Remediation details: roof claddings

Most of the focus of remediation design is on leaks through the wall cladding and associated features such as decks, parapets and windows. However, a smaller proportion of buildings with weathertightness failure have leaks originating through the roof cladding.

Extent of roof cladding problems

Roof cladding problems tend to be relatively minor and not widespread across the whole roof. Problems can arise with:

  • areas of flat or low-slope roof with a membrane roof covering
  • penetrations through the roof, such as chimneys and vent pipes, that have not been flashed properly
  • installation of aerials and satellite dishes on the roof
  • poor installation practice
  • water blowing back under flashings, particularly on low-slope roofs (including the upper portion of a curved roof) – this is exacerbated where turn-ups have not been formed on the end of the profile
  • broken clay or concrete tiles
  • insufficient flashing cover
  • inadequate fixing of flashings or unprotected holes in the flashing
  • buckling of flashings because thermal expansion and contraction has not been allowed for – this is more of an issue where dark-coloured roofing has been installed
  • laps in flashings (particularly those wider than 150 mm) that have a flat upper surface with no packing under to provide support
  • water flowing back into soffits where the gutter outlet has become blocked
  • internal gutters either as a result of poor installation or blocked outlets (or no overflows provided)
  • blocked valley gutters generally as a result of no maintenance
  • for older buildings, corrosion of metal roofing
  • loose asphalt shingles.

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Areas of damage

For most of the problems described above, the damage is usually limited to wet ceilings and insulation, with damage to framing being isolated, except in the case of:

  • leaks through membrane roofs that are usually a low-slope or no-slope skillion roof – the water gets into a concealed space with little possibility of drying, and serious timber degradation may occur
  • water that overflows unseen into soffits, which can then run down through the walls and even accumulate between the foil insulation and the flooring in buildings with a suspended timber ground floor.

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Membrane-clad skillion roofs

Where a skillion roof with a membrane has leaked, the inspection is usually done from below so that as much of the waterproofing as possible provided by the membrane can be retained until repairs have begun.

Typical failures

Typical failures to membrane roof systems typically result from:

  • poor installation practices/workmanship
  • stressing of the membrane causing it to deteriorate, usually along the lines of the joints in the substrate (this may also result from temperature differences in the roof as a result of water not draining from it)
  • undersized and/or insufficient number of outlets for the roof/deck catchment area
  • insufficient edge membrane upstand
  • glued sheet joints losing adhesion and allowing water in
  • flooding and overflow into the building from blocked drainage outlets
  • damage such as cuts to the membrane – this is often caused before the building is completed as a result of tradespeople walking on and working over the completed roof
  • for liquid-applied membranes:
    • lack of maintenance (recoating)
    • inadequate curing
    • insufficient film build
    • poorly installed reinforcing
    • inability to cope with substrate movement
  • deterioration of untreated plywood substrates where a leak has occurred.

Damage is likely to be greater where untreated plywood has been used as the membrane substrate.

Options for repair

  • Rebuild the existing roof with new framing, treated (water-based treatments only) substrate and membrane roofing system.
  • Rebuild the roof but increase the slope to allow the roof to be clad with a long-run profiled metal. Generally, a profiled metal roof that can be laid to a fall of 3° can be installed where there was a membrane roof originally. The change to the appearance of the building will depend on the existing edge detailing – often a new metal roof can remain concealed behind a parapet, but the owner may ask for eaves to be incorporated into the redesigned roof. With this option, more roof insulation may be needed to ensure H1 compliance – the metal has a lower R-value than the removed ply.

Where timber and plywood substrate damage is identified, the steps are as follows:

  • Cover the roof area with tarpaulins.
  • Remove the membrane.
  • Remove the plywood.
  • Remove insulation.
  • Identify timber to be replaced – at this time, the ceiling may also need to be removed depending on the extent of the damage.
  • Rebuild framing, checking the falls achieve the minimum recommended by the roofing manufacturer or E2/AS1 Third Edition or the roof cladding chosen:
    • For membrane roofing, E2/AS1 Third Edition allows a minimum fall of 1.5° but rebuilding the roof with 2–3° fall is recommended to ensure there will always be positive roof drainage.
    • For long-run profiled metal, specifying a trapezoidal profile will allow the roof to be installed at a minimum 3° slope.
  • Tie down the structure to framing below to resist wind loads.
  • If the ceiling remains intact, lay new insulation. Where the ceiling is removed, insulation is better installed after the roof has been made weathertight.

For membrane clad roofs, incorporate the following into the detailing (see figures below):

  • Removal of the parapet to the back wall to allow an external gutter to be used.
  • Where an internal gutter is retained, provide a minimum of two outlets with equally sized overflows.
  • Lay new plywood treated with a water-based H3 treatment, and incorporate anti-spill edge detailing to contain water within the roof area where the roof is not surrounded by parapets.
  • Before the membrane is relaid, it may be necessary to remove some or all of any wall cladding that is erected around the roof areas to allow membrane edge upstands to be achieved around the perimeter of the roof.
  • Relay new membrane in accordance with the manufacturer’s instructions.
  • Flood test.
  • Remove the tarpaulin covers.
  • Repair or install new ceiling (remember to install insulation).

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Metal roofs

Typical failures to metal roof cladding systems typically result from:

  • poor installation practices/workmanship
  • poorly installed penetrations, such as the boot flashing to a flue or pipe
  • flashed penetrations blocking drainage down a trough or pan
  • lack of flashing cover
  • omission of turn-up to roofing troughs or pans at the roof apex
  • lack of fall on low-slope roofs
  • purlin spacings too wide for the roof span, allowing the roof to sag
  • poorly installed nails or screws
  • lack of allowance for movement at fixing points particularly in long runs of roofing (over 12 m)
  • blow back (under flashings) on low-slope roofs in exposed locations.

Low-slope metal roofs

  • Redesign the roof so that it drains to an external rather than an internal gutter. 
  • Lay roof underlay.
  • At the roof perimeter:
    • Where there are parapets, before the metal roofing is installed, it is likely that wall cladding to the back faces of remaining parapets will need to be removed to allow the installation of apron flashing or the apron flashing can be installed. 
    • Where there are no parapets, finish the roof edge with a barge board or fascia with a cap flashing.
    • Ensure roofing turn-downs and turn-ups at sheet ends are provided.

Updated: 9 September 2014