Gutter Miter Corner Matters: How to Deal With Leaks

gutter leaks

You’ve heard the expression “the weakest link in the chain.” The phrase is often used to describe the one specific spot in a process where failure is most likely to occur. In the typical residential gutter system, that spot is the corner of the guttering run.

Table of Content:

  • What is a Gutter Miter
  • Why Gutter Corners Are Troublesome
  • Types of Miters
  • Signs of Gutter Miter Leaks
  • The Importance of Gutter Sealants
  • How to Repair a Leaky Miter
  • FAQs
Second-story gutter corner leaks might not be easy to spot from the ground.

What is a Gutter Miter?

A gutter miter is a type of joint used to connect gutters at corners and angled joints. When two gutters meet at an angle, such as at the corner of a roof, a simple butt joint with square ends would leave a gap that would leak. A gutter miter solves this issue by cutting each end of the gutter at matching 45-degree angles so that the ends fit together in a V shape with no gap. 

To create a gutter miter joint, each end of the gutter section is measured and marked at a 45-degree angle cut line. The ends are then cut along these angle lines, usually using a miter saw. The two angled ends can then be fit together to form a corner with no leakage. Gutter miter joints allow gutters to make 90-degree turns around corners while maintaining a waterproof seal. They are an essential technique for properly installing rain gutters on homes and buildings.

Why Gutter Corners Are Troublesome

Corners are the places where leaks will most likely form. One reason is because all corners must consist of at least two pieces of guttering that are affixed together; and the seam where they meet is a natural weak point in the system. Also, it’s unnatural for water to flow around a corner; so when a gutter forces runoff water to change direction, it can often result in uneven wear on a specific point over time.

Types of Miters

Generally speaking there are three different categories of miters that can be utilized in a gutter system.

Hand-tabbed Miter – This involves two pieces of guttering that are screwed together at two thin “tabs” that overlap each gutter on the inside; but these tabs can sometimes pull apart during temperature-related expansion and contraction of the metal gutters.

Pre-fab Box Miter – This is one corner piece that attaches to two perpendicular gutter runs; which of course means two different seams that must be fastened correctly.

Strip Miter – This is a skinny piece of aluminum that fits over the corner gutter seam and screws into both of them; but this strip requires a greater amount of sealant to prevent leakage.

Sealants are a gutter miter’s best friend.

Signs of Gutter Miter Leaks

Gutter Leaking at Seam or Corner

The most obvious sign of a leaking gutter miter is water dripping directly from the mitered corner or seam. This indicates the seal between the two angled gutter ends has been compromised. Water will drip from the gap or hole in the joint.

Ground Erosion Below Miter

Leaking miter joints often lead to noticeable erosion of the soil below where the water drips. The constant dripping wears away the dirt and causes a visible indentation. There may be a small ditch or hole in the ground below the leak.

Icicles Forming

In cold weather, leaking miter joints can lead to large icicles forming at the leak point. As water leaks from the gap and runs down the gutters, it freezes. Over time, the ice builds up into large icicles protruding from the miter joints. Their presence indicates a leak is allowing water to escape and freeze.

The Importance of Gutter Sealants

In short, each type of miter has its own unique “leak-prone” properties, which is why proper sealant use is so vital. Common sealant choices include caulk, silicone, or roofing tar; although so-called “super-sealants” like Unibond, Geocell 2300, or Vulkem 116 are often used by professionals. One important thing to remember about these products is to never apply a new sealant over an existing sealant material; otherwise, the bond won’t hold well and leaks are much more likely to form.

How to Repair a Leaky Miter

If you do come across a leak in your gutter miter, it’s highly recommended that you seal it promptly before it becomes worse and leads to water damage of your gutter, fascia, or lawn. Here is how to properly seal any type of gutter miter:

  1. Clear all debris out of the gutter miter.
  2. If the precise source of the leak isn’t clear, spray the area with a garden hose to identify the crack, hole, or gap where water is seeping out of the miter.
  3. Take an abrasive pad or wire brush and scrub the debris away from the leaky area. Remove any other dirt and grime with an all-purpose cleaner, then rinse the area clean and towel dry it.
  4. Using a putty knife and/or needle-nose pliers, cut away the existing sealant from the miter seam.
  5. Take your sealant of choice and apply it directly over the seam(s) of the miter.
  6. For other holes and cracks, use the sealant to fill them up as well.
  7. For a larger hole, cut a piece of metal flashing that’s large enough to cover it, then apply roofing cement both to the hole and over the edges of the patch that covers the gap. Allow the roofing cement to dry.
If you’ve got multiple corners in your gutter run, then you have to be very observant when looking for leaks.

Unless you can somehow redesign your home guttering system, you’ll have to live with one or more miter corners. The key is to be vigilant in inspecting them so that they don’t become leaky spots that can compromise the integrity of your gutters.

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How do I stop my gutter corners from leaking?

To stop gutter corners from leaking, you need to properly seal the miter joints. Clean out any debris, dirt, or old caulk from the corners. Apply a flexible gutter sealant made for miters to the inside corners and allow to fully cure. Aluminum pop rivets can also be used to reinforce the miter joints from the outside before applying sealant.

What is the best product to seal gutter corners?

The best products to seal leaking gutter corners and miter joints are gutter-specific caulks and sealants made for extreme weathering. Look for elastomeric or polyurethane “gutter seal” products that claim to seal corners and last through expansion, contraction, and temperature changes. They will bond best and flex with the gutters.

Why do my gutters leak at the seams?

Gutters most often leak at the seams because the joints have become loose or unsealed over time due to expansion, contraction, vibration, and temperature changes. Dried-out caulk, loose fasteners, or cracks from impact can also cause leaks at seams. Ensuring all gutter seams are properly caulked and reinforced with rivets or screws can help prevent seam leaks.


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