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FIBERBOARD SHEATHING, Celotex Homasote & Other
SHEATHING, FOIL FACED - VENTS
SIDING TYPES, INSTALLATION, DEFECTS
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STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIRS, RAILINGS, LANDINGS, RAMPS
STONE SURFACE CLEANING METHODS
STONE VENEER WALLS
STUCCO WALL METHODS & INSTALLATION
SURFACE GRADING, SITE DRAINAGE
THERMAL EXPANSION CRACKS in BRICK
TREES & SHRUBS, TRIM OFF BUILDING
TRIM, EXTERIOR CHOICES, INSTALLATION
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WATER BARRIERS, EXTERIOR BUILDING
WATER ENTRY in BUILDINGS
WINDOWS & DOORS
Fiber cement lap siding installation butt joint gap remedies are described in this article.
Companion articles explain where are clearance gaps required and how big should they be; where should there be no gap (butt joints).
The article series describes a problem of gaps at siding board butt joints that has been observed at some fiber cement lap siding installations.
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Cures for Exposed Gaps in Fiber Cement Building Siding - guidelines adapted from James Hardie, CertainTeed & Others
Separately at SIDING, FIBER CEMENT GAPS we explain why unsightly gaps may appear at the butt joints between individual pieces of fiber cement lap siding on a building.
Unless the builder erroneously and deliberately left such openings, the gaps are usually due to having installed fiber cement siding while it was at a "too high" moisture content.
Here we describe approaches to fixing both the cosmetic complaint "I don't like to see those gaps in the wall siding" as well as a possible leak complaint if the butt joints were not back-flashed.
First, it is important to listen to what the manufacturers of fiber cement lap siding have to say about the expansion/contraction (and buckling or gap formation) of this product:
Watch out: the HZ5® HardiePlank lap siding installation instructions include important details for joint treatment, including warnings against caulking at butt joints. CertainTeed fiber cement siding installation guides contain similar admonitions:
Watch out: the HZ5® HardiePlank lap siding installation instructions include important details for joint treatment, including warnings against caulking at butt joints. CertainTeed fiber cement siding installation guides contain similar admonitions.
Certainteed Corp. provides important advice for building owners troubled by gaps that appear in fiber cement exterior siding after the product has been installed and in service for some time.
The company points out that while these products expand and contract less in response to temperature changes than [some] other siding materials, thermal movement is normal, should have been allowed for at installation, and might produce noticeable gaps at butt joints. Numbers below refer to the page numbers in the product installation guide.
To see a table of the specified gaps for Certainteed's fiber cement building products as well as where caulk should or should not be used, see SIDING, FIBER CEMENT GAP & CAULK SPECS
OPINION: on textured siding products for which a coat of paint won't cover any caulk overspread, (or as the manufacturer may recommend), don't caulk these gaps.
You risk causing more trouble than it's worth, though I can imagine that meticulous work at each joint along with a new paint job may be cosmetically acceptable. You may also be voiding a product warranty.
HardiePlank Fiber Cement Siding Butt Joint Treatments: in 2008 James Hardie withdrew its recommendation for caulking butt joints in HardiePlank lap siding.The company's technical bulletin 9, added that
Watch out: any repair that is not specifically recommended by a product manufacturer may affect the product's warranty coverage. On the other hand, warranty coverage reports have an unfortunate history and often a poor payout rate anyway, so in our OPINION, you may not be risking so much.
Caulking: Repairs such as caulking or flashing-over and re-painting butt joints in siding, may protect the building from water intrusion and further deterioration but may leave cosmetic issues; Caulk applied in siding butt joint gaps should be top quality, rated for the application and materials, applied to a clean dry joint, in weather conditions permitted by the manufacturer; the company recommends a "... high quality, paintable latex caulk ... that complies with ASTM C834, ASTM C920 or better..." 
To improve the appearance of these open gaps at butt joints in lap siding installations where their fiber cement lap siding was used, CertainTeed recommends installing H-covers at the joint locations.
And we agree with CertainTeed that this approach is likely to be more attractive than blobbing or smearing caulk into butt joint openings - a procedure that changes the texture of the siding and its coating performance or re-painting performance and appearance in a way that can be far more ugly, leaving roundish smears at every butt joint so treated.
Open Butt Joint Repairs in Fiber Cement Flashing: using slip-on H-flashing or slip-in Back Flashing
Our photo (left) is a close-up of one of the bad fiber-cement siding installation job we describe at SIDING, FIBER CEMENT GAPS. Take a close look at our picture - that blue paper you see in the gap is housewrap.
There is no back flashing installed in this siding job, which means that blowing rain enters the wall at every butt joint gap. We are depending entirely on the housewrap to keep water (and later insects and rot) out of the building walls. Do you figure that the same fellows who installed this siding took great care to install the housewrap correctly and without leak points? Doubtful.
Several fiber cement manufacturers recommend the use of sliding H-flashing at fiber cement butt joints as a siding butt-joint gap repair. If the building siding is otherwise in good condition this option may be appealing.
Where to buy exterior siding butt joint back-flashing & H-covers for fiberboard siding products
Separately in "Fiber Cement Siding Best Practices for Effective Job Site Management" [5b] the company recommends
CertainTeed's advice on caulking siding butt joints and edges continues:
Separately in "Fiber Cement Siding Best Practices for Effective Job Site Management" [5b] the company recommends 100% acrylic latex primer or paint to seal cut ends of all siding boards.
Watch out: as with our warning about H-covers for butt joint gaps, a caulking or sealant product will most likely weather and change color at a different rate and to a different hue from those of the paint or coating on the siding boards. CertainTeed continues:
Should we remove & replace the siding? installing it correctly this time. The contractor at the job we investigated considered this approach, though there would be some lost material due to damage during removal. There the siding had so many installation errors, missing flashing, crooked, trim rot around windows and doors, building leaks, that a patch job was in our opinion and that of the owner and contractor, likely to lead to a lifetime of aggravation for the building owners.
(The owner later elected to toss the old siding and purchase new Hardieplank fiber cement siding - a step that was not without its own difficulties as we explain at SIDING, FIBER CEMENT MOISTURE LEVELS )
This is a very costly approach but makes sense in some installations such as the fiasco of a fiber-cement siding installation job shown at SIDING, FIBER CEMENT GAPS.
Details about when, why, & how to remove fiber cement siding from a building are provided at SIDING, FIBER CEMENT REMOVAL
Manufacturers point out that keeping water out of siding and avoiding too-close exposure to ground surfaces, patios, decks (where rain splash-up adds wear) and shrubs too close to the building are all sources of moisture trouble down the road.
Details about avoiding water damage to building siding are at:
Continue reading at SIDING, FIBER CEMENT GAP & CAULK SPECS or select a topic from the More Reading links shown below.
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