Recessed windows are becoming more popular and as a result they are very common to almost every community in California. The trend moving east rapidly. Arizona, Nevada, Utah and Colorado are just a few of the states that are seeing recessed windows included in the blueprints.

This photo shows no protection at the inside corners.

The details in most blueprints are very weak when it comes to flashing the recessed window condition (If a detail exists at all). Correspondingly, a large rise in rain related leaks has occurred in the homes built with recessed windows. It should be no surprise that most of these leaks are initially reported to be window leaks. The carpet under a window gets wet...window leak, Right? Wrong.

It is very common to have windows that are performing well, yet the recessed WALL condition around the window allows water to enter the wall system. To the untrained eye, this looks like a window leak. (Sometimes even to the trained eye this looks like a window leak.) Unfortunately, recessed wall leaks are much more difficult and expensive to repair than most window leaks.

The homeowners will wish it was a window leak by the time the repairs are through. Why do recessed wall conditions leak. First remember that in the most common wall systems being built today, some water travels behind the exterior shell of the home or building. The building envelope that is behind the exterior surface must be water-proof. The envelope must be built so that the water that travels behind the exterior surface is guided "Down and Out". In a recessed wall condition this is difficult at best. It is difficult because the wall system now must get water to travel a more difficult route. In a traditional flush mount window, water simply travels straight down to a weep screed and out the building. Gravity is its best friend. Gravity, along with good methods and materials will take the water straight down. In a recessed wall condition, this water must make some difficult turns before it hits the screed system. These turns, often ninety degree turns, had better be flashed and sealed well, otherwise gravity will direct the water into the wall system. Water will always follow the path of least resistance. It must be easier for water to follow the drainage path to the weep screed instead of into the wall system. The areas especially vulnerable to allowing water into the wall system are the inside and outside corners of the recessed wall.

By 2001 my small company, TLS Laboratories in Laguna Niguel, had responded to so many recessed window leaks that we recognized the need for an established method for flashing these areas. During the development stages of this process, we investigated hundreds of recessed window leaks. What we discovered was no two recessed wall conditions were flashed exactly the same. This was true even on the same window. The lower right corner and the lower left corner often looked like they were flashed by different people. Often, the upper corners were not given any thought at all. Peel and stick membranes were often forgotten, or never in the plan from the beginning. Solid backing at very important areas was not provided. And quite typically, no slope was provided at the lower horizontal ledge.

A recessed window installation in progress

We found that the inside and outside corners leaked for three reasons:

1. Holes existed in the innermost and outermost corners because flashing was not layered properly. Additionally, water-proof membranes often did not have the release paper pulled. This condition allowed water to travel laterally under the membrane where it got in trouble. Remember water must be guided down and out. (This condition is magnified when solid backing is not provided.)
2. Damage was often discovered to the flashing methods that allowed water to enter the home. Damage to the flashing system came from lath staples, furring nails, razor knifes and even from passing materials through the window rough openings. We found damage that appeared to be the result of contractors climbing through openings. Steel toed boots are not gentle to the flashing materials.
3. Deterioration to the flashing occurred because the wrong materials were used during the flashing process. On wall sections that are not vertical, in other words sloped away from the home, a water proof-membrane system 40 mils thick or thicker needs to be used. We know that building paper will deteriorate if it is subjected to prolonged exposure to water. Building paper is not the correct material for use where water might stand. Nor is traditional window flashing material. Water proof membranes were designed for this use

1. It must be effective 100% of the time. When correctly installed the process had to be rock solid. The penalty for anything less was too high.
2. It must be fool-proof. Simple and easy. It must be reasonable. If the method was too difficult, it would not stand to reason that it would be correctly installed by the typical tradesperson.

We concluded special flashing materials were required that would come out of the box ready for use. Correctly cutting and folding the materials was the most difficult part of the process. If we could solve that part of the problem, the rest would be easy. With these issues in mind, we developed the Corner Flash™ system. The Corner Flash™ system uses products called the GS100 A the GS 100 B and the GS Flex products. These are all pre-made inside and outside flashing corners. The alternative to Corner Flash™ is an “Origami" method. “Origami" is the Japanese art of cutting and folding. However, Japanese artists do not use sticky paper. This method can be successful if installed by trained craftsmen. It becomes more difficult when the release paper to the peel and stick membrane is removed. Achieving a properly flashed recessed wall condition using the “Origami" method is very difficult at best. It can be done...but the difficulty factor is huge. Lets be clear about one issue: if a talented tradesperson wants to use the “Origami" method of creating his own corners, that is fine. Just ensure this tradesperson is qualified and talented to do so. This process is time consuming; it often results in mistakes that require the process be stopped, materials thrown away and the process started over again.

