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Exterior building trim installation guide, installation methods, and details to avoid trim damage, rot, or loss.
This article provides examples of exterior trim rot, delamination, or other damage traced to choice of materials, nailing methods, or omission of adequate flashing or sealant. Our page top photograph shows delaminating hardboard gable-end trim at the exterior of a building .
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Exterior Building Trim Installation Details to Avoid Rot & Other Trouble
The best exterior trim details are designed to keep water
out but to provide easy drainage for any water that penetrates
the exterior. This is particularly important when
using trim materials that are vulnerable to decay or moisture
damage, such as non decay-resistant softwoods or
Caulking trim joints with sealants (CAULKS & SEALANTS, EXTERIOR) is a double edged
sword, since all caulk joints will eventually fail, and
when they do, the remaining sealant will tend to keep the
joint from drying.
Our photo (below left) illustrates Steve Bliss's point: the gap that has opened at the window sill and trim provides a ready path for rain to enter the building wall cavity.
Regular inspection and repair / replacement of failing sealant as well as use of a good quality sealant are needed in spots like this to avoid more costly building damage from wall leaks at windows.
Exterior building trim that is subject to frequent wettings from the
weather, such as corner boards, water tables, or wrapped
porch posts or balusters, is a good candidate for rain-screen
installation, where a ventilation and drainage space is left
behind the trim (Figure 1-36).
Make sure to leave 1/8 to 1/4-inch free at the bottom edge
of the trim to drain to daylight. [Note: for vertical trim ends approaching a horizontal or sloped surface, leaving more space, 1" or 1 1/2" is better - Editor.]
To create the drainage
space, either use shims (do not block the drainage path) or
synthetic drainage materials, such as Benjamin Obdyke’s
Home Slicker®. Foundation drainage materials can also
work. If shims are used, add metal screening or drainage
matting at top and bottom to block insect entry.
Regardless of the specific detail, the following principles
will help create long-lasting exterior trim on any building:
Wide roof overhangs, 8 inches minimum, at rake and
eaves keeps water away from the side of the house,
preserving siding and trim.
Avoid wide horizontal wood surfaces exposed to
water. Slope for drainage or cap with metal flashing.
Cut drip groove under edge to shed water.
Slope top edges of exterior railings and horizontal trim
boards, such as water tables, to shed water.
Avoid exposed end-grain facing upward in vertical trim
boards. Heavily prime all end grain and exposed edges.
Avoid exterior miter joints, which tend to open and
Use Z- or drip-cap flashing at horizontal joints, such
as above windows or where corner boards meet the
water table. Leave 1/8-inch clear above the flashing, and
do not caulk the horizontal joint. (CAULKS & SEALANTS, EXTERIOR)
In the window trim photograph above my [DF] key points to rotted brickmold trim around an exterior window. The rot and leaks at this window were temporarily repaired using a wood filler, sealant, and paint, but it's the combination of rain splash in this location and omission of a Z-flashing over the horizontal window trim that accelerate the trim rot in this location.
At FLASHING SIDING DETAILS we emphasize that improper detailing of the ends of J-flashing on vinyl or aluminum sided buildings also leads to window trim rot and window leaks that often originate at the window head trim.
Avoid caulk joints. Instead, flash well behind joints
and leave a gap of
1/8 inch for ventilation. Where caulking
must be used, apply a properly shaped caulk bead
(see “Joint Design,” below).
Our photo below illustrates soft, deteriorating wood fiber or hardboard exterior trim. In my [DF] experience, hardboard siding and trim don't fare well exposed to the exterior environment and are particularly vulnerable at cuts or surface penetrations. The hardboard trim in my photo was installed as the water table, a band of horizontal trim at the base of exterior siding around a building.
You'll notice that where the nails penetrated the skin of this hardboard trim the deterioration was most rapid. The absence of zee-flashing between the siding bottom and the horizontal surface of the trim board's top also contribute to trim deterioration: water runs behind and along the trim surface. My second photo (above right) shows the softening and beginning of deterioration where the hardboard siding nail penetrates its surface.
Vertical Trim Clearance from Roofs, Walks, or other Horizontal Surfaces
As our photographs illustrate just below, the exposed end grain of wood trim is particularly vulnerable to water absorption and subsequent rot as well as insect damage. At below left the building exterior has been ignored so long that not only is there extensive rot to vertical and horizontal trim, we expected (and found) leaks into the walls and wall structure damage from rot and insects.
I [DF] like to see an inch or inch and a half of clearance between the end of vertical wood trim and the nearest horizontal surface. Where that large gap is not aesthetically acceptable, leaving 1/8" to 1/4" clearance may suffice if the wood ends were sealed and if proper flashing was used.
