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.

 

 

 

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Timber Frame Defects

Common Timber Frame Problems

Neo-Georgian with Timber Frame

Neo-Georgian with Timber Frame

We were recently called to investigate some damp and structural issues to a 12 year old timber frame Neo-Georgian 3-storey apartment block across the water from mainland UK. The building had been affected by both water ingress and a number of structural issues for quite some time and two previous technical reports had reached broad agreement of the fact that the timber frame was showing signs of distortion due to shrinkage, shrinkage commonly occurs in timber frames after construction and one engineer estimated the height of the timber frame may reduce by up to 30mm, a degree of shrinkage that wouldn’t be replicated in the outer non-structural leaf of masonry. In fact the outer masonry leaf tends to expand as it takes up moisture during the first couple of years after construction, so it is in fact moving in the opposite direction to the timber frame.

Wall Ties

It is for these reasons that timber frame movement ties are specified for developments over four storeys, as these are required to accommodate the additional vertical movement in the timber fame and differential movement between the inner and outer leaf. However, this is a 3 storey development and so long as vertical movement stays within expected limits then a standard fixed wall tie should suffice.

Standard Timber Frame Wall Ties

Standard Timber Frame Wall Ties

Timber frame movement tie

Timber frame movement tie

 Structural Cracking & Movement

Significant structural cracking

Significant structural cracking

Structural cracking to the outer masonry leaf of timber framed buildings can often occur where this differential movement between the inner and outer leaf falls outside of acceptable limits due to inherent design flaws or poor build quality.

When inspecting the building externally we noted that door and window frames were often slightly deformed and out of square, which resulted in extreme difficulty in opening the softwood timber french doors leading out onto the apartment balconies. We also noted significant stepped cracking in a number of areas to the outer leaf of masonry.

Starting from the Top

It was initially thought that defective balcony detailing and waterproofing arrangements were the cause of water ingress into the building and in fact the initial instruction was very much about investigating potential balcony defects, but of course you must approach these investigations with a blank canvas and an open mind. Whilst there were a number of relatively minor issues with balcony upstand detailing and parapet wall box gutter outlets, it was clear that these were not responsible for the water ingress or the structural defects seen.

Logically, I like to start from the top and work my way down once I start the internal inspection  and starting from the top meant inspecting the balcony that fully surrounded the building at top floor level.

Keeping the Timber Frame Dry

Open bed joints to parapet copings and no throating detail to underside

Open bed joints to parapet copings and no throating detail to underside

I found a number of serious and critical defects relating to the high level parapet walls that in my opinion have been allowing rainwater ingress into the wall cavity for a number of years, possibly since the building was constructed. Of course, if this was the case and water ingress was as bad as I believed it to be then the the greater probability is that the timber frame has swelled and expanded, rather than shrunk. The net result of course is the same, which is the potential for excessive differential movement between the inner and outer leaf. Moreover, there is a further potential for timber decay in the structural timber frame and perhaps even structural failure as timbers are affected by fungal decay.

Open perp joints between coping stones

Open perp joints between coping stones

Defects Causing Consequential Damage

Adhesive and cohesive failure of sealant to coping bed joint

Adhesive and cohesive failure of sealant to coping bed joint

We noted that the parapet wall copings were not fit for purpose and had been poorly installed off centre so the outer wall face had a 70mm overhang, whilst the inner parapet wall face only had a 30mm overhang. To meet the requirements of BS5642 then a minimum 45mm overhang was required to either side. However, more critically there was no throating detail to the underside of the parapets meaning that rainwater would flow along the underside of the coping overhang and straight into  cracks or open joints that may exist to the coping mortar bed.

Lead apron proved not to extend across the width of the cavity

Coping stone removed. Lead apron proved not to extend across the width of the cavity

 

Of course, this shouldn’t be a problem, because there’s bound to be a physical damp proof course installed under the copings as a secondary line of defence… or at least there should be!  We removed a parapet coping and as we suspected there was no physical DPC installed. So water was entering the wall cavity from the underside of the failed bed joint to the copings and the open joints and cracks in the coping mortar perp joints.

On finding these defects we of course had serious concerns as to what effect this long term water ingress was having on the timber frame. On checking the timber moisture content to the head of the timber frame we recorded a moisture content of 21.2%, proving the real current and ongoing risk of timber decay to the structural timber frame.

High moisture content to timber frame

High moisture content to timber frame

 

 

 

 

 

 

 

 

We recommended and specified urgent works to correctly waterproof the balcony parapets and further recommended opening up sections of the wall cavity where cracking had occurred to inspect the integrity and condition of the structural timber frame.

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