Built-Up Roofing: Problems At Parapets

Procedure code:
Hspg Prepared For Nps - Sero
Thermal and Moisture Protection
Built-Up Bituminous Roofing
Last Modified:

This standard includes guidance on identifying problems at parapets associated with dampness of the parapet wall.

Signs of Excessive Wetness in a Parapet Wall

Dampness on the brickwork below the coping of the parapet, heavy efflorescence in the same location or frost splitting of the brickwork and/or sulphate attack of the mortar joints. The reasons for this include:

  • The damp proof course (DPC) under the coping is absent or ineffective. The worst condition occurs if there is no DPC, allowing direct penetration of rainwater into the cavity, particularly between the joints and the coping. A sagging DPC allows the penetration of rainwater into the cavity at laps or splits in the DPC material.
  • There is no adequate protection of the coping and/or there are no drips or the drips are faulty
  • Weepholes above the DPC are absent, leading to water collecting and saturating the brickwork rather than draining away.
  • Dampness internally at or near the junction of the wall and ceiling. Blistering of the skirting of the waterproof membrane prior to dampness occurring internally is usually indicative of damp brickwork behind the membrane. This may result from any of the following:
    • If associated with periods of heavy rain, dampness may result from rainwater bypassing an ineffective DPC.
    • If the outer leaf of the DPC does not extend to the face of the wall so that water can bridge it, it may then trickle down the underside of the sloping DPC to the inner leaf.
    • Faulty laps or other openings in the sloping DPC provide a path for water from the outer leaf directly to the inner weepholes in the exposed portion of the inner leaf, since water can dam up and then find its way through the sloping DPC. The exposed nature of parapets makes them particularly vulnerable to shortcomings in design or workmanship.
    • The roof membrane skirting may be bridged by accumulated debris.
  • Detachment and/or dislodgement of the pointing above the asphalt skirt.

Detachment or cracking of skirting. This may occur if the horizontal slot was inadequately formed and/or sized for proper asphalt tucking and mortar pointing above. Slumping and/or cracking of the skirting, or deformation of fillet will occur when:

  • There is inadequate thickness of the vertical asphalt (i.e. less than 1/2" or 3/4" when the skirting is greater than 12" high).
  • The vertical substrate does not provide adequate key for the asphalt, either by being too smooth or by being damp at the time the asphalt was applied.
  • The asphalt has become too soft owing to the lackof solar reflective treatment
  • The angle fillet is too small (i.e., less than the minimum of 2").
  • The skirting asphalt is not adequately supported at its base, in particular where a board foam or compressible board has been used for the insulation.

Inadequate fixing of felt at leading edge of top of parapet and/or lack of protective capping will result in the stripping of built-up felt from parapet to profiled steel external cladding and sometimes with stripping of built-up felt and insulation from adjacent flat roof as well.

Vertical or diagonal cracking (of masonry units and/or mortar joints) with or without dampness internally at or near the junction of the wall and ceiling can be caused by any of the following:

  • Thermal and/or moisture movements.
  • Differential movement between the roof deck and the upstand and the parapet. This is sometimes aggravated by the fillet being too small.
  • The parapet and/or roof length is excessive for the degree of exposure to heat and/or rain and for the properties of the units and/or the mortar.
  • Frost action and/or sulphate attack may be responsible for or may have contributed to the total movement.
  • No work is required if the parapet is safe and there is no water penetration.
  • If the parapet is unsafe, rebuild incorporating correct movement joints.
  • If the parapet is safe but allows water penetration, DPC's must be checked and replaced if damaged. See "MEASURES TO REDUCE DAMPNESS IN PARAPET WALLS" below.

Dampness internally at or near the abutment of a flat and pitched roof is most likely caused by the vortex action of wind. This can be particularly severe when the sloping abutment is facing in the direction of the prevailing wind. The effect of the vortex is:

  • To drive rainwater up laps further than would occur by normal head of water criteria, and
  • To pump rainwater into and over laps at joints where the waterproofing membrane is of a lightweight metal cladding (such as aluminum, stainless steel or zinc).

Measures to Reduce Dampness in Parapet Walls

If brickwork is simply wet, with heavy efflorescence:

  • Remove and reset the coping installing a proper DPC beneath it.
  • The DPC should project about 3/8" beyond the face of the brick and should be fully supported across the cavity by slates or cement tiles.
  • Introduce movement joints approximately 1/4" from corners and then at intervals of 13 feet minimum.

