Historic Preservation - Technical Procedures
Built-Up Roofing: Problems At Parapets
Hspg Prepared For Nps - Sero
Thermal And Moisture Protection
Built-Up Bituminous Roofing
Built-Up Roofing: Problems At Parapets
BUILT-UP ROOFING: PROBLEMS AT PARAPETS
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:
1. 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.
2. There is no adequate protection of the coping and/or
there are no drips or the drips are faulty.
3. Weepholes above the dpc are absent, leading to water
collecting and saturating the brickwork rather than
- 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:
1. If associated with periods of heavy rain, dampness may
result from rainwater bypassing an ineffective dpc.
a. 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.
b. 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
c. The roof membrane skirting may be bridged by
2. Detachment and/or dislodgement of the pointing above the
3. 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:
a. 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).
b. 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.
c. The asphalt has become too soft owing to the lack
of solar reflective treatment
d. The angle fillet is too small (i.e., less than the
minimum of 2 inches).
e. The skirting asphalt is not adequately supported at
its base, in particular where a board foam or
compressible board has been used for the
- 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
1. Thermal and/or moisture movements.
2. Differential movement between the roof deck and the
upstand and the parapet. This is sometimes aggravated by
the fillet being too small.
3. 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.
4. Frost action and/or sulphate attack may be responsible
for or may have contributed to the total movement.
5. No work is required if the parapet is safe and there is
no water penetration.
6. If the parapet is unsafe, rebuild incorporating correct
7. 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:
1. To drive rainwater up laps further than would occur by
normal head of water criteria, and
2. 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:
1. Remove and reset the coping installing a proper dpc
2. 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.
3. 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:
1. Take down the parapet and rebuild it, incorporating a
proper dpc and, if necessary, damp proof tray (dp).
2. 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).
3. 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
1. If trees are close by, ensure that debris is removed
regularly, particularly in the autumn.
2. If pointing above asphalt skirt is detached and/or
a. Remove the asphalt, 2 to 2-1/2" below the existing
b. Cut a new chase correctly sized and shaped.
c. Relay the asphalt with a weathered top and space
d. Point with cement:sand (1:3) between the top of the
asphalt and the underside of the chase.
3. 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.
4. For localized faults in the dpc:
a. Remove a short length of the parapet in the
vicinity of the fault in the dpc.
b. Repair the dpc and the parapet.
c. Form weepholes as necessary, every fourth course,
making sure the dpc is not damaged in the process.
d. 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
5. 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.
6. If the dpc is completely ineffective:
a. Insert a modified form of apron flashing (a tingle)
under the existing dpc. The tingle must be dressed
over the top of the asphalt.
b. 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.
c. 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.
d. After the tingle is inserted, the asphalt should be
relaid into the chase, and after it has cooled, the
tingle dressed over it.
e. 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
7. If above repair does not solve the problem, it may be
necessary to rebuild the parapet:
a. Take down the parapet and rebuild it, incorporating
a new dp tray with all laps fully bonded.
b. Ensure that the damp proof tray laps over the top
of the skirting of the waterproof membrane.
c. Introduce movement joints approximately 1/4" from
corners and then at intervals 13 feet minimum.
8. If the skirting is cracked:
a. 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
b. For concrete, the surface should be roughened; for
brickwork the joints should be brushed off rather
than deeply raked out.
c. In addition, cement:sand gauged with PVAC; or
bitumen rubber emulsion can be applied to the
d. The asphalt should be relaid 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
e. Where the deck is likely to move in relation to the
wall, use a free-standing curb detail, together
with an apron flashing.
9. If there is extensive cracking at the angle fillet:
a. Remove the asphalt for the full height vertically
and horizontally for about 6"-8".
b. 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.
c. Tack expanded metal lath on the face of the curb
and for 4" along the horizontal surface over new
d. Renew asphalt.
- Repair of stripped built-up felt from parapet:
1. Strip off felt to top and sides of parapet and for a
short distance (about 12") along flat roof.
2. Reinstate with new mineral surfaced or metal foil-faced
felt bonded to existing along flat roof and mechanically
fixed to top of parapet.
3. Cover top of parapet with a pressed metal capping fixed
to the wall cladding.
4. 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
1. 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.
2. 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.
END OF SECTION