Flat Roof Defects (Old and New)

Flat Roof Defects

We were recently appointed by a contractor to resolve a dispute between him and his client.

Flat roofed office building

He had constructed a new two-storey extension on a commercial building, which required the new flat roof deck to be connected to the existing flat roof deck.  Unfortunately, since completion of the work, the building had been plagued with water leaks.  Anecdotally, we were informed by the building owner that since they’d not previously had issues with water ingress, the new roof construction must be to blame. On arrival, we were steered by the building owner to only look at the new section of roof. But of course, we were not appointed by the building owner and it is important that these issues are investigated in a broader context.

New section of flat roofing with heavy water ponding due to poor falls
New section of flat roofing with heavy water ponding due to poor falls




















The roof was circa 300 square metres, and the new section of roof accounted for approximately 25% of the total roof area.

On accessing the roof, we noted a number of defects to the new section of roof, which included:

  1. Poor falls to the roof deck and subsequent heavy ponding.
  2. Failed lap joints to the deck and the parapet wall up stands.
  3. Poor junction detailing with the existing roof.
  4. Incorrect through wall (box gutter) outlet detailing
  5. Blistering of the roof to the deck and the upstands.
  6. Parapet Coping stones supplied and fitted that did not meet the British standard guidelines.
  7. Damp proof course poorly installed under the coping stones.
  8. Felt upstands not protected by a lead apron flashing.
Failed lap joint
Failed lap joint


Given the wide range of defects, it was clear that the new section of roof was not fit for purpose. There were numerous areas that could allow water ingress. So should we have stopped the investigation there?

In simple terms, no! In my view, that would have been irresponsible and not in the best interests of either party.

Existing Flat Roof

Parapet wall & coping stone defects
Parapet wall & coping stone defects

It was immediately apparent that the existing flat roof was as dilapidated as we’ve ever seen and was long overdue replacement. To my mind it simply wasn’t sensible to attempt connecting a new flat roof system to such a heavily damaged existing roof deck. Even if the new section of roofing had been constructed to a good standard, to my mind, there was always going to be a dispute relating to water ingress and who was responsible. If the existing roof felt was leaking, then water could easily find its way under the new section of roofing; either by direct flow, saturation of the roofing boards, or vapour transfer.

Dangerously loose coping stones
Dangerously loose coping stones

We noted a wide range of substantial defects to the existing roof, which included:

  1. Heavy blistering of the felt.
  2. Heavy moss growth causing poor rainwater run off.
  3. Failed lap joints and evidence or previous running repairs.
    4. Parapet wall coping stones not fit for purpose and all copings were dangerously loose.
  4. No damp proof course installed under the coping stones.
  5. Heavy ponding on the roof.
  6. Failed felt up stands which were not protected by a lead apron flashing.
  7. Poor falls to the roof deck.
  8. Roof boards very soft underfoot, suggesting they’d been affected by water ingress in some areas.

    Existing roof long overdue replacement
    Existing roof long overdue replacement

Both lists of defects make it very clear can we can expect water ingress to both old and new sections of the roof. Interestingly, the worst area for water ingress was close to a parapet wall section, where defects were found to both sections of the roof.

Correcting Poor Falls

A focus had been placed on the poor falls in the new section of roofing, and yes, these were poor, but so were the falls to the existing section of roof. Neither issue could be addressed without replacement of the whole roof. I did have some sympathy with the flat roofing sub-contractor, because it was impossible to effectively correct the falls on such a limited area of the roofing.

Poor box gutter outlet detailing
Poor box gutter outlet detailing
Bespoke through wall rainwater outlet
Bespoke through wall rainwater outlet

My advice to both parties is that this roof should be wholly repaired, with the owner being responsible for the existing sections of roof, and the contractor being responsible for his defective workmanship. Once the roof was stripped, and roofing boards renewed (as necessary), then falls should be corrected by the use of timber firring strips under the roof boards, or by installing tapered insulation, installed on top of the roofing boards.

Insufficient overhang to parapet copings
Insufficient overhang to parapet copings

Other Flat Roofing & Parapet Wall Works

Additionally, extensive works were required to the parapet walls. which included:

  1. Correctly specified parapet copings with a minimum of 45mm overhang and throating detail to the underside. (Requirement of BS5642:2)
  2. Physical DPC installed under the copings, with a minimum 150mm overlap between adjacent sections of the DPC, and a hard support bridging the wall cavity, so that the DPC does not drape into the cavity.
  3. Coping stones to be mechanically fixed to the parapet walls with stainless steel fixings.

Should Coping Stones Be Mechanically Fixed?

The latter requirement is something of a grey area that I’ll hopefully clear up now…

In 2014, BS5534 was changed to make the requirement that ridge tiles must now be mechanically fixed. It is no longer allowable for these tiles to simply be adhesively bonded on a bed of mortar. One might think that this requirement was extended to cover coping stones, but unfortunately it wasn’t.

However, BS5642:2, makes the following statement;

“The coping system should be designed to resist impact by displacement or pressure of ladders, or by normal wind conditions.”

Since a mortar bed alone, would not resist displacement, then clearly parapet copings should be mechanically fixed. The standard then goes on to state that clamps and fixings for copings should be of Austenitic stainless steel construction.

We also advised that the box gutter should be modified to have bespoke, ‘through wall’ outlets installed. This would make it far simpler to bond the torch on felt to the upstands, and also, those upstands, should themselves be protected at their head by a lead apron flashing. There is a fashion now for omitting these lead aprons, and simply expecting the torch on felt to fully water proof the upstand, but in my view, premature failure will always occur with this type of detailing. Occasionally the felt is turned under the coping stones, to also act the the parapet wall DPC, but again, this detailing is incorrect, and not fit for purpose.

Pre-existing defect close to main area of water ingress
Pre-existing defect close to main area of water ingress
Main area of roof leak
Main area of roof leak










I personally found it hard to believe, that the existing section of flat roofing was not also leaking, but it is a certainty, that the parapet walls were! Pragmatically, it was important that both parties addressed this problem with whole renewal of this roof, or I’m confident that the water ingress will be an ongoing issue.




5 responses to “Flat Roof Defects (Old and New)”

  1. Brenda Hales avatar
    Brenda Hales

    Much obliged such a great amount for sharing this marvelous information! I am anticipating see more posts by you!

  2. Garry Fleming avatar
    Garry Fleming

    Thanks for the great content, I have a question about the parapet. You mentioned that the lack of a DPC could have led to the internal water leak, should this have not been averted by a cavity tray in the parapet wall if it had been built correctly?

    1. Joe Malone avatar
      Joe Malone

      Hi Garry,

      Yes, absolutely, though this particular building had no cavity tray.

      There are generally three lines of defence in a parapet wall, the copings, the DPC below the copings, and thirdly, the cavity tray, though of course, the latter is only sometimes found in cavity walled construction.



  3. Michelle avatar

    I live in a 1930s art deco style flat roofed house and despair of finding a roofer with the expertise or inclination to tackle the ponding and deal with the parapet successfully. We last had it done about 15 years ago but none of the roofers we have contacted want to take the job on.

    1. Joe Malone avatar
      Joe Malone

      Hi Linda,

      I’d suggest that you decide on a roofing system, EPDM, Fibreglass, or high performance felt. Once you’ve done that, contact the roofing material supplier who will have a list of approved installers in your area.


      Joe Malone

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