The correctly flashed recessed opening will have the following characteristics:

1. Solid backing at all areas of the recess. This solid backing should extend up the sides of the recess at least 6".
2. A minimum dimension of 3" around the window rough opening. Anything less will not allow the necessary materials to be layered properly. When nail fin windows are used, the nail fins need room for attachment and the flashing must be allowed to travel from under the nail fin, across the framing to the recessed wall condition. If this minimum dimension is not used, the nail fin to the window may cut or otherwise damage the flashing materials.

   This photo demonstrates good preparation for recessed flashing: Solid backing, slight slope to the horizontal ledge, Corner Flash flashing corners in place with the 1st bib on the exterior wall. The next step is to apply the water proof membrane

3. The flashing at the lowest horizontal ledge of the recess wall condition should begin before the window is set. The water-proof membrane should be rolled on top the framing sill of the window rough opening. Remember, we are flashing two different conditions here, the window and the recessed wall. Both processes need to begin before the window is set.
4. The waterproof membrane components need to have a minimum thickness of 40 mils when the process is complete. Peel and stick membranes need to have their release paper pulled to prohibit water from traveling anywhere but down and out. When used in vertical applications the release paper needs to be pulled if the manufacturer of the membrane requires it.
5. All inside and outside corners need to be reinforced and carefully flashed and sealed. This means all the corners both above the window and below the window.
6. Some slope needs to be provided at the lower edge of the recessed wall condition. How much slope. 1" in 12" is a good minimum slope. More slope is better.
7. A bib should be installed on the outermost vertical wall, below the lowest horizontal recessed area. All membrane flashing should be layered on top of this bib. This allows the balance of the building paper to be tucked under this recessed wall flashing at a later date.
8. After the membrane flashing is successfully installed on the lowest areas of the recessed framing, an additional bib made of building paper should be applied just before the lowest window flashing. This 2nd bib should extend the over the entire horizontal ledge and extend down the vertical face of the outside wall. This bib serves to protect the membrane flashing from subsequent trade damage and provide protection from the installation of the exterior surface of the wall system.

An excellent product for flashing these areas is Corner Flash™ by TLS Laboratories. The GS100 Corner Flashing System is designed to make this process fool proof. Pre-manufactured corners are installed and counter flashed. The difficult inside and outside corners are pre-made and come out of the box ready for use.

Another view of a recessed windows installation.

Steps required to correctly flash a recessed window and wall condition:

1. Ensure the rough opening is correctly sized. Once the flashing is installed, any adjustments to the R.O. (Rough Opening) will be extremely difficult. Remember the flashing is rolled on to the framing sill. This will make floating the trimmers and leveling the framing sill practically impossible after the flashing is installed.
2. Ensure the recessed wall condition is properly built. The recessed area needs to have solid backing at all points of the lowest framing that extends vertically a minimum of 6". Ideally, all points of the recessed opening will have solid backing. More solid backing is better. This ensures that when the exterior surface is installed the fasteners do not create problems.
3. Check to see that the lowest horizontal framing is sloped away from the home. The minimum slope required is 1":12" or 8%. Once again, more slope is better. Remember, once the flashing is installed, the ability to slope the framing disappears.
4. Install a flashing bib horizontally across the lower ledge of the opening at the outside recessed wall. This bib may be made from building paper or window flashing. Do not use a peel and stick membrane. This will allow subsequent layers of flashing to be applied on top, then the rest of the building paper may be tucked under it at a future date.
5. Install the outside flashing corners on top of the bib at the outside edge of the recessed wall condition.
6. Install the inside corners under the window R.O.; these will rest upon the framing. They will layer on top of the ends of the outside corners.
7. Install the peel and stick water proof membrane in a manner that starts 4" or more down the exterior vertical wall and rolls all the way on to the framing sill of the window R.O. This layer of membrane needs to be layered without wrinkles; it should have tight corners at all the edges of the recess. Pay special attention to the inside corners!

   GS Flex installation

   
   
8. A sill pan under the window will be installed at this time. We encourage the use of GS Flex™ to create a Membrane Pan™ under the window. The GS Flex™ corners are installed on the framing sill before the water proof membrane is applied. The membrane is rolled on top of the GS Flex™ corners.
9. Now apply sealant at the edges of the water proof membrane all the way down to the outside vertical wall so that any straws that were created are terminated.
10. Next apply the 2nd bib over the recessed flashing. This bib serves to protect the flashing subsequent trades and materials.
11. Next apply the upper corners of Corner Flash™ at the inside and outside corners of the recessed framing.
12. Next apply the window flashing materials. (Assuming AAMA method B window installation). We do not encourage peel and stick membranes at these recessed areas because they are difficult to work with. Too many wrinkles are created as you attempt to layer a peel and stick membrane over the corners of a recessed opening. We believe products like Moistop Next are a very good choice. Layer the window flashing materials over the Corner Flash™ products.
13. Now prepare to install the window. If you are installing a nail flange window, ensure the window nail fin is not damaged. If it is damaged, repair as required and proceed to the next steps.
14. Sealant should be applied to all four sides of the nail fin. If a wood window with Brick Mould Casing is used, apply sealant behind the casing and the subsill. Other window Recessed windows configurations (bald wood windows for example) will need special attention. The use of backer rod and sealant may be required. Some theories exist that suppose all windows will fail someday; in these theories, the lowest horizontal nail fin should be left open, without sealant. If this theory is used, then the window framing sill needs to be sloped away from the building. How much slope? How does this slope affect the rough opening dimensions? We do not know. We do not currently subscribe to these theories. However, if correctly implemented, this method of window installation will work just fine.

    These photos show the basic flashing method with the GS Flex corners in the top photo; the 2nd bib in the middle photo, and the lower window flashing in the lowest photo.

    Remember to slope the framing sill, and roll a layer of water proof membrane on top of the sloped sill. DuPont Flex Wrap is a good choice in this application.
    
15. Install the window in the R.O. in a manner that creates sealant squeeze out from around all fours sides of the window. The window must be level, square and plumb. Three inset photos show the basic flashing method with the GS Flex corners in the top photo; the 2nd bib in the middle photo and the lower window flashing in the lowest photo Page 7 Recessed windows and plumb. Fasteners should be applied every 6-8 inches. After the window is installed, apply a second cap bead of sealant on top of the nail fin and butter the sealant flat. Pay special attention to any pre punched holes in the nail fin. Pre punched holes need to be completely covered and filled with sealant. Also look for cracks or damage to the nail fin and completely fill minor cracks with sealant. Major cracks that spider into the window frame require window replacement. Wood windows with folding nail fins need special attention at this time. The area on the jamb that allows the nail fin to pivot and fold back needs to have an application of sealant. This pivot area can allow unwanted water to enter the wall system. Even in wood windows where the nail fin is mechanically attached, this areas needs sealant applied.
16. Finish the installation by installing the top layer of window flashing across the top of the window. This flashing will be layered on top of the Corner Flash™ products and provide counter-flashing.
17. After the window is installed, ensure subsequent building materials such as siding or lath wire does not have fasteners installed through the nail fins of the windows

Here is an example of the good, the bad, and the ugly...

The Good: This looks like great preparation and flashing. The next step will be the 2nd bib and the window flashing

The Bad: This photo shows the damage to the water proof membrane caused by other sub trades. This home had serious water intrusion problems.

The Ugly: This photo shows a poor method of flashing the inside and outside corners of the recessed wall condition. When properly flashed, bare wood will never be seen. This home had serious water intrusion problems as well.

Recessed windows require special attention. If the special attention is paid then these conditions will not create problems. If special attention is not paid, these conditions will create nightmares for the unsuspecting builder that will remove his hard earned profits.

Recommended products:

• Corner Flash Products.
• Moistop Polyurethane Sealant or SM7100 Sealant.
• When using GS Flex producs and surface temperatures will exceed 120 degrees F, we believe a butyl based peel and stick membranes should be used (DuPont straight flash).
• If surface temperatures will NOT exceed 120 degrees F, then SBS modified asphalt peel and stick membranes may be safely used with GS Flex products (Fortiflash 40).

Gene Summy is the owner of TLS Labs and may be reached via e-mail at gene@tlslabs.com.