GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Siding with a step

Mike M | Posted in General Questions on

I have an addition that I have added to my home and am now in the process of finishing up the furring for the rain screen and getting ready to install siding.

The issue I’ve run into is that I am using vinyl siding to match the original house, but the addition has a basement and the main portion is slab on grade. Because of this, the addition has a rim board that extends roughly 12″ below the original slab/sill plate line.

When I go to install vinyl siding the starter strip on the old portion will not line up with the addition.

Should I just pull the starter strip higher on the addition since I’m already adding exterior insulation around the foundation and make the rim seem like it is below grade? Or is there a way to line them up. I thought about running J-channel at the bottom instead of starter strip, but was concerned that this would hold water. We have finished the other areas of the backer board with a mesh overlay and a polymer based coating that looks similar to gray stucco over concrete when we’re done,

Any ideas are greatly appreciated. I’m sure there are plenty of ideas out there that may work.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    There are lots of possibilities.

    1. Do a little math, and figure out how many extra courses of vinyl siding you can manage on the side shown on the left side of the photo. Install your starter strip at the correct elevation so that when these extra courses are added, you end up perfectly matching the course in the section on the right side of the photo. This approach may mean that your starter strip on the left side starts somewhat lower or somewhat higher than you might normally start it.

    2. Install water table trim on the section on the left. The water table trim needs to be capped with Z-flashing. Plan the width of the water table so that it allows the vinyl siding starter strips to line up.

    3. Install gray metal flashing (painted aluminum) to cover the rim joist area of the section on the left. Then begin your starter strips so that the starter strips line up on both sides.

    Flashing is your friend. A little bit of exposed flashing won't necessarily be noticeable.

  2. Mike M | | #2

    Some of the ideas may work. Right now the lower section is almost exactly 1 1/2 courses below. I could cut the siding half way and caulk it, but then it would be pretty rigid and might bow in my opinion.

    The picture is a bit fuzzy, but there is already a 4 1/2" Z flashing beneath that housepaper with 4" of XPS under. The Z flashing is about 2" above the sill plate, sealed and taped and then the two layers of XPS. Below the flashing you can sort of see the exposed Roxul. I took my chances with no rainscreen above grade on the Roxul and am covering it with backer board above grade. I have already done this in some areas as an experiment and the backer board and Roxul seem to hold up really well. Time will tell.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |