How to flash siding butt joints—AFTER the siding is installed

A common problem with cement board siding products (and some other types of horizontal lap siding) is that older installation guidelines called for caulking the joint instead of flashing the joint.

This has proven problematic over time as caulking the joint results in unsightly surface appearances due to the different textures as well as resulted in failure of the caulk joint and associated ongoing maintenance.

Since newer installation guidelines do not recommend caulking this joint but instead recommend leaving a small gap and flashing behind the ends of the siding, I think it is more prudent to add flashings when it is possible to do so.

If the corners of the siding boards have been face nailed, it can be a little more difficult (though not impossible for experienced siding installers).

Otherwise adding flashings is typically a piece of cake!

Here is what the detail look like.

In this first picture we see what a typical siding joint looks like on a home that was sided with cement board siding and there is no flashing behind the butt joints. The caulk has failed and water can find its way behind the siding.

Failed caulk joint---with no flashing behind

Failed caulk joint—with no flashing behind

The following series of pictures will detail how to make a simple flashing that can be slid up behind the joint. It requires no nails.

Depending on the “reveal” of your siding the flashing length will be ¼ inch less than the reveal. Dog ears will be cut on both sides approximately ¾ inch from the bottom and then folded to create a “stop” that will locate itself on the top of the underlying row of siding–but not be so long that it will run into the nails that hold the siding of the row being flashed.




flashing6aNote that the flashing does not go up high enough to hit the underlying nail but is still high enough to end up behind the next row of overlying siding.



Once the joint is flashed, it really will not look much different than before it was flashed but water cannot get behind the siding.


By Charles Buell, Real Estate Inspections in Seattle

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  1. Dean Vorhies says

    Very similar to flashing a center nail on natural slate roof. Thanks for the tip.

  2. I see small blue question mark symbols in a small box (but no pictures).

    I view your web site with an iMac airbook, running system 10.9.5.

    Interested in seeing your total presentation.

    Thank you,

    Dale Paegelow, AIA

    • Charles Buell says

      Do you have security settings or something you can adjust or try it on a non-mac device. I have no clue why you are not able to see it. Have not had anyone else complain about not being able to see the pictures. Not sure how to help.

  3. What material did you use for the flashing? We just bought our home last summer, and we are looking to possibly slip flashing under the joints, and carefully nail the edges of loose pieces. House is only eight years old, and we didn’t realize the severity of the issue when we bought the home. First time buyers, learning the hard way!



    • Charles Buell says

      It is 24 gauge painted steel. Let me know how it works for you—your pictures looks like a good application for them. Make sure you pre-drill for any face nailing you do.

      • Great, thank you so much for the quick response. Your article on this subject is the most helpful I’ve found on the web. I’ll let you know how it turns out! Happy 4th.

  4. Great idea, but how do can you please include a better picture of the dog ears? What’s the best way to get the flashing into place behind the butt joint without getting it stuck on the dogears? Would you also suggest caulking and painting over it for cosmetic reasons? Thanks!

    • Charles Buell says

      The dog ears show about as good as I can do in the pictures. The thing I should have indicated is they are only about 1/4″ tall and you have to drive a little shim under both pieces of the siding to the left and the right of the joint so that the dog ears don’t drag too much on the siding and flatten out. Once it is in place the shims are removed and the siding essentially holds the metal flashing in place.

      • Thank you so much for this very clear article on how to retrofit flashing under butt joints, and in particular, for this clarification on how the dog ears work and how to slip the flashing into place. I’m just wondering whether the dog ears wouldn’t work even better if they were folded on the lower side of the horizontal cut rather than the upper. And they would also be easier to slip into place. Do you see a downside of doing it that way?

  5. This is awesome! I just installed about 50 of them. On my house the there is metal siding on the bottom 42 inches. I can’t figure out how to flash the bottom joints. There is a metal J channel and then siding. There are only a few of these joints but I would like to finish the job.

    • Charles Buell says

      I think for the bottom you might have to just rely on friction and a little caulk adhesive to make it stay in place—not much for water to get behind on the bottom course and perhaps just caulking these joints will be OK as well.

  6. Thanks for the great advice. Do you have any suggestions for flashing a four foot vertical seam? I am going to install James Hardie 4′ X 8′ fiber cement panels on the exterior parapet walls of my garage. I’m using Z flashing along bottom edge, but I haven’t found any flashing for the 4′ vertical butt joints. I have only found 6″w X 12″h flashing. Any suggestions would be greatly appreciated.

    • Charles Buell says

      They make reglets just for this purpose—any cement board siding supplier should be able to help you out with this.

  7. Charles Bevan says

    This is genius! My house was built in 2006 and the butt joints were neither caulked nor flashed so i really appreciate this guidance. Mr. Buell, I have a few questions for you:
    1. How have these inserts held up over time? any water stains due to corrosion of the steel? I know it’s painted steel but the paint could be scratched or water could contact the cut edges.
    2. Is this steel readily available at a big box home store like Lowes or Home Depot? I don’t think i’ve seen sheets of painted steel.
    Thanks again!!

