Thursday, September 3, 2015

How to replace rotted T1-11 siding with a belly band

My wife and I live in the beautiful North West, near Portland Oregon. When we bought our first house last year we discovered during the rainy season that we had soft spots along the bottom 5 inches of our siding. We had two solutions, replace all the siding on the south side of our house or install a belly band / trim board. This tutorial will cover the basics of how to cut, replace and fix this issue. Hope this helps you in your DIY projects and best of luck!

I found a video from This Old House that gives some great starting tips!

Two things, from the video i'm not using lap siding, and my house's sheathing layer is just the T1-11, I don't have OSB and a moisture / vapor barrier. This presents some unique challenges.

Below is a picture of the side of my house needing repair, it might be hard to tell but when wet I could push my finger through the bottom few inches easily. While its dry it crumbles really easy when I pull up on it. I recommend doing the tap test to figure out how much needs to be replaced, I start from the top about 10 inches high and working my way down I tap gently to feel the density, its easy to tell where the soft parts start. Mines about 5-6 inches above the concrete foundation.

Skipped ahead a few steps to show you what the bad parts at the bottom look like you'd see the following that was cut away. 

The first thing I did was figure out my length and depth that needed replacement. The trim pieces go over your siding so you'll need to figure out what you want to do around the trim. I ended up doing the length up to the trim, and cut and replaced the 1/2 piece behind the trim.

My length was about 21 feet, the depth I chose was the depth of the trim 3/4" plus the depth of my T1-11 siding 1/2" for a total of 1-1/4". Therefor my new trim piece along the base will be equal depth to the existing trim.

Next remove all the vertical trim along the side you're replacing, So not to cut off the trim with the circular saw. Then take a chalk line and have one person (my wife hold it exactly 6 inches above the concrete. Move over about 10 feet and marked 6" up as well and drew my line with a chalk snap. Double check its flat with a level, its better to measure twice and cut once as my dad always said.

The cutting isn't to bad. Simply set the depth of your circular saw so the blade tip to the guard band is only the depth of your T1-11 siding. In my case that's 1/2 inch. DO NOT CUT YOUR STUDS!

A few learned hints,  cutting this on a 94 degree Fahrenheit day is not ideal. Second, if you're cutting make sure you do 4 feet segments, the T1-11 is in 4 foot siding interlocks and makes it easy to stop at these points. As you slowly move along the side of your house don't stop for too long in one place. I found the blade would heat up the siding and kind of make a warm/smoke smell. You don't want to catch your house on fire! I had a hose handy just in case and on occasion would spray a little water on my new cut seam right after cutting.

Once cut, take a hammer and pry bar and carefully chip away at your cut siding until removed. Should look something like the image below.

Next I needed to install a new moisture / vapor wrap over the exposed boards. For some reason my house didn't have a moisture barrier under all its T1-11 siding, only the lap siding on the front. This is not ideal. Anyways I used an asphalt roofing paper. The goal was to get 2-3 inches above the inside of the siding. This can be tough since so many times the siding is nailed down. A couple easy solutions that saved me tons of time was to gently pull up on the siding at the studs with a pry bar, careful enough not to damage the siding but enough to loosen a nail or two.

This next step I found online from a guy named Steve Hamilton. I love this idea! You pry out at each stud and right above drill a 2" screw in the siding and stud, this holds the gap while you install flashing and vapor barrier. Genius, and a must do!!!
Once that's done, paint and seal the newly cut under edge and add the vapor barrier as seen below. This is the best time to seal the newly cut board so it doesn't rot again. Put on a couple coats of paint or primer as needed.

Maneuver your moisture barrier up as far as you can and staple it to your studs and joists. Make sure to overlap the barrier strips at least 6 inches.

Before installing your new trim this is the prime time, pun intended, to prime and seal all sides of the new trim board before installing it to your house. Add one or two layers of paint until water proofed and looking good. While its drying we'll start installing the z-flashing.

Z-Flashing is basically the shape of the metal flashing. Make sure its just over the depth of your trim piece. The goal of the flashing is to provide a transition piece for water between the siding and the trim board. It also lets moisture behind the wall have a way out, we call this breath-ability. If this wasn't needed you could just caulk between the boards and be done. But breath-ability is very important. So don't caulk between the flashing and the T1-11 siding! Make sure there is room for the drip to settle and don't let the siding sit in the flashing. Leave about a 1/4 to 1/2 inch gap vertically between the just cut T1-11 and top of the flashing. Once nailed in, you can give it a slight bend down so water doesn't pool up on the trim. The bend is optional in my opinion.

