Building a structure which is attached to a house usually involves a plate which is bolted to the existing building, typically a 2x8, or 2x10. I understand how to structurally attach this, my concern is working with the existing vinyl siding.

If I just bolt my structure to the house, I understand I risk damage to the siding (shattering), leaks around the bolts, and it would just be unsightly to have the siding looking crushed around the edges.

But... if I cut the siding back, I'm not sure how to install flashing under the existing siding. Or will J-moldings be sufficient? I'm also not sure how I can cut siding properly while it's in place on the house.

Are there any known (good) techniques for adding a structure to a building with vinyl siding?

1 Answer 1


You'll absolutely need to cut back the siding and attach directly to the home. For the siding, you need two custom tools. The first is a small hook for unhooking a piece of siding from the piece below. Use this to remove all the pieces that are currently where you want to install the plate. Get a role of tyvek tape to cover up all the holes left from removing the existing siding. Then, snap a chalk line across the wall from screw to screw of the existing siding so that your reinstall will be perfectly straight.

And the second is a punch that is used to attach the siding to a special insert (undersill trim) that goes in the J-channel when you don't end at the exact top. You cut the siding to the height you need (measure twice) and then the punch creates small tabs that stick out from the siding (only 3 sides of the small hole are punched). The undersill trim goes inside the downward facing J-channel to receive this punched piece.

The top piece of J-channel (which is an inch or so long to line up with the side pieces of J-channel), you cut back an inch along creases to create a small tab that folds down into the J-channel on the sides. This helps create a water path that hopefully doesn't go behind the siding.

The next tip, when cutting, you can either use tin snips or a circular saw. We have a dedicated saw to siding on the job site and the blade is mounted in reverse. I believe this results in less tearing of the siding giving you a nice smooth edge.

One last tip, don't over-tighten the screws, the siding should be allowed to float right and left for expansion. The rule of thumb is that your fingernail should be able to get under the screw.

If this is confusing, go out and look at how siding is installed around a window.

Here are some of the tools that I referred to (I don't think you'll need the nail hole punch):

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And a good description could be found at this site.

  • So, there's no flashing behind vinyl siding edges, other than the tyvek sheeting they put on the walls? The J-channel is sufficient?
    – Scivitri
    Jun 17, 2011 at 18:48
  • I don't recall using any flashing for our installs, but most of our protrusions were windows that came with built-in flashing and were then sealed with a rubber membrane. I think a bead of caulk around the top and sides of the plate would be more than enough, but it wouldn't hurt to get a second opinion.
    – BMitch
    Jun 17, 2011 at 19:09
  • 2
    We always install a layer of Grace Ice and water shield from under the siding down and over the ledger (plate). I then also use some 4 or 6 inch flashing flared out from under the starter strip and slightly above the decking surface. It is absolutely necessary to divert water away from the back of the plate, or it will find it's way behind it and into your wall. Jun 19, 2011 at 12:26

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