Delaminating hardboard trim  (C) Daniel Friedman Details for Installing Exterior Trim on buildings
Which building details cause rot & hidden damage at exterior trim on walls, doors, windows?

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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 .

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Exterior Building Trim Installation Details to Avoid Rot & Other Trouble

Trim Installation details (C) J Wiley and Sons, S BlissThe first portion of this article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

[Click to enlarge any image]

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 hardboard.

  • 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.

Window leak at exterior sill leads to rot, mold damage, even insect infestation (C) Daniel Friedman

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.

Figure 1-5: corner board details (C) Wiley and Sons - S Bliss

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 absorb water.

Rotted window trim on an exterior wall, New York (C) Daniel Friedman

  • 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).

-- Adapted with permission from Best Practices Guide to Residential Construction.

Exterior Trim Damage Vulnerability Points

Horizontal hardboard trim

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.

Deteriorating hardboard exterior trim (C) Daniel Friedman Trim deterioration beginning at nail penetrations (C) Daniel Friedman

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.

Severe exterior trim rot on a New York Building (C) Daniel Friedman Wood rot at ends of vertical wood trim with inadequate surface clearance (C) Daniel Friedman Wood rot at ends of vertical wood trim with inadequate surface clearance (C) Daniel Friedman

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.

Window trim rot at sill - common damage location (C) Daniel Friedman Trim rot occurs also at horizontal wood trim ends (C) Daniel Friedman

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.

Window trim rot at sill - common damage location (C) Daniel Friedman

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 Cemplank fiber-cement siding and trim
  • James Hardie HardiePlank, Hardipanel, fiber-cement shingles
  • Nichiha Wall Systems Fiber-cement lap siding and simulated shakes, brick, and stone
  • Certainteed Fiber-cement lap, vertical, and shingle sidings, and soffits and trim
  • GAF Weatherside fiber-cement siding

Hardboard Trim

  • ABTco
  • The Collins Companies
  • Georgia-Pacific
  • Masonite Corp.
  • Temple-Inland Forest Products

Polyurethane and PVC Trim Product Sources

  • AZEK Trimboards, Vycom Corporation, 801 Core St., Moosicvk PA 18507, Tel: 570-346-8254 [illustrated above in this article]
  • Cellular Polyvinyl Chloride (PVC) Trim
  • Custom Decorative Mouldings (CDM)
  • Edge Building Products
  • Focal Point Architectural Products
  • Flex Trim Flexible polymer composite moldings
  • Fypon
  • Gossen Corp.
  • Mid-America Building Products
  • Nu-Wood Decorative Millwork
  • Marley Moldings
  • Outwater Plastics Industries, Inc.
  • Ras Industries
  • Resin Art Duraflex flexible moldings

LVL Trim Product Sources

  • Pacific Wood Laminates
  • Clear Lam (textured wood facing) and Socomi Lam (MDO facing)


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EXTERIOR TRIM INSTALLATION DETAILS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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