Updated: Nov 8, 2020
Ed is a do-it-yourselfer who is happy to share some of his ideas and experiences in this monthly column.
Everyone understands that a shingled roof won’t last forever, and most folks know that often a 2nd roof can be installed over the first one. (You shouldn’t put more than 2 layers of shingles on, however, because the weight is tremendous, and most building codes won’t allow it!) I’m also talking about what most of us call “3-tab shingles” and not “dimensional shingles” which really should only have one layer.
There are loads of information on the internet about how to nail shingles, install flashing, work around obstacles and valleys, etc. There is very little information, however, about how to best install a 2nd layer of 3-tab shingles, and believe me, there is a trick or two to be learned!
First, you need to assess the straightness of the existing rows, because with the method I’ll be discussing, you won’t be changing any alignment – only placing a second layer. Here’s a picture of a small roof ready for a second layer:
You will notice a few things if you look closely. The first is that the shingles were nailed in nice parallel rows. That’s a big plus. You might also see that there is no drip edge installed at the bottom row. Drip edge is very inexpensive, and keeps water from weeping backward and up under the bottom shingle edge. Here’s a piece of aluminum drip edge, ready to be cut and slipped under that bottom row, as should have been done at the initial installation.
It really should be installed along the outer edges of the roof, too. On a new roof, you’d put the lower piece on first, install ice guard/roofing paper over it, and then install the eave drip edges over the roofing paper. In our demonstration discussed here, the eave had drip edge installed already.
OK, so what’s the “secret” to putting on the second layer? It’s knowing a trick that will help the 2nd layer lie flat. If you just put another layer over the first without following my instructions, the shingles will look very thick and “bumpy” and everyone will know you put the 2nd roof on. SO,first, measure the exposure of the lowest row of shingles. Most were likely installed with an exposure of around 5”, as shown here:
We’ll begin by cutting away a 5” strip from the TOP of a shingle. That’s the piece you WANT, so you can discard the rest. Actually, you’ll need a group of these whose combined length is equal to the length of the roof edge. Better yet, if you want to use a ROLL of “starter strip”, cut the roll to length, then to the width (5”) of your shingle exposure.
That 5” strip will then be nailed on the lowest row of shingles, essentially making the “step” between the first two shingle rows “flat”. You can use that piece with either surface up, since there is no “glue strip” on it. I used this one “back side up” but it doesn’t matter. What does matter is that you should trial-fit a shingle over this 5” strip prior to nailing to be sure the spaces between tabs won’t line up too closely with a nail!
Now, doing the next 2 steps carefully will ensure a nice-looking job. Since the old shingle exposure was 5” (or whatever yours was), 2 rows would be 10” (measure to be sure, and use YOUR exposure measurement). Note that the picture was taken prior to the 5” strip being nailed in place.
Cut the top off of what will now be the bottom row of shingles, making it that width, (10” in my case). Install it with the normal nailing pattern (usually printed on the shingle bundle wrapper). The upper edge will butt up against the lower edge of an old row of shingles. So, THERE IS NO NEED TO THINK ABOUT ALIGNMENT, no need to measure, snap chalk lines, or anything like that, and everything will lie flat!
Finish that row normally. The next row (and all subsequent rows) will use full-width shingles, BUT because of what “leveling” you’ve done, the exposure of the bottom row will be less than the rest. If you can live with that, and believe me, they won’t see it from an airplane, the rest of the alignment will go easily and quickly. Here’s how the edge looks with that reduced exposure:
There is one more thing to think about, though. Since the top of the 10” was cut off so the shingle “filled in” the space, there won’t be any indexing marks visible. Shingles have small marks/slits at their upper edges so that as you butt one against another, then move to the next row, you can align with the mark. That keeps the shingle tabs all aligned perfectly and the roof will have nice straight vertical lines. You will need to align the 2nd row manually above the first row. From then on and upward, you can use the indexing marks.
Here’s how it all looks after a few rows. Again, the easy part is that you’re using the old shingle alignment to set the alignment of the new ones.
I hope the above wasn’t too confusing, and that if re-roofing, you try this method. You’ll be rewarded with a nice job, and you’ll save a lot of time!
I hope all your projects go well. Thanks for reading, and happy restoring!