Flat Roof Leaks caused by Skylights

A 3D video explaining why skylights start leaking

Watch this video to see where a skylight could be leaking when the gasket seal is broke

When a roof is leaking, then it could be either the flashing or the skylight that is causing the leak. Skylights that have been on a roof for over 10 years will likely start leaking. A glass top skylight has a rubber gasket between the glass and the metal frame that holds it in place and that gasket will eventually fail.

Flat roof leaks are mostly caused by a faulty skylight and chimney flashing

There are many areas on a roof that can lead to a leak. So often the roof itself is blamed for a leak. The most common cause for leaks are obstacles on a roof like air conditioners, vent pipes, skylights, railing posts, siding, and doors sills that lead onto a roof.

The following can cause leaks on a flat roof:

There are many areas on a roof that can lead to a leak. So often the roof itself is blamed for a leak. The most common cause for leaks are obstacles on a roof like air conditioners, vent pipes, skylights, railing posts, siding, and doors sills that lead onto a roof.

leaks and skylights are part of flat roof problems
An Atrium Style Skylight. The gasket between the glass and the metal frame is what cause leaks

Skylights on Flat roofs causing leaks

When a flat roof is leaking it could be due to a skylight, chimney, vents or any other object on the roof. When an object like a skylight is part of the roof, then flashing is involved. It’s fair to assume that flat roofs leak more than a shingle or slanted roof. The reason is that water doesn’t run off as quickly and have a better chance to find its way in.

Two types of skylights

Skylights can be divided into 2 categories; those that are glass and those that are plexiglass. Skylights that are made of glass, should always be installed with a slant or at an angle. The reason is that if it would be totally flat, the frame that holds the glass will prevent water from running off. It will take time for water to evaporate and dirt will accumulate on top of the glass. The chances of water leaking through the gasket seal also increase.

Plexiglass dome skylight

Plexiglass skylights are mostly round. They are meant for flat roofs because no water or debris can collect on top of them. Plexiglass does not last forever in the sun. These domes must be replaced every 10 years. They crack and become brittle.

Plexiglass Tunnel Skylights are seldom seen
This is an Plrxi-Dome style skylight

Plexi dome skylight could leak at the flashing

Most of the time, a plexiglass type of skylight could leak where the flashing meets the roof. In this case, the roofing material was brought up on the side of the curb of the skylight and tucked under the framework.

Plexiglass dome skylight will leak at the flashing
The flashing on this skylight curb can cause leaks

Skylights and roof pitch and roofing material  – which one is wrong?

  1. The roof pitch is too low for shingles – should have been a rubber flat roof instead
  2. Skylights installed on shingles that made the problem worse
  3. Replacing the shingles with a rubber roof will fix the problem
Skylights causing leaks on a shingle flat roof
Shingles on a flat roof plus adding skylights just increase the chances of leaks

Skylights on a low pitch shingle roof

This is often the case. When the roofing contractor is someone off the street and does not understand what type of material to install on what slopes. The slope is too low on this roof. Moss is growing due to water being trapped in the shingles.

skylight on low pitch shingle roof causing leaks
More examples of shingles and skylights installed on a flat roof. See the moss grows

Preparing skylight flashing for a roof replacement

When a roof replacement is considered, the biggest part of the job is cleaning the flashing on all the objects. This is a tedious labor intensive job to remove all the old roofing material from the flashing.

skylight flashing cleaned for re roofing
When replacing a flat roof, the most time consuming part is cleaning and preparing the skylight flashing to receive the new roof

Skylight flashing ready for the new roof

Once the flashing has been cleaned, the base layer of the roof is installed under the flashing. It is very important to remove all the roofing debris for the flashing. In the case of EPDM rubber or TPO, they will not be able to use the existing flashing. This type of installation is only suitable for a Modified Bitumen Torch Down Membrane

Skylight flashing on top of base roof layer
This is what the skylight flashing looks like after cleaning it for the new roof installation

Erik is explaining how not to do flashing on a skylight

Flashing on a Skylight explained

Skylight glass seal was leaking on the left corner

This skylight developed a leak between the frame and the glass. Instead of caulking it, we added another layer of protection to the seal. Caulking does not last on glass and metal. Too often we see the caulk peel off.

Skylight glass seal was leaking on left corner
An Atrium Skylight with a metal frame that causes leaks

Repairing a skylight with EPDM cover tape

We do not promote EPDM rubber for roofing, but we did use it for this repair. We have never tried this type of repair before but tried it on this application. most people will use caulking. From our experience, we know caulking doesn’t last that long. This image was taken 10 years ago and we have never heard back from the owner.

leaks on skylights calls for repairing a skylight with EPDM cover tape
This was how we repaired skylight leaks, We now use Super Silicone Seal instead

After repairing the skylight with a cover tape

This is what it looks like after we have sealed the skylight seal with cover tape. The tape cannot be seen from the outside.

After repairing a skylight with cover tape
The repair on this skylight was done with an EPDM cover tape. We now use Super Silicone Seal instead

Skylight leaking on a metal roof

There were several of these skylights leaking on the same commercial building. Installing a skylight on a metal roof is not easy. There are the standing seams of the roof that makes for an uneven surface and the transition from the skylight flashing to the metal roof is not simple.

For the most part, the skylight flashing and the roof overlap each other and a sealant is used to seal the 2 metals together. This sealant will eventually give and water will find its way in.

Skylight leaking on a metal roof
In this image, tar was used to fix the flashing

Removing everyone’s attempted repairs

When the sealant fails on a metal roof, then it’s not easy to make a repair. The skylight will have to be completely removed and all the metal cleaned and then be re-installed. This will be a costly exercise. Instead, we came up with a system that works great on metal.

We first have to remove all the tar and sealants everyone else has used to try and solve the problem. Then we cut away some of the standing seams to allow for a flat surface around the skylight. This is so the water can freely flow and not get trapped on the top end of the skylight.

In this image we are removing the tar an cleaning the flashing

Cutting the Rubber membrane to fit around the skylight

The reason why the tar and sealants don’t last is that of ultraviolet exposure. Tar poured on a metal roof will dry and crack in a short period of time. Because TAR is a petroleum-based product, it will dry very fast. Our method is to apply the same rubber we use for Torch Down Roofing, for the flashing. We have found a secret trick to be able to attach the Modified Bitumen Rubber to metal and make it watertight.

Cut a rubber membrane to size to use as flashing
In this image we are sizing the rubber membrane to be installed around the skylight

Rubber is adhered to the metal roof and flashing.

With a special adhesive, the rubber was adhered to the flashing and metal roof making the transition watertight. At the same time, water could flow freely around the skylight. This type of repair will last a long time.

Attaching rubber to metal flashing
This is what the repair look like after we done. Notice no more tar that will crack

Round Domed Skylight

Notice how the roofing material is attached to the skylight base. This is one of the most important tricks we have. This image was taken more than 10 years ago and the evidence is still there that our flashing system iswatertightt.

Round Dome Skylight
A Round Dome Plexiglass skylight
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