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Gaps at butt joints of HardiePlank siding on an 8-year-old home (C) Daniel FriedmanFiber Cement Siding Clearance Gap & Caulking Specifications

  • POST a QUESTION or COMMENT about the cause & cure of butt joint gaps in fiber-cement siding & about fiber cement siding installation

Fiber cement siding gap & caulk requirements, where not to caulk:

Where are clearance gaps required and how big should they be; where should there be no gap (butt joints). Where should caulk be applied on a fiber cement siding job and where should it be omitted? Our siding close-up photo at page top illustrates what may be another clue in the history of a failed installation. We can see a hard-dried layer of caulk that was a bit under 1/8" wide inside of the now wide-open butt joint between two siding boards.

This article series includes field reports of fiber cement board lap siding butt joint gap problems, fiber cement moisture problems, and other fiber cement lap siding installation or in-service defects, their diagnosis & repair. We include diagnostic inspection photos and text explain how to recognize, diagnose, and cure this problem.

We organize & include fiber cement siding gap repair advice from the manufacturer as well as from builders and building inspectors. We provide a table of all fiber cement siding installation clearance or gap specifications, caulking requirements & storage or moisture requirements.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Fiber Cement Siding Gap & Clearance & Caulking Specifications

The following information is adapted & expanded a bit from Certainteed Corporations installation instructions for the company's fiber cement siding products, as this is the most comprehensive guide we could find for caulking & clearance instructions when installing fiber cement siding. The table below was constructed by collecting specifications spread through longer documents provided by fiber cement siding manufacturers.

Fiber Cement Siding Gap Specifications Adapted from CertainTeed
Clearance or Gap Location between fiber cement siding & other building components1 Clearance Gap Specification Caulk Specification Comment / Installation Instructions page#
Blind Fastening  1/8" from all trim before fastening Always Make sure the panel overlaps 1-1/4", p. 33, 55ff
Butt Joints between horizontal siding boards Never leave a gap at a butt joint Never2 p.5, p. 36, p. 37,
Notes 1 & 2 below [5a]
All butt ends/joints should be installed in moderate contact with one another. Note 2 [5b]
Durable, non-corrosive back flashing that is non-reactive with fiber cement is required for use at all butt
joints. Flashing must span 3" to the left and right of the butt joint and 1" over the previous course of siding.		 Note 2 [5b]
Concrete walks, patios 1" vertical clearance   p. 55 ff
Door trim 1/8" gap between siding and trim surrounding windows and doors

Always if trim boards

Never if J-channel

 p. 4, 42
Dissimilar materials 1/8" There should be a gap between dissimilar materials. Fiber cement should be separated from other materials such as brick, stone, wood, and metal. Always

p. 25, 42
Face Fastening 1/8" from all trim or other materials before fastening Always Make sure the panel overlaps 1-1/4", allow for structural movement, p. 33
See J-channel
Gaps at butt joints that open after installation  Install H-covers Note 8 p. 55ff
Ground surface, finished grade 6-inches   p. 55 ff
Horizontal zee-flashing at siding bottom or at horizontal band trim boards 1/4" clearance between the bottom of WeatherBoards Fiber Cement products and the horizontal flashing. Caulk should not be used at this location. - Never

p. 29
J-channel, F channel etc. at windows or trim   Never

Re-seal cut edges with primer or paint before insertion; flash all corners; p. 35, 41
Note 3
Note 4
See also J-CHANNEL ERRORS on FIBER CEMENT SIDING
Roof surfaces 1-inch  

p. 55 ff
Shakes Never leave a gap at a butt joint Never p. 53, 54, 55ff
Note 6

1/8" gap between trim & first & last panels

  

p. 46
1/4" keygaps between shake component sections   p. 41
Note 5
1 1/2" keyway spacing   P. 43
Trim, Vertical 1/8" gap between siding ends & vertical trim, for caulking Always Refers to trim boards, not J Channel
p. 40, 41, 42
Note 4.
Wall surface distance to plants & shrubs no contact, "... should not reach"   We recommend 2ft or in areas of wildfire hazards 5 ft. - Ed.
Window trim 1/8" gap between siding and trim surrounding windows and doors

Always if trim boards

Never if J-channel

 p. 4, 42

Notes to the table above

1. Contact the CertainTeed corporation or download the company's Fiber Cement Siding product installation manual (www.certainteed.com/resources/fc017.pdf Certainteed website: certainteed.com, Certainteed consumer Help Line: 800-782-8777 [5a]

FC Siding InstallationSpecifications

2. The specification to omit caulk at siding butt joints is for new installations and is discussed in detail by the manufacturer. "Do not caulk fiber cement butt joints/edges. Applying caulk to the butt joint is ineffective because the gap is not large enough to accommodate the sealant.

Also, on prefinished products, caulk may leave an unsightly looking finish. Spreading or feathering the sealant into a thin film will create a noticeably different appearance, and it can remove the needed thickness required to withstand UV exposure and joint movement." - p. 37

3. Face Fastening: do not caulk between the siding and the channel, either before or after siding installation as this may restrict water movement around the opening. Make sure all corners are properly flashed.

4. Vertical trim gap & caulking: Leave a 1/8" gap between the siding and the trim or other materials to allow for structural movement.
Always caulk between the siding and the trim.

Do not caulk between siding and any built-in receiving channels located at or around windows.

Always prime or paint any cut edges that are inserted into a window J-channel. - p. 41

5. Shake installation:

this 1/4" gap specification is for individual shake style siding, aka "random square siding" not other materials described in the installation manual.

