and we thought it would be a good idea to have a look at the workmanship of some of Britain’s finest roofers.

With a combined 30 years experience in the construction industry, we’ve certainly seen a fair share of questionable flat roof installations in our time.

We’ve scoured the web and found 5 worryingly common mishaps to share with you.

This page all about flat roof bodge jobs, disasters and shoddy unprofessional workmanship.

As a bonus, we’ve provided some suggestions on how the work should have been carried out and a recommended course of action.

1 – The Outlet (or lack thereof)

As a general rule of thumb, it’s a good idea to ensure that the flat roof’s water outlet isn’t the highest point of the whole roof.

You know, there is that little thing called gravity.

Also, the outlet shouldn’t be so small that it’ll block easily.

water on flat roof

Outlet placed too high

Here is the view from the opposite side of the flat roof:

Fancy a swim, anyone?

Water pooling on flat roof. Images sourced from DIYnot

As you can see from the photos above, the roofer hasn’t done a bad job of torching the felt down but clearly hasn’t given much thought to how the water will run off the roof, which is very worrying as this is pretty basic stuff.

Also, given the small size of the outlet, a minor blockage of leaves and moss etc could cause some major flooding of this roof.

Here’s What We Would’ve Done Differently

While it’s difficult to see from just two photos, it does appear that the water on the roof is flowing away from the outlet.

Here’s a list of things we would have investigated first:

  1. could the outlet be relocated to where the water is pooling?
  2. could the roof decking have been raised so the water flows towards the outlet?
  3. could the outlet be lowered by stripping out those layers of built-up felt and installing some thinner lead?
  4. could the decking and/or roof timbers directly underneath the outlet be trimmed to help lower the outlet?
  5. could the outlet be made wider?

Assuming the homeowner doesn’t want to strip the entire roof and start from scratch (and depending on what the spirit level tells us) we would suggest focusing on the outlet and try to lower it and widen it as much as possible.

From experience, we know that some leadwork along with some creative trimming of the timbers should help us drop the outlet by around 2 inches, which might be just enough based on what we can see from the photos.

2 – The Internal Outlet

Has anyone ever seen an internal outlet that doesn’t get blocked with leaves, moss and twigs etc?

A few years ago we saw a nasty example of poor workmanship where a builder had extended a property and rather than relocate the rainwater pipe, had left it in between a cavity.

Needless to say, the homeowner was none too impressed when it blocked and overflowed, leaving him with saturated ceiling boards, wet plaster and ruined carpets.

Blocked flat roof outlet – a common problem

A Different Approach

It most cases we would advise the customer of the pitfalls of internal outlets and suggest a gutter be installed to one side of the roof and the decking adjusted so the water flows freely off the roof and into the gutter.

If the customer insisted on keeping costs to a minimum, then a recessed outlet with a filter is a better option to minimise blockages:

Recessed flat roof outlet

3 – The Raspberry Ripple

I’m fairly sure that buried somewhere in the installer’s guidebook, there’s instruction on how to stick a rubber roof membrane to the deck with adhesive.

While I’ve seen a few rubber roofs with creases and ripples, this one surely holds the world record:

Ripples and creases in rubber flat roof. Images sourced from DIYnot

Where Shall We Begin?

Perhaps there wasn’t any adhesive applied at all?

Or perhaps it just failed?

Maybe the deck was soaking wet and the moisture evaporated and lifted the felt?

Maybe the roof membrane was stored at a low temperature just before installation and during the summer, when the temperature has increased, it’s expanded?

Either way, we think the best approach would be to rip it off and start again.

4 – Good Firm Consistent Bonding with Sufficient “Bleed”

We love the attention this flat roofer applied when bonding the two layers of felt together.

When two bitumen felts are bonded together with a gas torch, the roofer should ensure there is enough “bleed” which is a way of saying it should be melted together properly.

Incorrect application of heat and lack of bleeding usually result in leaks.

Poorly sealed flat roof felt


Flat roof not sealed

Needless to say, the poor bonding of the two layers is the issue and we wouldn’t be surprised if the roof leaked like a sieve when it rained.


The fix for this issue will depend on the age of the felt and how much, if any, rot there is to the decking and timbers underneath.

At worst, the homeowner will be looking at a complete strip off, decking replacement and new felt.

5 – No Bleed-out or Details

When I was a wee nipper, one of the first things I was taught was:

No bleed-out = no manufacturer’s guarantee


No detail edgings = no manufacturer’s guarantee.

In other words, there is zero chance of the manufacturer accepting a claim under any guarantee if the felt goes all the way up the wall or down the drip edge without any separate details.

The same goes for bleed-out, if there isn’t any, then it’s not stuck down sufficiently.

Felt not hot sealed correctly

For those not in the know, this is how it should look:

Good bleed on overlap joints


Separate detail edgings

Why We Would Do Things Differently


The bleed-out ensures that the felt is adhered correctly and prevents water ingress.

Without it, water seeps under the felt lap and during the winter will freeze.

Because water expands when it freezes, it will lift the felt.

This process repeats itself throughout the winter months until the felt fails and water enters the property.

It could take a few years or longer but without good adhesion, the felt’s life expectancy is shortened.

Detail Edgings:

Long lengths of bitumen felt have a tendency to expand in the hot summer and shrink in the winter.

This is referred to simply as thermal expansion.

By laying the bitumen felt in one section, down the drip edge, across the roof and up the wall, there is no allowance for thermal movement and the following may happen:

  1. a fold or crease will appear in the roof
  2. a split or crack near the wall
  3. a split or crack near the drip edge
  4. the felt coming loose from the wall

Go On – Keep Reading. We Dare You.

Below is our flat roof repair guide which covers all the popular materials currently used in the UK:

Job Price’s Flat Roof Repair Troubleshooter

Below you’ll find Job Price’s full price list which includes flat roofs:

Job Price’s Flat Roof Cost Guide

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