Gaps at butt joints of HardiePlank siding on an 8-year-old home  (C) Daniel FriedmanFiber Cement Siding Clearance Gap & Caulking Specifications
what are the specs for fiber cement lap siding butt joint & trim joint gaps, caulking & clearances?

     


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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.

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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

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:

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]

Also see "Certainteed WeatherBoards™ Fiber Cement Siding Best Practices for Effective Job Site Management" [5b]

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 Caulk 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 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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