How to Tie A New Roof Into An Existing Roof

While tying into existing roof shingles seems complicated, it’s a reasonably simple task to accomplish as far as roofing goes.

By simple, we do not mean that this project doesn’t require diligence and hours of hard work. Instead, knowing how to tie in a roof to an existing roof correctly isn’t as complicated as one might think.

Why Should You Tie into an Existing Roof

Key Terms

First, to understand how to tie in a roof to an existing roof, it’s beneficial to understand the construction terminology related to this project.

Here are some key terms:

  • Ridge: Horizontal beam on the top of the roof that connects two roof planes.
  • Rafters: Sloped beams connected from the ridge to the wall plate. Rafters support the shingles on a roof.
  • Wall Plate: Horizontal lumber that denotes the start of the wall and supports the rafters.
  • Truss: The roof frame that forms the layout, which usually consists of beams connected by nodes.
  • Valley: The intersection where two sloping roofs meet and allow water to flow off the roof.
  • Eave: Part of the roof that hangs over a houses’ walls.
  • Jack Rafters: According to Residential Reports, jack rafters are “a short spar that runs from a hip to the eave or from a ridge to a valley.”
  • Run Length: Distance between the ridge and a wall.
  • Chalk Lines & Snapping Lines: A string with chalk to make straight lines on a slanted surface.
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Steps to Tie a New Roof to an Existing Roof

Step 1: Frame the Roof

Set your trusses or rafters up on the new roof so they match up with the eaves of the existing roof. Also, mark the shingles with chalk lines to indicate the area where the new valley goes.

You can remove the shingles in the marked area since that is the area that the roof from the new addition will cover. Whether or not you decide to strip the old shingles is your preference. However, it might be wise to remove them if they’re not in good condition.

roof trusses

Step 2: Install a Valley Board

Now, you can install a valley board to the existing roof. But, first, you will need to calculate the roof pitch, which is the slope of the existing roof.

To measure the pitch, measure the distance between the ridge and wall and the roof’s rise. Then, you can use a calculator to determine the angle.

With your valley board nailed down, you should put roofing cement on the nails and along the outside edges, where the valley board touches the existing roof. This helps prevent leaks from occuring later on.

Step 3: Sheathing & Underlayment

Next, install plywood sheathing on the new valley board with nails. Make sure to use nails that are the proper size to penetrate the plywood and valley board.

When placing the sheathing, leave a 1/8th inch gap between each piece. For the nails, place them six inches apart and about 3/8th inches from the edge of each plywood sheathing.

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After securing the sheathing, lay a waterproof underlayment over the sheathing.

plywood sheathing

Step 4: Remove the Shingles near the Valley Board

You will want to lay down a snapping line two inches up from the valley board on the old shingles. Now, strip the shingles in this area and lay more waterproof underlayment over the site.

Step 5: Tie into Existing Roof Shingles

Start by removing two shingles from each side of the new valley and continue removing them until you reach the bottom. You will use these later, so remember to number them and set them aside.

Begin by laying shingles along the edges of the roof and use snapping lines to ensure proper spacing. When installing shingles where the new valley meets the existing roof, make sure the shingles touch both surfaces by at least a foot.

Next, grab your labeled shingles and reinstall them. Finally, secure each shingle by laying down a 2-inch-wide roofing cement 4 inches up from the valley’s center.

Why Should You Tie into an Existing Roof?

If you don’t tie in a roof to an existing roof, you risk the possibility of air and moisture seeping into your home’s structure because of openings in your roof. If water is continuously leaking into your home, irreparable damage, mold, and mildew will form.


The cost for a project like this depends on the size of your roof. According to Fixr, you should expect to spend $50 to $450 per truss. You will also need to purchase the tools and materials required to complete this project.

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How to Tie In a Roof to an Existing Roof – Steps


What can go wrong?

  • If a roof isn’t tied into an existing structure properly, you may encounter water damage to your homes’ construction.
  • To avoid damage to your old or new structure by having too much weight on your addition, consult with an engineer to help you calculate the permissible weight of your new roof.
  • When working on your roof, serious injury is possible, so take precautions to ensure safe working conditions.

What nail size should I use?

According to Build My Own Cabin: “Roofs are typically sheathed with 5/8″ exterior grade OSB. Nail the sheets with 8d nails at 6 in. o.c. at the edges and 12 in. o.c. in the field.”

Can I cover up roof damages with new shingles?

You shouldn’t cover an existing roof with new shingles, as it is only a temporary fix for a bigger problem.

How do I blend the new roof with my existing roof?

The best way is to remove a shingle from your existing roof and take it to a hardware or roofing supply store to find a good match.

Nails or staples?

You can use staples for plywood sheathing and waterproof underlayment, but use nails for securing the shingles.

Final Thoughts

It’s important to note that not everyone can tie into an existing roof, and sometimes it is better to ask for help or hire a roofer. That said, hopefully, you learned some key tips on how to tie in a roof to an existing roof and are on your way to expanding your home!