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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.
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
Comment / Installation Instructions page#
1/8" from all trim before fastening
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
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
1/8" gap between siding and trim surrounding windows and doors
Always if trim boards
Never if J-channel
p. 4, 42
should be a gap between dissimilar materials.
Fiber cement should be separated from other
materials such as brick, stone, wood, and metal.
p. 25, 42
1/8" from all trim or other materials before fastening
Make sure the panel overlaps 1-1/4", allow for structural movement, p. 33
Gaps at butt joints that open after installation
Ground surface, finished grade
p. 55 ff
Horizontal zee-flashing at siding bottom or at horizontal band trim boards
between the bottom of
Cement products and
the horizontal flashing.
Caulk should not be
used at this location. -
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
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Questions & answers or comments about the cause & cure of butt joint gaps in fiber-cement siding.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
 " Anyone else have HardiePlank siding on their house?", Early Retirement.org online blog, retrieved 1/14/2013, original source: http://www.early-retirement.org/forums/f27/anyone-else-have- HardiePlank-siding-on-their-house-19475.html
 - DIY Chatroom, retrieved 1/17/2013, original source http://www.diychatroom.com/f2/ certainteed-weatherboard-vs-HardiePlank-4638/
 Certainteed Weatherboard fiber cement siding and trim products - see certainteed.com/ or see certainteed.com/resources/sidingandtrimspecsheet.pdf
[5a] "CertainTeed WeatherBoards™ Fiber Cement Siding Installation Manual", CertainTeed Corporation, 2012, CertainTeed Corporation, PO Box 880, Valley Forge PA 19482, Professional Help Line: 800-233-8990, Consumer Help Line: 800-782-8777, Website: Certainteed.com www.certainteed.com/resources/fc017.pdf retrieved 4/11/2013, [copy on file as Certainteed_Fiber_Cement_Siding_Install_fc017.pdf ]
[5b] "Certainteed WeatherBoards™ Fiber Cement Siding Best Practices for Effective Job Site Management", CertainTeed Corporation, retrieved 4/15/13 original source: www.certainteed.com Copy on file as Certainteed_FC_Handling Brochure-Contractor_FC063.pdf
[5c] Personal communication, M. Smith, CertainTeed Corp, Customer Service, to D Friedman 4/16/2013
 "Moisture Control in buildings: Putting Building Science in Green Building," Alex Wilson, Environmental Building News, Vol. 12. No. 5. [Good tutorial, "Moisture 101" outlining the physics of moisture movement in buildings and a good but incomplete list of general suggestions for moisture control - inadequate attention given to exterior conditions such as roof and surface drainage defects which are among the most-common sources of building moisture and water entry.--DJF]
 J. Tibbets, "Green Houses", NCBI, retrieved 1/17/2013, original source: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1469482/
 Gleeson et al, "Fiber Cement :Building Materials with Low Density Additives", U.S. Patent 8,182,606, B2 5/22/2012, [adding low volumes of volcanic ash & hollow ceramic microspheres to cellulose fiber-reinforced building materials.
 Thanks to reader Marie Carr for James HardiePlank siding photograph and case history information.
 JamesHardie HardiePlank® Lap Siding information can be found at the company's web page on this material:
 "30-Year Limited Warranty
HardiePlank® HZ5® Lap Siding, HardiePanel® HZ5® Vertical Siding,
HardieShingle® HZ5® Siding, HardieSoffit® HZ5® Panels", 1-800-9-HARDIE
10901 Elm Avenue Fontana, CA 92337, retrieved 1/18/2013, original source: http://www.jameshardie.com/pdf/warranty/hz5.pdf [copy on file as Hardieplank_Warranty_hz5.pdf]
 James Hardie Building Products, James Hardie CustomerLink™ Service Centre, 10 Colquhoun Street, Rosehill NSW 2142, Tel: 13 1103, Outside Australia 61 2 8837 4709,
Fax: 1 800 818 819. Hardie has operations in Australia, Asia, Canada, Europe, New Zealand, United States, and the Philippines. The company is a significant producer of fiber cement siding and backerboard. Email: info@JamesHardie.com and Website http://www.jameshardie.com/
[12b] "Technical Bulletin #9, Joint Flashing at Field Butt Joints", JamesHardie, retrieved 4/15/2013, original source: www.jameshardie.com, copy on file as James_Hardie_9-joint-flashing.pdf - quoting: This Technical Bulletin is an explanation supporting the
announcement made by James Hardie on September 8th, 2008
withdrawing its recommendation on the use of caulk at field butt
joints for HardiePlank® lap siding.