My flashing was purchased at HomeDepot but you can find it at many hardware stores. Mine was 10 feet long so it doesn't meet the total length needed. Overlap by 4-6 inches for transitions and caulk between the flashing transitions. Use metal snipers to cut the final piece. BE CAREFUL WEAR GLOVES!

No need to caulk between the z-flashing and trim piece. Remember you need a way for water to get out if it got behind the flashing and trim.

When you go to nail your trim in, use 2 1/2 inch or longer galvanized nails.

Once your trim is hung and leveled, use exterior paint-able caulk to seal the sides and pieces (see the red lines)below for where to seal. Between all transition pieces. You don't want to seal behind the trim board and the siding. You want it to be breathable just in case moisture got behind it. If water were to run down the front, you don't want it to go behind everything.

Once its all caulked, make sure you seal all the nails with caulk or paint. I put a layer of white paint that I had laying around over everything. Using the color of your house is recommended, so don't worry we'll be painting soon.

Thanks for checking out my post, if you have question please let me know below.

Helpful links:


  1. Hi Brian. I just put a bid in in a home that has T1-11 siding. I had read up on the downside of this siding here in Pacific NW. Sure enough there were same issues and you described. Very interested in same kind of repair you showed. Most of the damage does appear towards the bottom on back wall but did extend up higher on side wall. Would you just make belly band same height on all walls? How high up can you go with this kind of repair? (Vs just replacing entire wall of siding) thanks the all the pics and instructions. Most of them I understood. And at least maybe my SIL will understand! :o) You were a tremendous help!

    1. Hello Darice,

      Glad it helped you. Yes I would make all sides the same height, for look and appeal purposes of the house. I'd say you should go up no more than 8 inches. Most of the exterior trim boards are 4-8 inches wide that I saw. Honestly any higher might look funny too. At some point it might be cheaper to just replace the whole siding if your soft spots run really high up the wall. Doing everything myself I figured a was saving a few hundred dollars, but having a contractor do the work they may get a better price on siding. Either way, best of luck to you and thanks for reading.


  2. I live in Kansas City, and this looks like the exact issue my house has and seems to be a very viable option to replacing the entire panels. Thanks for posting this!

  3. This is very very helpful. I considered simply putting a trim board over the siding, but that didn't make a lot of sense to me. I started looking on This Old House and a couple google searches brought me to your page. I consider myself pretty handy, but your tutorial is the piece I needed.

    This is exactly the thing I needed! I start work tomorrow!!!

    1. Glad i could help, let me know if you have any questions while going through my tutorial. All projects will be slightly different, but hope it helps! Let me know how it goes...


  4. Hi Brian

    I also live in Portland and have a house almost identical to yours. I had the same problem on my house's south side, which gets the bulk of rain (weather system generally come from the south-west). I had a contractor install this and he did more or less the same as you, except he caulked between the siding and z-flashing. After considering your point I think that was a bad choice. All other Z flashing on my house is not caulked. Fortunately the fix is easy. I'll just, very carefully cut off 1/4 inch of T1-11 using my Dremel SawMax, which is a small circular saw ideal for fine cutting.
    This is an excellent post thanks. I might add two points:
    1) From the pictures you show it appears you have LP 'Smart' siding, not T1-11. T1-11 is textured plywood, whereas LP is basically OSB.
    2) T1-11 and LP is in your and my case wall sheathing and hence part of the building structure. In the picture of the cut-way siding you see the mud sill on the bottom, your 2x6 tongue in groove floor boards, in the middle and the wall bottom plate stud on the top. To my understanding the wall sheathing is supposed to be attached to the wall bottom plate. I think you might have cut away too much?

  5. This comment has been removed by a blog administrator.

  6. Brian: I live in the Southeast and we encounter the same issues here.
    My plan when I begin this repair later this spring, is to nail a piece of strapping to the siding for my saw to ride on while making the horizontal cut. Any thoughts?
    Thanks for your article.
    Russ Johnson -

  7. Thanks Brian, this will work on my shed....

    FYI, you can take the cutaway piece and bring it to any paint store and they can match the color exactly

  8. Thank you so much. I believe this will work well for me. The only difference with mine is...I don't have a full on concrete foundation on bottom. It is a manufactured home and currently has horrible vinyl siding that I plan to replace. I probably will replace it with treated plywood or hardy board. This will need to be attached to the rim joist so I wonder if I should just place the entire treated wood from under the z flashing all the way to the ground?

  9. I don't have a rotten sliding side, I am just building a shed and I have been eyeing the bottom of the sliding, thinking about should I calk it or seal it up. But I think I can just wait, it will keep dry here in California most of the year.
    Thanks for the information, Nice to know more.