Roof valleys are like canals through which two sloppy section roof’s runoff water is discarded. Sometimes the water of such a situation is normally discarded using overlapping shingles, but this is just a temporary solution and often results in leakage.

Roof valleys are more of a permanent and long-lasting solution. But, how to cut a valley into an existing roof shingle can be a question.

It’s okay if you do not have any idea about the procedure of the work. This article will hopefully help you get a brief knowledge about the work. So, let’s begin.

Required  Tools

To go with this process, you have to manage some tools. And here is the list of those tools.  

  • Nail gun
  • measuring tape
  • utility knife
  • caulking gun
  • caulk line
  • tin snips
  • composition shingles
  • roofing cement
  •  roofing nails
  • flashing boards

How to cut a valley into an existing roof: Step by Step Guideline

To cut a valley into an existing roof is not hard as it seems, but it is better to work with an assistant or get some help to hold up your flashing boards or to draw a line. In this way, this work can be more accurate and effective. 

cut a valley into an existing roof

These are steps that you need to follow if you are thinking about cutting a valley into your existing shingles.

Step 1

For an existing roof, first, remove the shingles in the slope of the two roof junction area. 

If you are getting ready to install a valley, the second thing to do is roll down the ice shield. Ice shield is like a sticker, you pull the back of it and stick it down to the wooden roof. 

Step 2

After the ice shield is all stuck to the wood, the next part is to install the valley flash. 

Make sure your valleys are straight before you nail them. Draw a line about one and a half-inch from the center. Draw a checking line all the way to the other end, two men are needed to hold both ends.

Usually, the valley flashes are 12 feet long, so obviously that does not cover the whole roof section. As a result, overlapping occurs, in such a case you have to be extra careful so that there will not be any water leakage or anything that can cause damage to your roof section.

Step 3

Make sure to cut the valley flashboard one or more inches longer in the edges of the roof so that water can run through the board and go directly down the roof without clogging and leaking.

Step 4

Now, it is time to put the shingles on top of the valley flashboard, the layering of the shingles will be done afterward. Align the shingles through the valley to the line drawn, and nail the shingles down. 

You have to start your shingle attachment from the bottom and put one layer on top of each shingle securing them with nails.

Step 5

Lastly, cut through the excess part of shingles and come up with a clean and beautiful look.

Importance of roof valleys

Many traditional homes have layers of roofing papers in the valleys, which can deteriorate over years.

In other circumstances, shingles are overlapped to form a valley, and rainwater can get underneath the tiles. So, it is very necessary to have a valley installed on your roof for proper roof water management. 

If you live in a very rainy area, then it is recommended to put some roof cement on top of the valley on the edge on each side, before starting the layering process. This is done to minimize water leakage into the roof. 

Safety Measure

  • While using a ladder, make sure to be very careful.
  • As this roof valley installation needs you to work on top of the roof, so make sure that you have enough support. Otherwise, you might fall off.  
  • Valleys are needed to be placed on the slopes of two roof joints, so always hold a good grip.
  • always work on a sunny day, as the snowy or rainy day the roof becomes more slippery.
  • Nail down smoothly to prevent any injuries. If any nail is to be put correctly, use a crowbar to pull it out and nail it down again. 

Final Words

Installing a valley on your roof will extend the lifetime of your roof preventing leakage, damage, and other problems. Existing roof shingles just need to be cut and a valley board needs to be placed underneath them, and you are done.
This little effort can make your life much easier. So, let’s not be afraid of how to cut a valley into an existing roof. Follow these simple steps and it will be completed.


As a civil engineer and roofer, I love to share the experience that I have gained through the last couple of years. In the roofing industry, practical experience is a very crucial fact that can help you a lot. Hence, I want to help you with my blog.

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