Details that Keep Walls Watertight

Careful detailing of house wrap and flashing connections will reduce maintenance and lengthen the working life of structural components in wood-frame structures.

by Paul Fisette – © 2009

I vividly remember the job where I learned that even properly installed siding isn’t waterproof. It was an expensive home I had contracted during the early days of my career: A large Gambrel on the peak of the highest hill in town. Start-to-finish the job was a plum. The clients were overjoyed with the finished product and moved in ahead of the scheduled closing date. But 6 months after they moved in, a wind-driven rain converted my plum into a prune. A late-night phone call explained in detail how many drips-per-minute were falling from the head jamb of the east-facing double-mullion window. A next-day fix was promised.

Well, fixing leaks can be a tricky proposition. And this one ran true to form. This leak occurred once a year for 3 consecutive years. Each time there was a heavy east wind driving the rain directly against the window and wall surrounding it. During this 3-year period, I sprayed water, applied caulk and tightened every joint and flashed connection on the east wall of the home – with no luck. Finally, out of desperation, I removed every course of bevel siding from the east wall. Then the obvious hit me.

During construction, this window was back-ordered, so I had decided to pre-wrap the entire wall with 15# felt and pop in the unit when it arrived. Later, as habit dictated, I installed the window z-flashing above the window head casing and sided the wall. Here was the cause of the leak. I had attached the window flashing against the felt paper rather than tucking it under the paper. Heavy east winds pressurized the wall and guided rain water through the butt joints of the siding above the window. The water ran down the face of the felt paper, behind the window flashing and head casing. Lesson learned: properly-installed siding isn’t water proof!

Moderate and severe weather can drive water through the smallest of cracks in any siding. A second line of protection should be provided in all homes, especially those exposed to wind-driven rain. I have investigated structural failures and repaired homes for many years. Some failings are chronic. The details provided will improve your chances of preventing leakage, even in high exposure conditions.

House Wraps

Too often, builders neglect to wrap their houses with some type of water resistant membrane before they apply siding. This seems especially true when houses are sided with vinyl. Many builders are convinced that nothing gets past vinyl siding. But vinyl siding is not waterproof, as I learned while inspecting a house in frigid temperatures after a heavy, wind-driven rain. The sidewall had icicles protruding from beneath one section of the vinyl siding where rain had penetrated the siding and was frozen in its tracks trying to escape.

Patchwork

Poorly installed housewrap is just as bad as no housewrap. Another builder I met with, used a combination of Tyvek (R) and Typar (R) on the same house. No real problem here, but the lack of consistency caught my eye. What did concern me was how little scraps of house wrap were cobbled together. Especially, how some upper pieces were installed first then overlapped by pieces installed later at a lower point in the wall. If rain penetrates the siding on this house, it could easily find a path behind the lower sheet of house wrap.

Step Flashing

Many homes have sloping roof lines that intersect taller sidewalls. For example, when the gable end of a garage is attached to the gable end of a 2-story house. In this application, the garage roof shingles are step-flashed against the sidewall sheathing of the house. In cold climates snow will drift and collect at the point where the garage roof joins the sidewall of the house. When the drifted snow melts, water can find its way through butt joints, knots, cracks and other spaces in the siding. The solution is to place the garage roof flashing directly against the bare house sheathing, then make sure the housewrap overlaps the step flashing.

Wrapping Windows

Recommendations:

  • use house wrap or felt paper on ALL houses newly built or resided.

  • the wrap should be continuous and carefully installed. Give wind-driven rain the respect it deserves. Plan on rain penetrating the siding under certain weather conditions.

  • to gauge your house wrap details: imagine droplets of water running down the surface of house wrap. Be sure that all pathways into the envelope are protected by an overlapping arrangement of house wrap and flashings.

  • provide water that penetrates the siding with an unrestricted path down and out of the space behind the siding.

  • wall membranes should overlap by 3-inches horizontally and 6-inches vertically. All seams should be taped.

Windows and Doors

This makes sense. But there is a little more involved when working this detail: Don’t bury the bottom of the splines beneath a layer of house wrap. The bottom of the spline should be lifted and placed over the felt or house wrap that runs horizontally beneath the window. Guide any water that penetrates the window casing/siding joint down the spline and over the felt or wrap that covers the wall.

Windows with nailing fins (vinyl, clad and metal) are becoming more popular, replacing traditional wood-cased units on many job sites. When these units are properly installed, they offer more protection against water infiltration than wood-cased units because the house wrap membrane can be double-overlapped in a weather-tight detail :

1) install a strip of felt across the bottom of the rough opening and fold the top of this strip inward (sill flashing) 2) install splines along the sides of the rough opening, overlapping the sill flashing with bottom edge of splines 3) install window unit 4) wrap entire building (tuck lower wrapping under sill flashing)

Head Flashing

Head flashing is a critical detail for any window installation. The top edge of window fin or Z-flashing should be protected by an overlapping membrane. When using a house wrap like Tyvek (R) or Typar (R) it is not as easy to weave the flashing under the wrap as it is with felt. With these wraps, I would either: 1) pre-wrap the entire house, install the windows and flashing, and then tape the top of the flashing to the house wrap(product like 3M contractors tape or W.R. Grace Vycor); 2) install splines, install windows and flashing, and then wrap the house. Be sure the splines don’t lead rain water beneath the house wrap in any way (tricky detail! ) ; 3) wrap the walls before the windows are installed and slice the wrap above the window so you can slide the top of the window flashing beneath the house wrap. But be very careful here: On windy days, water collects on top of the window flashing. It will be blown sideways along the top of the window and may leak when it reaches the window’s edge. Be certain that the window flashing leads water over the house wrap(not behind it) at the ends of the window.

The use of rigid plastic foam sheathing is also becoming more common in residential construction. If you tuck window flashing under the foam sheathing you may create a leak. As described above, water collects on the top of windows and is forced to the window’s edge where the flashing leads water beneath the foam. The flashing is installed over the foam and the foam is notched to receive the top of the flashing.

Recommendations:

  • protect the top of the window flashing with overlapping wrap.

  • double-overlap house wrap around nailing fins of vinyl or clad windows.

  • splines must direct water over underlying house wrap.

  • leave a gap >1/4″ between the window flashing and bottom edge of siding to prevent capillary suction.


Corners

Not all leaks result in water dripping on the carpets of nervous clients. Some leaks slowly compromise the structural integrity of a building. Rip off the corner boards of enough old houses and you will see your share of rotting corner posts. In general, corners of buildings experience the greatest effects of wind pressure. Special care should be taken to protect this area from exposure to rain water.

Last month I had the opportunity to visit the construction site of a very expensive new home. The quality of construction was unsurpassed by any crew I have seen. Joints were tight. Framing was cleverly laid out. And the design was intelligent. But one detail troubled me. The 1-by wood corner boards were applied directly over the plywood sheathing. There was no house wrap beneath the corner boards. Over time, the corner boards and the wood siding will shrink and expand. Certainly the intersection between the corner board and siding will not be water tight. Yet there was no protection provided at this point.

This example demonstrates the overconfidence builders have with well-installed siding. Many builders fail to place house wrap beneath corner details. An often-heard strategy involves caulking the joint between the siding and corner board. In my opinion, caulking only makes matters worse. It retards the drying process when water penetrates the corner detail. I think that building a forgiving corner joint is the best design. Sealants should not be used as a primary membrane material in wood-frame construction. The bond between most sealants and wood deteriorates within a few years. Faster in severe exposures.

A better detail is to extend any type of house wrap youÕre using around the corner before installing the corner board. Corners of houses are often dinged and damaged during construction. So I would recommend that a second vertical strip of felt paper or house wrap be applied over the corners of the house just before installation of the corner boards. A double layer is cheap insurance.

Recommendations:

  • install house wrap beneath all corner details.

  • double-wrap corners.

  • don’t caulk the joint between the siding and corner board.


Siding

There have been many books written about siding applications. So, I won’t explain in any comprehensive way how to prevent siding failures. But I will provide a sampling of some failures I’ve investigated in recent months.

Diagonal Board Siding

Horizontal Board Siding

I’m not crazy about horizontal board siding in windy or exposed locations either. If the architectural design calls for horizontally applied, tongue and groove or ship lap siding, be sure to install it so the tongues point upward . If you have knots, splits or defects in the tongues or grooves you run the risk of allowing water to pass through the siding when driven by wind or gravity. Good house-wrapping details are critical. Use a high-quality, defect-free grade of siding for horizontal applications. Vertical installations provide better protection.

Panel Siding

Another common failure involves improper installation of plywood or panel siding. Problems typically occur at the horizontal joints between upper and lower panels. Often plywood panel siding like T-111 is used to economize so structural sheathing is not installed beneath the siding panels. Very often house wrap is not installed either. Typically, in panel applications, either Z-flashing is installed at the top of the lower panel or; the top and bottom edges of adjoining panels are beveled to “prevent” leakage. These connections are not weather tight. House wrap should be used. Z-flashing should be installed at the top of the lower panels. And the house wrap should overlap the z-flashing. Be sure to leave at least a 1/4″ gap between the upper and lower panel to prevent capillary suction.

Splashback

We’ve all noticed how siding is discolored just above grade. Splash-back is unavoidable on most homes. It doesn’t matter what type of siding you have, this part of the wall experiences heavy exposure to water. And the sheathing must be carefully protected with a continuous layer of house wrap at this point. Installing a strip of self-healing bituthane membrane like Ice and Water Shield¨ under the siding at the bottom of the wall may be a worthwhile investment. Carefully detail the area at the bottom of the wall where the sill connects to the foundation. This intersection is critical. Be sure that water runs down the siding and is carried past this juncture so that wind will not drive water under the sill. A 2-inch minimum overlap is required.

Recommendations:

  • don’t install board siding on a diagonal bias.

  • use top-grade boards & fussy house wrap details for horizontal applications

  • point tongues upward for T&G applications.

  • use house wrap and flash horizontal joints in panel siding.

  • protect lower part of wall sheathing with bituthane membrane.

  • overlap sill-to-foundation joint with a minimum 2-inch overlap.

As is the case for all detailing: think about the application. Gauge the exposure. And be conservative in your estimation of the forces influencing your design. Plan on water penetrating your primary line of defense and develop a plan that offers solid backup protection.

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Last updated: April 29, 2009