6. Shake installation:

Do not caulk fiber cement siding butt joints/edges. It is never acceptable to leave a gap of any size at a butt end/joint. Butt end/joints should be installed with factory sealed or factory-prefinished ends butted together in moderate contact.

CertainTeed recommends (but does not require) the use of a butt end/joint flashing made of a durable, non-corrosive material that is compatible with fiber cement siding (e.g. #15 felt, trim coil, Bear Skin joint flashing). Check if local code requires backflashing at the butt joint.

7. Shake installation:

Do not caulk. Applying caulk to the butt joint is ineffective because the gap is not large enough to accommodate the sealant. Also, on prefinished products, caulk may leave an unsightly looking finish.

Spreading or feathering the sealant into a thin film will create a noticeably different appearance, and it can remove the needed thickness required to withstand UV exposure and joint movement. - p. 54

8. See FIBER CEMENT SIDING RETROFIT-REPAIR TIPS for butt joint or other gaps that may open in service.

9. To Identify Fiber Cement Siding as CertainTeed, Hardieplank or Brand "X" Fiber Cement Siding

see SIDING, FIBER CEMENT IDENTIFICATION.

To identify fiber cement siding products as CertainTeed, Hardieplank or Brand "X" Fiber Cement Siding please

see SIDING, FIBER CEMENT IDENTIFICATION.

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-07-01 - by (mod) -

Todd

Thank you for your comment. In fact we cite all of the manufacturers of this type of siding and their specifications that we can find, including JamesHardie - the company you tout.

Unfortunately in installing and evaluating the condition of various FC siding products (installed in a northern state) we found this type of shrinkage Gap across pretty much every product.

On 2020-07-01 - by (mod) -

Re-posting


Todd said:
This page is completely out of date and irrelevant. Your main source of information is from CertainTeed Fiber Cement, which doesn't even exist any more. It was dismantled and sold off to Elementia following the loss of a $103MM class-action law suit and resulting bankruptcy in 2014 (http://www.certainteedfibercementsettlement.com/). The class-action suit was for excessive product failures. They were using fly ash in their fiber cement, which was causing brittleness and dimensional instability (shrinking and expanding as seen in your photos)... This is not the same product as James Hardie fiber cement.



On 2019-02-19 - by (mod) -

Sorry thnx,

But we don't sell anything. To protect reader trust that there are no conflicts of interest this website has no Financial or other relationships with any product or service that we may discuss.

On 2019-02-18 by thnx

need 2 c where we can purchase eoof tiles...we in Fl?
Firedette

On 2018-05-29 - by (mod) -

Bill,

Please see our article SIDING, FIBER CEMENT REMOVAL inspectapedia.com/exterior/Fiber_Cement_Siding_Removal.php

I appreciate the situation but I'd prefer to pull off enough siding to get at what you want without cutting it.

Trying to saw a cut through the FC siding in-place, without lifting and removing boards, regardless of whether you are using a small circular saw or tile saw (that can cut a nice clean straight line) or a reciprocating saw (that I guarantee you is going to cut a horrible sloppy wavy cut line through the boards) you almost certainly

1. will not be able to cut the upper part of the board to be removed and that is underneath the next FC board course above

2. will not be able to cut the bottom edge of the siding boards without nicking the board in the course below.

3. will have shortened the boards by the thickness of your saw kerf. While you'd prime the ends that were sawn, and would back-flash such a cut, you may end up with an unsightly "caulked" butt joint that's generally not recommended and will not look so nice.

In my OPINION sloppy end cuts in a fiber cement sided wall are likely to look really ugly unless the cut can be moved to abut vertical trim where it will be covered with caulk.

Watch out: some articles say "use a pry bar" to loosen the bottom edge of siding but you need to take care not to damage the siding board below the one you're prying. Use a wood shingle or similar buffer when loosening siding if you're not starting with the bottom course. Sometimes I use a hacksaw blade to cut off nails that are difficult to remove.

Remember that for most siding installations nearly all of the boards are nailed to the wall through the upper edge of the board. If you're not careful with the pry-bar, or if you use a really fat prybar instead of a thinner model, you risk breaking the board during the prying operation. I use a thin wide flat bar and I drive it up under the board I'm removing, prying gently. When I find a nail I tap the prybar up so that its notch is (hopefully) around the nail, then I pry gently loosening it but DO NOT pry up so much that you break the board above.

You need to loosen the entire board and often have to cut its nails before you can drop it down and out from the wall.

Take care using a reciprocating saw to cut nails - recommended by some fellows who I think not actually tried what they recommend. It's really easy for the reciprocating saw to damage the siding boards below.

You should not have to remove ALL of the siding ALL of the way across the wall, but depending on where the existing butt joints are, you'll need to remove a lot.

On 2018-05-29 by Bill Engle

I have a situation with a Hardie sided wall at the house, where water has penetrated the wall at or below an upstairs window and into a 10 foot wide 3-bay window 5or 6 feet below. The wall is 30 feet long, so I want to cut out off-set sections to expose the siding above the bay window up to the window above. The question: is there a recommended method to cut in butt joints on Hardie while on the wall straight enough to fit in the new pieces, or does all the siding have to be removed all the way across? Can not find anything anywhere to assist with this problem

On 2017-10-16 by Richard Philley

What will cut the hardie siding in the center for recepticals

On 2017-09-07 by tony

hardee fibre cement lap siding installed 2 years ago now has butt joint gap from 1/8- 1/4 . Any suggestions how to repair. Is moisture be an issue? between the gaps


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