Wall corner siding & trim flashing details:
Tthis article gives specifications for proper flashing & sealing detailing at building siding joints, corners, and window or door openings to provide an attractive and durable job that doesn't leak or rot.
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This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
[Click to enlarge any image]Adapted/paraphrased with permission from Best Practices Guide to Residential Construction. Steven Bliss.
Proper detailing at joints, corners, and openings makes for an attractive and durable job. Key details follow: Lap Joints. The IRC requires that horizontal lap sidings have a minimum one-inch lap joint, or 1/2 inch if the siding is rabbeted.
Weather-proofing Butt Joints in Building Siding
In most climates, it is a good idea to slip a small spline of asphalt-felt paper behind each butt joint in horizontal sidings. Layer the spline so it overlaps the piece of siding below, directing any water out onto the siding (see Figure 1-12 at above left - click to enlarge this or any image at InspectApedia.com).
All end grain in the siding should be sealed after cutting with a water-repellent preservative (WRP) or primer.
Building Siding Flashing & Finish Details for Building Corners
Use overlapping 1x4s or 1x6s at outside corners or use 5/4 stock for a heavier look. Use a felt paper spline, wrapped around the corner and extending 6 inches beyond the corner board, to protect the joints where the siding meets the corner boards (see Figure 1-5 at left).
Use a square length of 5/4 stock at inside corners with a spline underneath. All end grain in the siding should be sealed after cutting with a water-repellent preservative (WRP) or primer.
With the spline, there is no need to caulk the joint. With no caulk, the joint is free to dry out when wet.
Flashing Details for Windows and Doors
If windows and doors are properly protected with splines of felt or flashing tape, there is no need to caulk the joints where siding meets the side casings.
At the top of a door or window, always direct the sheathing wrap over the head flange or cap flashing. Never caulk the joint between the siding and the head casing or the sill, leaving these joints open to drain any trapped water.
Also see WINDOWS & DOORS.
Siding Details At Roof-Wall Joint Step Flashings to Avoid Rot
Stop wood sidings at least 1 inch short of the bottom leg at step flashings and other roof flashings. [Click to enlarge any image]
Otherwise water will wick up into the flashing leading to paint failures and decay (see Figure 1-6 above left). Our wall siding photo (above right) shows the beginning of deteriorating wood clapboards that were installed touching the surface of an abutting lower asphalt shingle roof.
ALso see RAIN SPLASH-UP SIDING DAMAGE.
Vertical and Plywood Siding Butt Joint Details
Avoid horizontal butt joints in vertical siding. Where a butt joint is necessary, use a scarf joint sloped down toward the building’s exterior.
With plywood sidings, use a Z flashing at horizontal joints to shed water to the outside (see Figure 1-13 at above left).
-- Adapted with permission from Best Practices Guide to Residential Construction.
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Frequently Asked Questions (FAQs)
Question: clearance between corner boards and lower story roof
8/10/14 Pat said:
second story has cornerboard that abbuts lower lst story roof. Should cornerboard be cut back so it doesn't touch lower roof? There may be some rot in it.
Yes, Pat, I'd want to see a inch and a half or more between the bottom end of vertical trim and the roof surface below. Particularly if the corner boards are made of wood, the end grain of wood is like a sponge and will easily draw up water into the lower end of the board, leading to rot.
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