Flat Roof Parapets

In construction, parapets are external dwarf walls that project above and around roofs and balconies, providing adequate vertical surfaces and edging for proper roof coverings, as well as serving a practical safety feature in some instances. Traditionally, these parapets are made from masonry, often featuring decorative coping stones atop them. In more modern flat roof construction, parapets can be partially constructed from timber. Here is how this procedure is carried out. 

What are parapet walls?

Parapets are traditional masonry dwarf walls that project above the surface of a flat roof to create a form of retention. On flat roofs with access, they are used as a safety feature to prevent people from falling off. They can also be used as decorative features on flat roofs without access. Flat roofs with parapets will typically feature some sort of box gutter inside the perimeter of the walls in order to remove rainwater, as typical plastic guttering doesn't work in these applications. In modern construction, parapet walls can be formed with timber. For masonry parapet walls that are rendered or assembled from face brickwork, a lead flashing must be installed on the inside surface of the wall in order for the roof covering to properly be sealed. With modern roof covering systems such as GRP or Alwitra, the material is typically taken all the way up the inside face of the parapet and over the top, with a cover trim being installed on the outside edge of the roof. In order to facilitate this and provide an adequate surface for adhesion, the masonry parapet must be faced with plywood.  

What are the criteria of a ply parapet?

Plywood must be installed flat on the faces of the parapets in order to provide an adequate surface for the roof covering to be adhered to. The edges of the plywood segments must be straight in order to maintain a professional aesthetic. The sharp edges of the plywood should be taken off with a plane or sandpaper to ensure that the roof covering is not damaged over time due to elemental exposure. Appropriate fixings should be installed at regular intervals to fix the plywood to the masonry. Appropriate 18mm WBP plywood should be used in these external scenarios. All of the joints and intersections between the different plywood segments should be tight and without gaps to ensure no disruptions or visual distortions to the roof covering. 

How are ply parapets formed?

For ease of installation, the plywood facing segments should be installed before the creation of the box gutter components, which we will look at shortly. The most common assembly method for these parapets sees plywood strips that face the internal vertical surface of the parapet wall, with a top cap of plywood being installed afterwards. In a box gutter, the plywood will extend from the top of the joists to the top of the masonry parapet. A small tolerance is allowable here in order to correctly align the plywood with the wall, as any gap between the bottom of the plywood and the top of the joists will be hidden by the box gutter components. These plywood segments can be cut from whole sheets of plywood either lengthways or widthways, though the longer the lengths are the less time consuming the installation is. Tapered lengths of plywood may be required down the sides of the roof as it runs down, depending on the order of assembly of the roof. Alternatively, the parapets can be faced before the main roof is assembled, with the buildup of the deck rising up inside the parapet. This method reduces the amount of precise cuts to be made on the plywood, though it requires the masonry parapets to be constructed to the correct height before assembly begins. Once all of the internal face pieces are cut, they can be fixed to the masonry with plugs and screws, with the top edge of the plywood aligned with the top edge of the masonry.  

Once all of the inside face pieces are installed, the width of the top cap pieces can be determined and cut accordingly. These pieces are easier to install as they simply lay flat on top of the parapet and can be moved into alignment with the top end of the facing plywood. Screws can be installed through the edge of the top cap and into the top edge of the facing plywood, provided that the holes are predrilled to avoid splitting the facing plywood. Packers on shims should be installed in the gap on the external face of the masonry to provide a slight inwards slope of the top cap towards the main body of the roof, to prevent water from pooling on the top of the parapet. With all of the plywood installed the edges and corners should be rounded over with a plane or sandpaper to prevent damage to the roof covering. At this point, the installation of the box gutter is ready if applicable.