Seattle Home Inspector's Blog


How to flash siding butt joints

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 caulked joint in siding

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 your 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.


Butt joint flashing


Note that the flashing does not go up high enough to hit the underlying nail but is still high enough to 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.


Charles Buell, Real Estate Inspections in Seattle

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Comment balloon 11 commentsCharles Buell • October 25 2015 01:00PM


Thank you  Cement board is the prefered siding material of many builders In  \my area  I wondered why they were moving from flashingto  caulking.  THANk YOU for the post!

Posted by Nancy Robinson Ranked #6 in MI, #11 Century21 Realtor in the US, #16 in the World (Century 21 Town and Country) almost 5 years ago

Charlie, This is awesome. Perfect example of how to retrofit a but joint flashing.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) almost 5 years ago

Hi Charles Buell I have never applied cement board siding but I have done a couple of structures with log siding  and I think this would be perfect for that type of construction too.  I do have two questions.  How is the flashing attached  -  nails, staples or ?  And where do you find this product?

Posted by Gary Coles (International Referrals), Latin America Real Estate (Venture Realty International) almost 5 years ago

With that title line, I simply had to read the entire blog to make certain it wasn't pornographic or illegal. . .tsk, tsk!

Posted by Myrl Jeffcoat, Greater Sacramento Real Estate Agent (GreatWest Realty) almost 5 years ago

That is best practice for sure, but that adds a ton of time for the installer to make a bunch of those by hand... my bet is that it will get "overlooked" unless someone checks as they go!

Posted by Fred Hernden, CMI, Albuquerque area Master Inspector (Superior Home Inspections - Greater Albuquerque Area) almost 5 years ago

Charles Buell This is an excellent post and much appreciated by me. I got a kick out of Myrl Jeffcoat 's comment!

Posted by Sandy Padula and Norm Padula, JD, GRI, Presence, Persistence & Perseverance (HomeSmart Realty West & Lend Smart Mortgage, Llc.) almost 5 years ago

Sorry I missed this Charlie!

When I do a JH inspection I put my knife blade into the joint to see which flash they used.  JH now allows a plastic flash, but metal is certainly the better.  Why they would allow plastic is beyond me, except cost.

Caulking is a no no, and a sure sign the installers didn't know what they were doing.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) almost 5 years ago

P.s.  When I went to Indianapolis years ago to a Firestone continuing ed week, there were many home inspectors there.  We all knew each other.  And pretty well, from on line stuff.

Coming out during a break they were all gathered in one area.  "Hey Jay!  C'mon over!"  I got into the crowd and they were measuring butt crack temperatures with a thermal camera.  "Pull your pants down Jay.  We're seeing who has the hottest ass!"

No kidding.  No, I did not pull my pants down.  Dale Duffy had the hottest ass.

I mean temperature wise.  Otherwise it was pretty ugly.

Boys and their toys...

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) almost 5 years ago

Go ahead, don't believe me.  True story!

Oh, Croakster had the coolest ass.


Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) almost 5 years ago

I’ve just had Hardieboard lap siding installed, and I’m worried.There is no butt flashing on a single joint.Contractor informed me it is not necessary because he used marine caulking,but I can not visibly see any. What is the flashing material that you show.Any direction would be appreciated..... Thank You

Posted by Richard about 2 years ago

The flashings in the repair is metal flashing.  As far as I am aware, all climate zone intallation instructions call for flashings behind the butt joints.  Caulk used to be acceptable but no longer.  Go on line and find the Hardi installation instructions for your climate zone.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) about 2 years ago