Connecting new roof to existing - how to seal valley?

I’m new to this board and need help, hopefully by Tues. morning when the shingles are to be delivered.

We are building an addition and now have a new roof, which has to be connected to an existing roof at right-angles. Here is a picture:

(that’s a piece of “ice&rain” running along the top of the valley, which can be removed). How do we make a valley that won’t leak? Cut on the intersection line, lift the existing shingles and slide a piece of flashing underneath? Run the new shingles up under the old and seal with roofing cement/tar?

thanks for any help.

First thing is to get all that underlayment to lay flat.
Next remove the part of the existing roof that adjoins the addition.
Shingle the addition as usual.
When you have shingled the addition up to where the addition meets the existing roof, run the addition shingles up and onto the existing roof.
Once the addition is completed replace the shingles on the existing roof, these will be the cut side of your valley, so you will have to cut the shingles along the valley line 2" from center.
Keep all nails a minimum of 6" away from the center of the valley, measured perpendicular to the center line of the valley.
Apply a fairly thick bead of tar under the cut side of the valley, roughly 2" in from the cut so that no tar is exposed.
The instructions on the package should fill in what I omitted.
A fairly easy and common detail for any roofer.
If the applicator is not an experienced roofer it still shouldn’t be very difficult for someone with average problem solving skills and mechanical ability.

There is no good way to leave the existing roof intact (where it joins the addition) and scab it together, if you try it will look horrible and will leak. :smiley:

Yo Ax, your nuts are showing. Heh.

Ax is right. You will need to take part of the existing roof off in order to do a proper tie in with the new addition.

How far onto the existing roof?

The “applicator” talks a good game but is NO roofer from what I have already observed. I do have those attributes you mention but until this week knew nothing about roofs at all…

That much I have learned, but thanks for the reminder anyway :smiley:

Do I have to take off the existing shingles (there is either one or two layers) or can I just put a new layer over the existing roof, which is in decent shape?

Also, should I use flashing (or ice & rain) in the valley before shingling, or just go with the shingles and the bead of tar?


When you bend the shingles from the addition onto the existing roof, the shingles will conform to the valley in such a way as to be at an angle on the existing roof.
The end of the shingle that ends on the existing roof will start on the addition.
This will result in the top (unexposed) part of the shingle to be closer to valley centerline than the exposed part, on the existing roof side.
Run your shingles from the addition onto the existing roof so that the top (unexposed) part of the shingle is no less than 12" from the centerline of the valley.
The exposed part of the shingle will be more than 12" from the centerline…
Keep your nails a minimum of 6" away from the centerline of the valley, on both sides.
I hope I described it in a way that is easy to visualize and understand.

You don’t have to take off all of the shingles, just the ones that are within 2’-3’ of the valley.

If you have 2 layers and the roof you have to tie into isn’t very large it would be better to do the entire side.
You can just remove the top layer to tie it in but then your Ice & water shield won’t be on wood on the existing roof side…

You can remove both layers within 2’-3’ of the valley, install ice and water shield in the valley, remove old felt so that the Ice & water shield is directly on bare wood.
Then fill in the lower layer from the existing roof, carefully cutting them at the valley ( in such a way so that they don’t go onto the addition), these going to smooth everything out and are going to be covered by the second layer.
now you filled in the gap, the second layer from the existing roof is not installed yet, it is the cut side…
Install the addition roof, valley and all.
Now it is time to install the second layer on the existing roof, proceed as described earlier.

If you choose to use unexposed metal in your valley, install Ice & water shield first then put the metal over that.

Rather labor intensive any way you look at it.

ditto Axiom,
i would ad that bein the exsisting shingle side is so small i would reroof/recover to the peak instead of tryin to tyin along the valley.
thats a little harder to do.
ok its a lot harder to do.

good luck.