    • Charles Buell says

      I have no data as to how they hold up by I see no reason they would ever rust or deteriorate and I would anticipate they would last longer than any siding they go with. The amount of exposure they actually have is almost nil. The steel is just normal enameled steel flashing material and is readily available any most any big box hardware store—like the Big Orange Tool Box. It comes in rolls, sheets, bent shapes and small pieces.

      • Charles Bevan says

        Fantastic. Thank you for the prompt response.

      • CHARLES BEVAN says

        Mr Buell, one more follow up question: given that the flashing does not adhere to the Hardiplank and there’s a small air gap between the flashing in the Hardiplank isn’t it possible that condensation or ice may form on the backside of the flashing especially in cold weather? Since it’s made of metal I would think it would get colder than the Hardiplank and be more susceptible to condensation and frost

  8. Ronald G Finney says

    Thank you for your good advice. I did a little donation out of appreciation

  9. David Stevens says

    Great idea! After doing this, is caulk no longer necessary?


  10. Hi Mr. Buell,
    Great site! I am paranoid about water getting behind the siding on my house and creating
    major issues. I am the second owner of this house which was built in 2007. The original
    owner never did anything to the exterior. It is sided with Hardie Plank. This siding was
    poorly installed. During the process of buying my home I hired a home inspector. After the
    inspection he advised me that when the home was built no flashing had been installed and
    the butt joints had not been caulked. What I found through much research is the best
    choice for caulking Hardie Plank is polyurethane caulk. I live in a hot, humid climate with
    very little ice or snow. What is your opinion regarding my caulking the joints and with polyurethane
    I read your info. about installing flashing on already installed siding. Great info. I feel caulking
    will help a lot and enhance the appearance of the house. My neighbor’s house has Hardie Plank
    siding. It was painted by a contractor who caulked the joints before painting. About 6-12
    months later the butt joints looked awful. All the caulk was coming out and exposing the
    joints. I suspect low quality or an improper type of caulk was used and poorly applied.
    I appreciate any comments or suggestions you have. Thank you

    • Charles Buell says

      In my opinion caulking of joints should be avoided and I recommend flashing the joints as discussed in the post. No matter what caulk you use there will be texture, and sheen differences that will result in the joints showing up more than leaving them uncaulked.

  11. Good job! Thanks for your help as a moderator with the Professional Home Inspector Forum! Your insight adds so much value to our Facebook Forum!

    If you are not a member, feel free to stop in:

  12. Great post; thanks! One concern I’d have is that if the dog ears are cut too large, they could extend farther than the thickness of the siding and slice or penetrate the building paper underneath. Easily avoided by making them relatively small (they just need to “catch” on the top of the lower board, no more), but I thought it was worth noting. Do you agree?

    Again, thanks!

    • Charles Buell says

      Not sure how much a little pin prick of the dog ears if you got them too long it would not be too much different than the gazillion nails holding the siding in place.

  13. We are first time home buyers and just bought a newly flipped house 5 months ago. Already we see the siding shrinking (?) and a lot of spacing/reveal between siding. Is this from the climate? We live in san diego so weather isnt that extreme so we are quite surprised. This seems like a great idea however no one in my household is really that handy.. would just caulking be sufficient? I should probably expect the wood to shrink more??

  14. I was able to slide 1 inch wide pieces of flashing up into my joints.
    Occasionally, when things were too tight, I would use pieces smaller than an inch.
    Sometimes I would have to pry the Hardie plank slightly, carefully away from the house.
    Pry as far away from the nails as possible because the boards can crack around them.

  15. I the picture it looks like a section of hardie board has been cut out and then the flashing placed behind the joint

    But in the text you imply these flashings are simply slid up behind the joint ?

    I’m confused

    • Charles Buell says

      Sorry I do not have a video of one being installed (perhaps I should include one). Think of the pictures as a “drawing” of the idea, not so much how it is done. If the siding was all in place it would be more difficult to show what it looks like behind the siding.

  16. Thanks for the reply, a couple more questions, does the flashing have to be metal ?

    Will it be necessary to pull the boards out a fraction to slide these flashings in ?

    What about the possibility of damage to the boards while inserting these flashings ?

    • Charles Buell says

      It is going to be hard to make the tabs on anything besides metal, yes the boards have to be pulled out maybe 1/4″ (carefully). If you do it right there will be no damage to the boards.

      • Thanks for the reply

        Unfortunately, I have more issues with my siding installation, in addition to the butt joints not being flashed but caulked the installer:

        Did not install flashing where the bottom board of the gable ends meets the stucco

        Did not overlap or ‘kick out’ that bottom board, it’s actually flush with the stucco

        Did not attach the hardie board to plywood so there’s just the house frames, them Tyvek paper, then the hardie board nailed to the frames

        Caulked under each board the entire length of the gable

        Cut the h boards for fitting purposes in the middle, not the ends as recommended for a uniform fit

        There are probably other issues

        I am so concerned about the job I’m considering tearing out and starting all over again

        Any thoughts ?