Where no clearance space is feasible, such as at vertical window trim abutting a window sill, it is important to seal the end and all other sides of the wood trim before nailing it in place, followed by painting or staining, followed by use of a compatible sealant (caulk) around the joint of wood trim to the horizontal surface.
Below you can see significant rot at vertical wood trim along the side of a window at its sill abutment. Home inspectors know that if they were going to select one location to inspect around a window for trim damage, this would be a good one. In my OPINION, if my pen can penetrate the trim, it was badly damaged before I touched it - this is not "destructive" probing.
My second window trim rot photo (above right and probed at below left) shows that even if the vertical trim was given good clearance above a roof top or other horizontal surface, a horizontal abutting wood trim end grain will also pick up water and subsequently rot unless the connection is properly prepared with sealant and then kept caulked and painted.
Special Exterior Trim Installation Details for LVL, Fiber Cement, PVC, Plastic & Other Exterior Trim Products
Additional Exterior Trim Installation Materials Choices & Special Installation
Requirements are detailed at TRIM, EXTERIOR CHOICES, INSTALLATION where we discuss best construction practices in the choice, installation, painting, and maintenance of exterior trim on buildings.
We discuss solid wood trim, finger-jointed wood trim, hardboard trim, LVL trim, fiber cement building trim, polymer trim products, and cellular PVC building trim products and details.
Where to Buy Building Exterior Trim Products
Fiber-Cement Siding and Trim Suppliers
Cemplank fiber-cement siding and trim
HardiePlank, Hardipanel, fiber-cement shingles
Nichiha Wall Systems
Fiber-cement lap siding and simulated shakes, brick, and
Fiber-cement lap, vertical, and shingle sidings, and soffits
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Questions & answers or comments about how to choose, install, maintain or repair exterior building trim.
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.
Webb, Craig, "What's In this Stuff? Composit and PVC Decking", Pro Sales Magazine, 6/10/2011, retrieved 8/10/14, original source: http://www.compositology.com/news/industry_news/pro-sales-what-is-composite/
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
Azek Building Products, "Installation Guidelines for AZEK Trim and AZEK Moulding", Azek Building Products, 801 Corey Street,
Scranton, PA 18504, Tel: 877-ASK-AZEK (877-275-2935)
 "PVC trim holds paint well, resists rot, and looks just like wood", Jim Blahut, The Journal of Light Construction, September 2002. [copy on file].
Allen, Edward and Joseph Iano. Fundamentals of Building Construction: Fourth Edition. Hoboken: John Wiley & Sons, 2004. pg. 91 [LVL lumber]
APA - The Engineered Wood Association, 7011 So. 19th St., PO Box 11700, Tacoma WA 98411-0700, Tel: 253-565-7265. APA provides an HDO/MDO Plywood Product Guide that offers details about these products., provides an HDO/MDO Plywood Product Guide that offers details about these products. Product support help desk: 253-620-7400. Email the APA at email@example.com. Web search 09/13/2010, original source: http://www.pacificwoodlaminates.com/img/PDFs/PlywoodGuide.pdf
Azek Technical Bulletin - Paint, November 2007, explains the use of Sherwin-Williams VinylSafe™ paints, Pittsburgh Paints - PPG Architectural Coatings, Duron Paints, ICI Paints, on AZEK trim products. web search 11/10/2010, original source:
Georgia-Pacific Building Products, 133 Peachtree Street NE, Atlanta, GA 30303. Telephone:
(404) 652-4000 , Wood Products: 800-284-5347. Gypsum Products: 800-255-6119. LVL lumber: GP-LAM, web search 09/14/2010, original source: http://www.gp.com/build/product.aspx?pid=1392
Laminated Veneer Lumber, Overview of the Product, Manufacturing, Market, Department of Forest Products Marketing, wood-Based Panels Technology, Finland [PDF] web search 09/14/2010, original source: http://www.hochstrate.de/micha/finnland/reports/replvl.html
John Rudy, Advantage Home Inspections, Flemington N.J. 08822 home inspector, 908-806- 6364, Home, Radon & Termite Inspections, Central & Parts of North New Jersey, email: firstname.lastname@example.org
Malco® Products siding tools are available from that company, including the SideSwiper II SRT2 discussed at Malco's website. Websearch 09/07/2010 http://malcoproducts.com/product/roofing-siding-gutter/siding-vinyl/siding-tools-vinyl/sideswiper-ii. Malco also produces other vinyl siding repair tools such as aprons, awls, hole punches, saw blades, and tools for for fiber cement products including power-assisted cutters
Mark Morsching, Everflashing, Tel: 800-550-1667, Email: email@example.com. The Everflashing product comes in G-185 and Stainless Steel and is intended for use with treated lumber with copper in it. Everflashing produces a variety of specialty flashing products including flashings for use with decks at deck ledgers and deck perimeters.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones