Coping for Flat Roof Parapet Walls (Perimeter)

Neither brickwork nor concrete is meant to take rain directly on its top surface. Water would soak into the top of the wall, freeze in winter and slowly break the masonry apart. On the outside you would see damp patches, flaking render and wet stains on the ceiling of the top floor.

Coping therefore has several jobs:

• Cover the top of the parapet across its full width
• Send water back onto the roof, not down the façade
• Cover and protect the waterproofing that runs up the parapet from UV light and damage
• Close the joint between roof and parapet into one watertight whole

The metal and the waterproofing work as one system. The membrane is taken up the inner face of the parapet and the coping covers it from above. If one of these layers is done badly, the other suffers too. The coping also has to run continuously around the whole perimeter, including corners and ends. Every gap is a place where water will find its way in.

The material affects how long the coping lasts, how it looks and how it connects to the roof covering.

• Galvanised steel: a traditional, sturdy option. Without any further coating it corrodes over time, so it is used less often today.
• Pre-coated (painted) steel: galvanised steel with a coloured protective layer. It resists corrosion better and can be matched to the colour of the façade or windows.
• PVC-coated steel: used on roofs with a PVC membrane. The membrane is hot-air welded straight onto the metal, so the joint between coping and waterproofing is truly watertight. Do not confuse it with ordinary painted steel, which a membrane cannot be welded to.
• Aluminium: light, rust-free and holds its shape well. It suits modern buildings where low weight and a clean look matter.
• Titanium zinc: very durable, with a grey patina. It needs an experienced tinsmith, because it is worked differently from steel.
• Copper: a very long service life and a distinctive look, used mostly on prestigious and historic buildings.

Think about neighbouring elements too. Copper and galvanised steel should never meet in a way that lets water run from the copper onto the zinc, otherwise the zinc corrodes quickly. Fixings should also be made of a compatible material.

Systems differ as well. Traditional coping is folded to size in the workshop or on site. Proprietary systems come with pre-formed coping profiles, corner pieces and end caps, which speed up fitting and reduce the risk of mistakes when shaping the metal.

The order of work follows one rule: each layer must overlap the one below it, so water always runs downwards over the joints, much like roof tiles.

1. Preparing the base. The tinsmith or roofer checks the top of the parapet. It must be sound, clean and dry, with no loose pieces. On an insulated parapet a timber board is often fixed on top to give the metal a flat base.
2. Creating the fall. The top of the parapet is sloped towards the roof. A gentle slope is enough, usually a few degrees according to the system manufacturer. Water then runs onto the roof and into the outlets.
3. Bringing up the waterproofing. Bitumen felt or membrane is taken up the inner face of the parapet, ideally right over the top edge.
4. Fixings. Cleats or starter strips are fixed to the base. The coping hooks onto them, so there is no need to drill through the top, where every screw would be a possible leak.
5. Laying the metal. Individual lengths are lapped or joined with a seam. At set intervals, depending on the material and manufacturer, expansion joints are made so the metal can move freely as temperatures change.
6. Corners and ends. Internal and external corners, abutments with walls and ends are handled separately, ideally with ready-made pieces or carefully made fittings.
7. Sealing the joint with the waterproofing. With PVC-coated steel the membrane is welded to the metal. With other systems the joint is covered and sealed according to the manufacturer's method.

Parapet coping is work at height on the edge of a roof. It needs scaffolding or other fall protection, so leave it to a tinsmith who has the equipment and experience.

Even good metal will not help if the shape is wrong. The width of the coping depends on the thickness of the parapet, including any insulation, and on how far the metal should overhang on each side.

On the outside the coping must form a drip edge, a folded lip that stands off the façade. Water drips off it into the air instead of running down the render. If the drip is missing or sits too close to the wall, dark streaks and damp appear on the façade over time.

Why does the fall go inwards and not outwards? Water from the roof is collected in outlets and downpipes anyway. If the parapet sloped towards the façade, all the rain from its top would run down the wall. The fall must be even, with no spots where water can sit and freeze in winter.

How demanding the work is depends on several things:

• Length and shape of the perimeter: a straight parapet without corners is simple, while every corner, recess or penetration takes time
• Condition of the base: crumbling masonry or a missing fall must be repaired first
• Removing old coping: stripping and disposing of the old metal is extra work
• Height and access: a tall building or a tight site means more complex scaffolding
• Material: titanium zinc and copper take more skill to work than standard steel

In a block of flats, decisions about parapet coping are made jointly, so speak to the building management or owners' association first. If the work changes the appearance or construction of the roof, for example when the parapet is insulated, check with your local building authority what local building regulations require. Do not assume that no approval is needed.

• Fall towards the façade or no fall at all: water sits on the parapet, runs down the wall and the masonry gets damp.
• Missing or short drip edge: streaks from running water appear on the façade.
• Coping too narrow: the edge of the parapet or insulation is left exposed.
• Screws through the top without seals: every hole lets water in.
• Rigid fixing without expansion joints: the metal buckles in summer and winter and the joints crack.
• Poorly finished corners: badly lapped corner joints are a typical source of leaks.
• Waterproofing stopping short of the top: a gap is left between membrane and coping.
• Incompatible metals: the corrosion only shows after a few years.

At handover, look from the roof as well as from the ground and check:

• The coping runs continuously around the whole perimeter with no gaps
• The top has a visible fall towards the roof
• The drip edge stands off the façade along its full length
• Corners and ends are tight and neatly aligned
• Joints are lapped or seamed and expansion joints are in sensible places
• There are no unnecessary holes or unsealed screws on top
• The waterproofing runs up under the coping and the joint is closed
• The metal is not scratched, dented or distorted
• After rain no water stands on the parapet and no new marks appear on the façade
• The roof and surroundings are cleared of offcuts and screws