Plant Screens & Parapets

Parapets are the low walls that rise above the roof at its edge: masonry, concrete or framed, finished with a coping. Plant screens are the walls, louvres and panels that hide rooftop plant and are usually steel framed and fixed to the roof structure. Both are exposed on every face, both carry high wind loads at the edge of the building, and both stand over the street or a public area on many buildings.

A parapet does several jobs: it terminates the roof membrane and supports its flashing, it can serve as the guardrail for roof access if it is high enough, and on many buildings it is a fire separation or carries ornament. Its coping sheds water off the top; a damp-proof course under the coping stops water going down into the wall.

What shortens life:

• Two-sided exposure: parapets get wetter and colder than the wall below.
• No damp-proof course under the coping and open coping joints.
• Expansion of the roof slab on concrete-roofed buildings.
• Fixings through the membrane for screens and cappings.
• Screens added without engineering for wind loads.

Lean and bowing

Parapets lean when frost, sulfate attack and wetting expand one face more than the other, and when the roof slab or structure behind pushes or pulls them. Tall unrestrained parapets are cantilevers from the roof level and fail in wind when their mortar is weak.

Roof slab expansion

A concrete roof slab heats and expands more than the walls below it. Where the parapet and wall are built tight against the slab, the slab pushes them outward, cracking the wall horizontally at roof level and at the corners and displacing the parapet.

Coping failure

Coping joints open, water runs into the wall below, and copings loosen and slide. Stone copings crack at their fixings; metal cappings lift in wind when fixings corrode.

Roof junction leaks

The membrane turns up the inside face of the parapet; where the upstand is low, the termination fails or the flashing pulls out, water enters at the roof edge and appears inside at the top of external walls.

Screen fixing failure

Screen posts are bolted through the membrane to the structure or to plant platforms. Corroded bases, loose bolts and fixings designed for less wind than the screen now carries let louvres and panels loosen and blow off.

Tie corrosion in cavity parapets

Cavity parapets are wet on both leaves and the ties in them corrode first. With little load on top to hold the outer leaf, it lifts and leans more readily than the wall below.

Concrete parapets

Concrete upstands and parapets carry reinforcement close to both faces; carbonation and chlorides reach it quickly. Spalling follows the bars, and corner cracks open where the parapet has no movement joints.

Parapets as edge protection

Where people work on the roof, a parapet often serves as the guardrail. Many older parapets are lower than fall protection rules require, and some have been lowered in past repairs; they then need guardrails added.

From the street and from aerial imagery

• Parapets visibly out of plumb; bowing along the roof line.
• Horizontal cracks in the wall just below roof level; cracks at corners at the top of the building.
• Copings displaced, open joints, vegetation; missing cappings.
• Loose or missing louvres; screens added since construction.

On the roof

• Cracks and open joints in parapet masonry on the roof side.
• Membrane upstand height and termination; flashings pulled away.
• Screen post bases corroded; bolts loose; louvre blades rattling.

Inside the building

• Damp at the top of external walls under the parapet.
• Cracks in top-floor walls at slab level.

In the records

• Parapet observation reports where required.
• Screen engineering.
• Coping and flashing repair records.

Reading the direction of lean helps. Sulfate and frost attack lean the parapet towards the drier side, often away from the weather. Slab expansion pushes outward, with a horizontal crack at the slab line and stepped cracks at the corners. A parapet leaning inward over the roof may have lost its ties to the structure behind.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A façade or structural engineer inspects close up.

• Plumb survey of parapets.
• Close inspection of copings, joints and the roof face.
• Opening up under copings to find the damp-proof course.
• Screen fixing inspection and wind load check.
• Water testing of the roof junction.

The report should state parapet height and plumb by elevation, coping condition, the presence of a damp-proof course and ties, screen fixing adequacy against current wind loads, and any parapets that do not meet edge protection height.

• Coping removal at sample positions to inspect the damp-proof course, dowels and fixings, and the wall head below.
• Height measurement of parapets against edge protection requirements wherever people access the roof.
• Moisture readings on internal walls below the parapet to map water entry.
• Cover and carbonation testing on concrete parapets.

• Rebuild leaning parapets with a damp-proof course and restraint ties to the roof.
• Reset copings with new joints and fixings; metal cappings with proper fixings.
• Slip joints where slab expansion is the cause.
• Renew upstands and flashings.
• Re-engineer screens with new fixings or independent frames.

• Restraint: steel or stainless ties and brackets connecting the parapet back to the roof structure where rebuilding is not needed.
• Guardrails on top of or behind low parapets where roof access needs edge protection.
• Concrete repair to spalled concrete parapets with an anti-carbonation coating.

• Coping replacement with weathered and throated copings, dowelled and with sealed or lead-tray joints.
• Metal cappings with concealed fixings, joint covers and a fall to the roof side.

Parapet and coping work is priced by linear metre; screens per bay or as a lump sum. Scaffold or access dominates. Observations recur annually where required.

Rebuild is lifecycle; make-safe and observations are code compliance; repairs are repairs and maintenance.

Parapet works over a street need scaffold or edge access with footpath protection and permits, which can exceed the cost of the masonry itself. Screen re-engineering is usually a lump sum per screen with crane hire. Restraint ties are priced per tie. Annual parapet observations, where required, are a small recurring compliance cost.

• Australia. Masonry follows AS 3700 and wind actions AS/NZS 1170.2, called up by the National Construction Code; where a parapet is the roof edge protection, its height must meet the fall protection requirements under the work health and safety regulations and AS 1657.
• United Kingdom. Approved Document A and the Eurocodes (BS EN 1996, BS EN 1991-1-4) apply; parapets used as edge protection fall under the Work at Height Regulations 2005.
• United States. New York requires annual observation of parapets facing a public right of way under 1 RCNY 103-15, with a plumbness check; International Building Code Section 705.11 covers fire parapets; ASCE 7 sets wind loads.

Rebuilding heritage parapets and their ornament needs heritage consent in all three jurisdictions, which sets materials and methods.

• Parapet observation and façade inspection reports.
• Screen engineering.
• Coping and flashing repair records.

• Wind load design for screens.
• Roof access and edge protection records.

• Heritage listing.
• Records of roof works that disturbed parapet flashings.
• Council or insurer notices about falling masonry.
• Previous coping repairs and the materials used.