Flat Roof Box Gutters

In all aspects of roofing, proper rainwater redirection is a primary consideration. In smaller flat roofs constructions such as garage roofs, the flat roof covering folds neatly over the drip edge and into a plastic gutter system. For larger flat roofs without plastic gutters, custom gutters may be built as a part of the flat roof in order to direct rainwater away from the construct. Here is a description of the implementation of these custom flat roof gutters.  

What is a flat roof box gutter?

A flat roof box gutter is a custom assembly that is built into the main portion of a flat roof, typically in warm roof systems. These gutters or “troughs” collect water that is directed by the furrings on the main portion of the roof, in turn using their own furring systems to direct the water off of the roof and into a hopper or downpipe. This negates the requirement for conventional plastic gutters, additionally removing the necessity for soffit and fascias. This in turn allows for surface finishes like cladding or render to rise all the way to the top of a building, with a custom roofing cover trim finishing the intersection. Alternatively, these gutters work very well in combination with parapet walls, a combination that is very popular in modern construction for its sleek design.    

What are the criteria of a flat roof box gutter?

Flat roof gutters must be installed in the correct orientation and position in relation to the run of the main furrings within the flat roof. The gutter itself must run from a high point to an outlet at the low end of the roof. Any insulation that is removed due to the installation of the gutter must be replaced underneath the warm deck between the joists to maintain correct heat retention and insulative properties. An appropriate width should be maintained for the gutter to allow for the roofers to effectively install the roof covering.  

How are flat roof gutters formed?

When constructing a flat roof that features a box gutter, the gutter components must be installed simultaneously with the different layers that make up the flat roof. In this example we shall look at a simple straight box gutter that runs from one end of a small flat roof into an outlet at the low end. The first step in constructing a box gutter is to establish the interior and exterior kerbs around the perimeter of the roof that will house the primary layer of insulation. This exterior kerb is installed in the same method as with a standard warm deck flat roof. The interior kerb is installed to house the insulation also, with a channel being created between the interior and exterior kerb to form the gutter. In order to allow the roofers easy access to the inside of the gutter with their tools during the application of the roof covering, this gutter should be a minimum of around 300mm in width where possible. 

The height of the interior kerb should be the thickness of the insulation + the thickness of the first layer of plywood (18mm), so that the second layer of plywood will sail over the top of the insulation and onto the top of the interior kerb. This timber may need to be ripped down to a specific size from a wider length in order to accommodate this height, depending on the makeup of the roof. This timber kerb could be installed on top of the first layer of plywood, as the exterior kerb sometimes is, though there are some downsides to this. By installing this kerb first before the furrings, we achieve a few advantages. With the kerb in place, we have a solid registration point for the end of the furring to receive into, as well as the first layer of plywood. Additionally, we can ensure a positive fixing through the kerb straight down into the joists, whilst also keeping the kerb perfectly upright.

Once the kerb is in place, the thin end of the furrings can be placed into it and nailed in place. The first layer of sheathing can then be installed with a small tolerance between the plywood and the kerb for expansion. With the first layer of plywood installed, the exterior kerb can be run around the roof with the vapour barrier installed on top of this in accordance with the regular warm deck procedure. Once the insulation is cut in between the kerb the final layer of plywood can be installed. The plywood should come to the interior edge of the interior kerb and be nailed securely to it. With the main section of the roof assembled in preparation for use with the box gutter, the gutter components can be installed. 

The next components to be installed are the furrings and plywood of the gutter. Depending on the length of the gutter and the available fall offered by the furrings, there might be room for rigid insulation within the gutter. As we can see in the drawing, the primary body of the roof features 150mm of insulation, which leaves a total depth from the top deck to the top of the joists of 186mm(2 x 18mm plywood and 1 x 150mm insulation). In this particular instance, 90mm of insulation can be installed within the gutter b[6] etween 2 layers of plywood for a total height off of the joists of 176mm. At the high end of the trough, this is a 50mm furring, 2 thicknesses of 18mm plywood, and the 90mm rigid foam insulation. In scenarios where the run of the gutter is longer, the furrings may taper from a larger thickness such as 75mm. This means the total thickness of insulation in the gutter must be reduced to keep the top level of the high end of the gutter below the main body of the roof. Depending on the exact makeup of the roof, insulation may not be present in the gutter at all, instead being insulated from below, as shown in the diagram.  

In either scenario, the first step is to install the furrings within the trough to taper to nothing at the outlet point of the roof. In the diagram we can see that the facing plywood of the parapet is already installed, and so the furrings will sit in between this and the internal kerb of the gutter. Plywood is ripped down to thickness with a few millimetres of tolerance to sit on top of the furrings. These strips are nailed to the top of the furrings. At this point the insulation and top layer of plywood can be installed if relevant. Once the last layer of plywood is installed, the outlet hole can be drilled through the parapet plywood using a 3” hole saw. The bottom of this hole should sit just below the top layer of plywood so that there is no lip in the gutter when the covering is installed. A prerequisite for this outlet is a lintel and void left in the masonry to allow for the plastic outlet to pass through the masonry and project out of the building into a hopper.  

There are a few variations and alternative procedures associated with forming box gutters, the most common of which can be seen in the annotated drawings.  

Out the side and into a hopper

Through a condom and out the bottom directly into a pipe

Returning both direction into a central hopper.

Flat gutter built in above a soffit overhang

Flat gutter going round corners