Water Drainage and Glass Roofs – A Comprehensive Guide

Glass roof and canopy water drainage details are something that clients may not spend much time thinking about when they set their hearts on an all glass structure, yet it is crucial to the longevity of a building.


06 Sep 2022


Simon McAuliffe

Like much of the design and specification process, it’s up to the architect and glazing specialist to plan for water drainage: these subtle but vital aspects of a glass roof from the very start of the design process, to ensure there will not be problems further down the line.

This is even more pertinent when it comes to glazing structures, for reasons of safety and upkeep, and is something that can prove a real challenge when frameless glass is required. How can you direct and filter off rain water and accompanying detritus, without adding distracting components to a pane of glass? 

There are a number of ways to achieve this, and it’s valuable for architects to be up to speed with potential solutions when it comes to water drainage from glass roofs and canopies, when approaching their designs.

Why drainage is important for glass roofs

To understand why roof water drainage is so crucial, it’s first important to note that glass bows under its own weight. To counteract this, glass roofs are designed at a pitch of up to 3 degrees – any less than that, and there will be a problem with ponding. 

Pooling, or ‘ponding’, of water on a flat roof (whether glass or not) is never a good thing. At best it could leave stains and smearing, at worst: leaking and damage to the components of the structure, caused by a build up of silt over time. In the case of glass specifically, water and other debris need to run off smoothly and quickly to ensure visibility and natural light aren’t impacted. 

It’s also important to note that while a pitch of 3 degrees will prevent ponding, it doesn’t necessarily mean that water will simply run off the roof. Picture a car windscreen in the rain – even with a pitch closer to 45 degrees, water droplets still form. Put simply, the bigger the pitch the better, but for a glass roof to be considered ‘self cleaning’, and for water to drain away effortlessly off the panel, the pitch should be at least 15 degrees. 

Along with the pitch of the roof, you also need to consider where exactly you want the water to go. These decisions affect not only which direction you want to pitch your glass, but also what kind of drainage system you will utilise.

There are a few available design choices for fixed glass roofing (opening rooflights can also suffer from water pooling, but they have different ways of dealing with the issue) where a lower pitch is preferred, and these depend on how your roof is attached to the rest of the building. 

Glass to Glass Connections

When glass is connected to glass, whether it is a vertical glass wall, or another pane of flat glazing further down, it is vital to control where the water travels, even before it gets to a gutter. Without effective planning, the water would splash in sheets down the sides of the glass or harshly onto surfaces below, which would create unpleasant visual deluges, and can lead to water marks after the fact. 

It is possible to design a glass roof with an extended overhang of the horizontal pane, but in these cases it’s vital to be aware of capillary action: water rolling over and under the overhang edge.  For that reason, in many modern installations, a fully flush edge is preferable both practically and aesthetically. This isn’t to say a drip edge is inherently a bad thing, but it tends to give a more Victorian appearance, compared to the sleek visual impact of a flush edge. 

Another consideration for glass to glass drainage would be if the glass below the lip has installations in place which could be damaged by runoff water, such as pivoting opening doors or windows, in which case a structural gutter may need to be used or the glass pitched in another direction.

Glass Connected to Solid Wall Structures

This is one of the easiest scenarios to fix water drainage on a flat glass roof (as it can take advantage of the existing drain system in place for the rest of the building), but if your glazing is going to sit directly up against a solid wall, it’s going to be more weathering compared to other drainage systems.

There are a few options that can be designed into the structure to ensure longevity. If you’re face-fixing an aluminium angle to a wall, for instance, this should be covered with lead flashing to ensure the installation will last. 

In the case of new builds, however, it’s possible to chase the glazing into the brickwork. This creates a completely minimal visual appearance, but involves the challenge of effectively removing a brick course, and in many cases the structure won’t be sturdy enough to handle this on its own. The workaround typically involves inserting a steel channel. 

If the rooflight is to be built on an upstand, this will need to be weathered; usually it will need to rise to at least 150mm, and the glass placed on a 3 degree pitch, usually over the smaller dimension. (Otherwise, it will need to be built upwards).

In instances where heavy rainfall is expected the glass may need to be altered slightly or stepped, to guide the water towards the lip, to make sure that the system can cope and to ensure that the water doesn’t simply fall in sheets at the other edges, although this is only a possibility on projects with smaller pitch angles.

Structural gutters

There are a few instances when a structural gutter can be installed – notably, when the glass roof is attached to a wall that is higher than the glazing panels, in the case of a party wall, or when there are no other means of removing the water. 

They work twofold: as well as facilitating drainage, they assist in connecting the glass to the structure, providing added support to the glazing. The size and functional design of the structural gutter will depend on the size and weight of the glass that it will be supporting. 

These are bespoke fixtures made for specific projects; at Cantifix, we use Eltec, a certified structural gutter which can be fixed into the wall (providing it’s strong enough).

It’s possible for the contractor on your project to fit this, but in many cases it makes more sense (and is preferable) for your glazing specialist to install this as part of their work, to minimise the number of parties involved in the design and installation of the solution and ensure a cohesive design. 

The inclusion of a structural gutter does have other implications when it comes to things like thermal performance; these products aren’t typically thermally broken, so it’s important to factor this into corresponding aspects such as insulation. 

This kind of system can be found in many Victorian side return extensions, due to the height of the adjacent walls. Extensions can be pitched for the water to run back towards the building, with structural guttering to then carry the water away. This can be used either to make the extension appear bigger, to aid in hiding the drainage system with building finishes, or because the front of the extension contains doors or openings that may not perform well with other forms of water drainage. 

Other Considerations for glass roof drainage

As well as solar coatings and treatments to help us get the most out of our glazing, there is also the option for a low-maintenance glass coating, which minimises the need to clean your glazing, making it perfect for fixed glass roofs. The coating is added while the glass is still being developed and works in a similar way to something that has been waxed: water and other debris will simply not be able to grip onto the glass and will instead slide off. This will help with the movement of water but also lessen the time that it is in contact with the glass, a major factor in the creation of unsightly spotting and water marks.


Before deciding on how to remove the water from a flat glass roof, it is important to consider other aspects of the construction, such as what the supporting or adjacent walls are made of, if the glass will sit atop, or be attached to, these walls, and where you would like the collected rainwater to fall. 

As well as this you will need to work in conjunction with other drainage systems, such as those that may be found at floor level, as with a flush threshold or if there is anything that the water may splash down onto, on its way to the ground. 

It’s also valuable to note that depending on the specific nature of your project, it may be possible to design a unique solution to improve drainage performance. In the case of glass boxes, for instance, drip details may be suitable rather than unsightly gutters; these can deflect water away to the edges of the box to drip peacefully away when it stops raining.

Final thoughts

As the drainage system is an integral part of the design of a glass roof and will need to be designed to work elegantly – rather than splashing water everywhere – it’s always best to talk to your specialist about what will work best for your particular design. A simple rule applies: water will always travel in the direction you want it to, so a strong level of expertise is required to understand how it can be effectively and elegantly controlled. 

Canopy drainage details: a success story

One of our recent canopies, for the cider house at The Newt, had a pitch of just 3°, yet was able to successfully drain away any water. We achieved this by silicone bonding an aluminium flat to the edges of the glass, to guide the water down the slope and away from the building.

We all know how transformative a large glass rooflight can be to a building, and Cantifix have designed hundreds of such systems over the years. Our in-house team designs and engineers every glass roof to ensure lifelong high performance when it comes to water drainage.  Contact us today to get started on creating the perfect glass roof for your home.