 Instructions for application of HardiePlank lap siding can be found at http://www.jameshardie.com/homeowner/products_siding_hardieplankLapSiding.py?openTab=jsnavLink4
 "Homeowners Care and Maintenance Tips", 1-800-9-HARDIE
10901 Elm Avenue Fontana, CA 92337, included with product warranty information, retrieved 1/18/2013, original source: http://www.jameshardie.com/pdf/warranty/hz5.pdf [copy on file as Hardieplank_Warranty_hz5.pdf]
 "HardiePlank HZ5 Lap Siding Installation Requirements - Primed & Colorplus® Products",JamesHardie, November 2012, 1-800-9-HARDIE,
10901 Elm Avenue Fontana, CA 92337, retrieved 1/18/2013, original source: http://www.jameshardie.com/pdf/install/hardieplank-hz5.pdf, [copy on file as Hardieplank-hz5_Install.pdf]
 Technical Bulletin #9,
Joint Flashing at Field Butt Joints, James Hardie corporation, retrieved 1/19/2013, original source: http://www.jameshardie.com/pdf/technical-bulletins/9-joint-flashing.pdf
 Technical Bulletin #17,
Fastening Tips for HardiePlank® Lap Siding, James Hardie Corporation, retrieved 1/19/2013, original source: http://www.jameshardie.com/pdf/technical-bulletins/17 -fastening-tips-for%20-plank.pdf [copy on file]
 Technical Bulletin #8,
Expansion Characteristics of James Hardie® Siding Products, James Hardie Corporation, retrieved 1/19/2013, original source: http://www.jameshardie.com/pdf/technical-bulletins/8-expansion-characteristics.pdf [copy on file]
 Eric Galow, Galow Homes, Lagrangeville, NY. Mr. Galow can be reached by email: email@example.com or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions. Personal communication 3/20/2013.
 Johnston, Colin D., Fiber-reinforced cements and concretes. Vol. 3., Taylor & Francis, (CRC Press) 2000. ISBN-10: 9056996940
ISBN-13: 978-9056996949, "This book summarizes and simplifies the results of a considerable body of research and practical experience with a wide range of fiber-reinforced cementitious composites."
 James Hardieplank, personal communication with Daniel Friedman, 6/6/2013. We called James Hardie to ask for guidance in determining just how much moisture was "too much" in Hardieplank siding, after observing that siding at a job arrived visibly wet from the factory even though still inside its original sealed plastic wrapping. We called James Hardie's technical support line where a courteous representative told us he would refer us to Hardie's expert Benjaman Batres. Our call to Mr. Batres was returned by Stephanie (declined to give last name) from James Hardie's customer warranty service department. Stephanie informed us that there are no moisture numbers, that it is not possible to measure moisture in fiber cement siding (utter nonsense!) and repeatedly advised "Just read our instructions" or "So file a claim". Don't count on much help from James Hardie's customer warranty department on siding shrinkage or butt joint gap concerns. Gaps at Hardieplank siding butt joints continue to appear in the Hardieplank installation.
Paint Handbook: testing, selection, application, troubleshooting, surface preparation, etc., Guy E. Weismantel, Ed., McGraw Hill Book Company, 1981, ISBN-10: 0070690618, ISBN-13: 978-0070690615, [Excellent but a bit obsolete paint theory and practice, also a bit light on field investigation methods, out of print, available used-DF] How to select and apply the right paint or coating for any surface. The first major reference to help you choose the correct paint or other finish to do the job best on a particular surface exposed to a particular environment. Experts in the field give full advice on testing surface preparation, application, corrosion prevention, and troubleshooting. The handbook covers wood, metal, composites, and masonry, as well as marine applications and roof coatings. A ``must'' working tool for contractors, architects, engineers, specification writers, and paint dealers.
Paint and Surface Coatings, Theory and Practice, R. Lambourne & T.A. Strivens, Ed., Woodhead Publishing Ltd., William Andrew Publishing, 1999 ISBN 1-85573-348 X & 1-884207-73-1 [This is perhaps the leading reference on modern paints and coatings, but is a difficult text to obtain, and is a bit short on field investigation methods - DF]
Provides a comprehensive reference source for all those in the paint industry, paint manufacturers and raw materials suppliers, undergraduate and postgraduate students, and industrial paint users. R. Lambourne was in the Research Department at ICI Paints Division and the Industrial Colloid Advisory Group, Birstol University, UK.
Seeing Through Paintings, Physical Examination in Art Historical Studies, Andrea Kirsh, Rustin S. Levenson, Materials in Fine Arts, 2000 ISBN 99-051835 [ forensic science, technical reference, focused on art works - DF]
Sealants, Durability of Building Sealants (RILEM Proceedings), J.C. Beech, A.T. Wolf, Spon Press; illustrated edition (1995), ISBN-10: 0419210709, ISBN-13: 978-0419210702 This book presents the papers given at the RILEM Seminar held at the Building Research Establishment, Garston, UK in October 1994. The book provides an opportunity for researchers to review up-to-date progress towards the achievement of the objectives of the standardisation of laboratory techniques of sealants in the variety of service conditions to which they are exposed.
Staining, Prevention of Premature Staining in New buildings, Phil Parnham, Taylor & Francis; 1996, ISBN-10: 0419171304, ISBN-13: 978-0419171300 The appearance of ugly staining early in a buildings life, ruins an otherwise pleasing appearance, tarnishes the image of the owners and gives rise to costly refurbishment works. In this book Phil Parnham raises a number of questions that should be considered whenever a new building is being designed or built. These are: * why has staining become so prominent; * what causes premature staining; which parts of new buildings are likely to be affected; * how can it be avoided? By using a number of highly illustrated case studies, the author answers these questions and ends by suggesting measures that should be taken by all design and construction professionals to prevent premature staining.
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
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