If frost splitting and/or sulphate attack is present:

  • Take down the parapet and rebuild it, incorporating a proper DPC and, if necessary, damp proof tray (dp).
  • All lap joints are to be fully bonded (and if a new dp tray is also installed ensure that it laps over the top of the skirting of the waterproof membrane).
  • Introduce movement joints, approximately 1/4" from corners and then at intervals of 13 feet minimum.

For internal dampness at or near the junction of the wall and ceiling:

  • If trees are close by, ensure that debris is removed regularly, particularly in the autumn.
  • If pointing above asphalt skirt is detached and/or dislodged:
    • Remove the asphalt, 2 to 2-1/2" below the existing
    • Cut a new chase correctly sized and shaped.
    • Relay the asphalt with a weathered top and space for pointing.
    • Point with cement:sand (1:3) between the top of the asphalt and the underside of the chase.
  • If the DPC is sound but there are no weepholes, form weepholes at every fourth course, making sure that the DPC is not damaged in the process.

For localized faults in the DPC:

  • Remove a short length of the parapet in the vicinity of the fault in the DPC.
  • Repair the DPC and the parapet.
  • Form weepholes as necessary, every fourth course, making sure the DPC is not damaged in the process.
  • It may be advisable to introduce movement joints in the joints of the coping, approximately 1/4" from the corners, and then at intervals of 13 feet minimum.

Clad the inner face of the parapet so that there is continuous dampproofing from the skirting of the roof to the front of the coping. Faulty DPC's can be left undisturbed, since they will not be redundant.

If the DPC is completely ineffective:

  • Insert a modified form of apron flashing (a tingle) under the existing DPC. The tingle must be dressed over the top of the asphalt.
  • The insertion needs to be carried out with extreme care. In most cases it will probably be impossible to insert the tingle without removing short lengths of the brick course above - no more than three bricks at a time.
  • It may also be necessary to remove the asphalt in the chase for about 50 mm below it. The chase itself will probably need to be enlarged, taking care not to damage the DPC while so doing.
  • After the tingle is inserted, the asphalt should be re-laid into the chase, and after it has cooled, the tingle dressed over it.
  • If however, the upstand is a low parapet, it will be more economical to dismantle the whole of the parapet and rebuild it with a new DPC and apron flashing.

If above repair does not solve the problem, it may be necessary to rebuild the parapet:

  • Take down the parapet and rebuild it, incorporating a new DP tray with all laps fully bonded.
  • Ensure that the damp proof tray laps over the top of the skirting of the waterproof membrane.
  • Introduce movement joints approximately 1/4" from corners and then at intervals 13 feet minimum.
  • If the skirting is cracked:
    • Remove the asphalt for the full height vertically and horizontally for about 50 mm from the angle fillet. Provide an adequate key to the vertical substrate.
    • For concrete, the surface should be roughened; for brickwork the joints should be brushed off rather than deeply raked out.
    • In addition, cement:sand gauged with PVAC; or bitumen rubber emulsion can be applied to the vertical surface.
    • The asphalt should be re-laid on isolating felt where it is horizontal so that it is level with the existing asphalt; 1/2" minimum thickness of asphalt should be laid vertically with a 2" minimum angle fillet.
    • Where the deck is likely to move in relation to the wall, use a free-standing curb detail, together with an apron flashing.

If there is extensive cracking at the angle fillet:

  • Remove the asphalt for the full height vertically and horizontally for about 6"-8".
  • Form a pre-screeded woodwool or timber curb with a 1/2" minimum movement gap between the back of the curb and face of the upstand or parapet.
  • Tack expanded metal lath on the face of the curb and for 4" along the horizontal surface over new isolating membrane.
  • Renew asphalt.

Repair of stripped built-up felt from parapet:

  • Strip off felt to top and sides of parapet and for a short distance (about 12") along flat roof.
  • Reinstate with new mineral surfaced or metal foil-faced felt bonded to existing along flat roof and mechanically fixed to top of parapet.
  • Cover top of parapet with a pressed metal capping fixed to the wall cladding
  • Ensure that butt joints along the capping are properly butt strapped and sealed.

For internal dampness at or near the abutment of a flat and pitched roof:

  • At best: Where the case is one of built-up felt or asphalt waterproofing having been taken up behind tile or slate cladding, making the lap of the flat roof waterproofing longer will usually suffice.
  • At worst: Where the case if one of lightweight cladding being the waterproofing membrane on both flat and sloping roofs, the remedy will almost certainly involve a change in the design at the junction of the two roofs and most likely the use of a jointless waterproofing membrane at the junction. There may be a need for a wind tunnel test to determine the severity of the problem and hence the nature of the remedial work in detail.
Last Reviewed: 2017-12-11