        • Charles Buell says

          Well it sounds like you have some things that sound like legitimate problems—others not so much. For example it is acceptable to install hardi over tyvek right over studs. This is very common on gable ends above the house side walls. I agree, I don’t like it either, but is allowed. Do you have the Hardi install instructions for your climate zone? Also distinguish between what is required as opposed to recommended. Hardi makes distinctions between the two.

          • Thanks for the reply

            I have not seen the specific hardie install directions for my region

            I’m in Tampa, Fl

            What is your opinion on the most significant problems with my installation (No flashing on butt joints or between the bottom board and the stucco, and of course no plywood behind the Tyvek wrap)

            I was planning on installing, flashing after the fact but after discovering that plywood is usually recommend to attach the h board I’m contemplating a complete year out and re-do

            A real disaster and massive expense.
            My big concern with no plywood is a lack of strength/ water and impact resistance associated with our tropical weather and hurricanes

  17. There is also a conduit with an electrical cable inside that continues into the attic through a hole the contractor made through the hardie board

    Looks like he nailed the end of the conduit to the board but it’s pulling away now

    Can I reattach it by screwing into the hardie board ?

  18. Seattle Homeowner says

    Hello Charles,
    HardiPlank was installed on our Seattle 1940’s bungalow ten years ago. The contract says,”Field-flash all siding joints and trim joints using mechanical slip sheeting for water intrusion prevention”. I can’t find any information on the web as to what this would mean. The joints now have separated and I can see a white paper below. I don’t think the joints caulked when it was installed. I don’t see any residue. Should I attempt to install metal flashing? I appreciate any advice you can give.

    • Charles Buell says

      I am guessing the white and blue you see is a flashing of sorts—maybe pieces of house wrap. While likely a cosmetic issue, it does not look like the ends of the boards are painted and perhaps painting the joint would make it look better and protect the ends of the boards—depending on how exposed the joints are to the elements. Certainly installation of a metal flashing as described in the OP could also be done. Joints should not be caulked IMHO.

  19. Bothell Home Owner says:

    Hi Charles,
    Thank you for a great article.
    Any tips on what particular tool you would recommend to carefully loosen up the siding? Also, what type of shims to allow for the flashing to be inserted?

    Thank again,

    • Charles Buell says

      A couple of those miniature wonder bars or a couple of door frame shims work fine. Be very careful with any where the corners are nailed—the nail has to be removed prior to any prying. It really does not take much lifting and depending on the amount of exposure it can pretty much be done by hand—especially with a couple of people.

      • Charles,
        Thank you very much! I’ll give it a try.

        • Update

          After much deliberation I decided to redo the entire job with a reputable contractor

          Job is about halfway done and they’re doing a good job

          Flashing at butt joints and between stucco and the bottom board and most important, plywood under the siding !

  20. How would it be possible to add flashing if the corners are face nailed?

    • Charles Buell says

      You would first have to very carefully pull the nails—or you might just have to caulk the joints of the ones that are nailed.

  21. tom cordier says

    You recommended painted 24gauge steel. Would you discourage using painted Aluminum?
    I think premade Aluminum ” shingles” are used on new cement siding installations

  22. Hello Mr. Buell: Thanks for letting me know how wide the flashing pieces are in the photos above. I read one comment from an individual who used 1″ wide metal flashing pieces. Do you recommend wider? Very much appreciate your taking the time to answer and will make a donation!

  23. Hi – I bought a house with 4×8 board and batten panels installed 2 years ago. I just found out it required flashing. Instead they covered the seams with a vertical trim piece. Can these be oulled back and flashing installed? We have a tall house so its 3 levels of panels. The other thought is if the middle panel can be replaced, we can flash above and below, then replace the bottom trim add pvc boards and flash those. Otherwise, I was told it was a complete tear off for 2 year old siding and $52k.

  24. Charles Buell says

    MY guess is the verticals are not a problem, but certainly the horizontals are wrong. You may want to discuss this directly with the siding mfg.

  25. Yea sorry, meant to say covered the horizontal butt joints with horizontal trim. The question is can those be removed and then flashing slid under the horizontal butt joints in the 4×8 panels. One installe told me he would have to rip it all out which I thought was a bit much if flashing can be inserted somehow, or middle panels (1 of 3 layers) replaced and flashed on top and bottom.

  26. These are Hardie panels FYI.

  27. Hi, there! We are buying a home and our inspector said there is no flashing behind the butt seams. We are attempting to determine how much it would cost to repair this. The house is 3200 sq ft, 2 story and both side, excluding the eaves, are hardi siding. Would you have any estimate (I know it is a major guess with so few details) as to how much it might cost to repair? They also didn’t install flashing at the bottom of the siding next to the brick foundation. They caulked that seam. I know that isn’t per manufacturer’s instructions, but as long as we keep that caulked well, is it ok? Other than these few things, the inspector said the house was beautifully done. Are these things that should cause us to walk away from buying? Thank you so much!

    • Charles Buell says

      It is hard to imagine it ever being an issue but the MFG requires flashing currently and has required caulking in the past. As to how easily they can be retrofitted with flashings varies from house to house and how the siding was installed. For example, if the corners are nailed it will be difficult. I would ask the siding mfg what they recommend, and have repairs done by a qualified cement board siding installation contractor.

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