Window flashing IRC 2015 R 703.4 Flashing requirements. How to make window flashing logical and mechanical.
Your window installer wants to wrap your water-resistive barrier (WRB) aka House Wrap inside onto your window jamb. You worry about water entering the window from your drainable WRB channels. Let's look at code requirements and give this process a review
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By Murray Wennerlund published 4-24-2022 updated 8-25-2022
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The Code I will be reviewing as a homeowner today is IRC 2015 R703.4 Flashing and my focus will be on windows and the window rough-in framing. I'll be using the abbreviation WRB which stands for water-resistive barrier instead of using trade names or the words house wrap. This is for new construction interpreted from reading and observing framing practices. Window Flange and Nail Fin are interchangeable and only their roles are different. Nail Fin is typically found on new construction windows and flanges are found on replacement windows.  

IRC 2015 Section R 703.4 Flashing. Approved corrosion-resistant flashing shall be applied shingle-fashion in a manner to prevent entry of water into the wall cavity or penetration of water to the building structural framing components. Self-adhered membranes used as flashing shall comply with AAMA 711. Fluid-applied membranes used as flashing in exterior walls shall comply with AAMA 714. The flashing shall extend to the surface of the exterior wall finish. Approved corrosion-resistant flashings shall be installed at the following locations:

  1. Exterior window and door openings. Flashing at exterior window and door openings shall extend to the surface of the exterior wall finish or to the water-resistive barrier complying with Section 703.2 for subsequent drainage. Mechanically attached flexible flashings shall comply with AAMA 712. Flashing at exterior window and door openings shall be installed in accordance with one or more of the following:
    1. 1 The fenestration manufacturer's installation and flashing instructions, or for applications not addressed in the fenestration manufacturer's instructions, in accordance with the flashing manufacturer's instructions. Where flashing instructions or details are not provided, pan flashing shall be installed at the sill of exterior window and door openings. Pan flashing shall be sealed or sloped in such a manner as to direct water to the surface of the exterior wall finish or to the water resistive barrier for subsequent drainage. Openings using pan flashing shall incorporate flashing or protection at the head and sides.

The parts in BOLD are what I will cover in this article. I want to point out I'm not making suggestions but I am going to make questions to the logic of some processes. 

First thing as a homeowner we understand that when you as a contractor tell us to "Caulk and Seal windows yearly or when leaks are detected" you're actually telling me to hang out my attic windows and apply a perfect line of caulk to the top and sides of my single hung window. That's not going to happen in my lifetime. 

So what are my alternatives? Can I have a window installed that doesn't require caulk or sealant as it's main water intrusion preventative? 

I can take any manufacturers "How to install guide" and understand that it's all based on the quality and skill of the trades before the installation. 

What does this R703.4 code statement "Pan flashing shall be sealed or sloped in such a manner as to direct water to the surface of the exterior wall finish or to the water resistive barrier for subsequent drainage." mean to a homeowner or contractor? Allow me to rephrase this reference to code R703.4.

  1. Pan flashing shall be sealed in such a manner as to direct water to the surface of the exterior wall;
    or if using WRB
  2. Pan flashing shall be sealed in such a manner as to direct water to the surface of the water resistive barrier WRB;
    or
  3. Pan flashing shall be sloped in such a manner as to direct water to the surface of the exterior wall;
    or if using WRB
  4. Pan flashing shall be sloped in such a manner as to direct water to the surface of the water resistive barrier WRB.

The sill pan catches all the water that your window leaks from the top and sides and then allows the water to drain away from inside walls with a 6 to 8 degree slope to your sill pan. This draining process follows the shingling method, directing the water over the next layer of WRB.

If you use a preformed pan sill and then follow the nailing and flashing of that pan you're going to find the pan will seal and it also will have a slope outward. That's the key, even with ABS or PVC plastic sill pans they are still saying to have the sill sloped. 

The next thought is why window manufactures say "Don't seal the bottom flange of your windows. This is to allow water that has penetrated the sill to escape to the outside weather barrier or sheathing." Sounds like after all your flashing the manufacturers all agree that you're going to get leaks one day so allow the water to drain out the bottom of your window sill. Again, this places my focus on a sill plate being installed with a slight angle directing the water outwards and away from the inside of your home. 

By reading just a few window, flashing and AWB manufacturer installation guides you may have noticed that the most important wall component is the sill pan which is built up on top of the window sill plate that is put in by your framer. I don't find many references to framing that say to make the sill sloped outward and level side to side. It would make the window installation and prep work so much easier and technically correct if the sill plate was constructed in a way that flashing tape could be installed directly per code 703.4 without having to add lumber to level or slope the window sill plate.

If you could get a perfect seal in the top and sides of your window you wouldn't be concerned about the bottom of the window or the sill at all. But it's not always perfect. You might have flashing against your OSB that protects your OSB from water. When you nail or screw your window into place the flashing forms small holes and imperfections in the material and could allow water to channel between your OSB > to Flashing > to Window nail Fin. To reduce these types of leaks we want to add a sealant between the window nail fin and the flashing or WRB layers to prevent water from penetrating. So now the shingle layer looks like, OSB > to Flashing > to Sealant > to Window nail Fin.

So far it sounds logical but we have a weak point again, you have to use a specific sealant for windows and the window material. Will a silicone sealant work with PVC (vinyl) windows? Some PVC window manufacturers say not to use silicone and to use a polyurethane sealant. Will your polyurethane sealant adhere to the material below the nailing fin of your windows? That's the weak point at this level, when you depend on a liquid sealer you're tapping into my experience with liquid gaskets and engine water pump repairs. I never saw a sealer last as long as a good gasket in any vehicle so in this case I draw my focus on gasket material and the sealant is only being used as a gasket material leveler to fill in any imperfections between the two surfaces of the materials. 

Let me clarify the gasket material reference. In framing you're going to have imperfections. Your window nail fin may not sit flush against your sheathing. This is when gasket material or sealant would fill in any small imperfections. It's better to start with a good foundation, so correct any framing issues with framing materials and avoid using sealant as a leveler. You need to have a good flat surface around your window opening to be sure you have a good tight seal for your window. If you have to build up areas that are small you can layer flashing tape but limit your time taping up mistakes. It's far better to fix the framing.

To recap our layering;

OSB under > Flashing under > Sealant under> Window Nail Fin; 
or
OSB under > Flashing under > WRB house wrap under > Sealant under > Window Nail Fin;

The WRB has to be overlapped per it's specific but often very generic installation guidelines. The overlap directs the water from one layer over the next so it stays away from the framed wall components. 

When we WRB a home the horizontal overlap is created to allow water to drain from one sheet to the other in a downward direction. When we use a vertical lap which has to follow the shingle style drainage path and is interrupted by a window we have to look at overlap from a vertical point of view.  

Vertical overlap of WRB is typically 6" of house wrap under the adjacent house wrap. This isn't a difficult process. Then, seam tape could add to protection by closing the vertical line opening.

So let's add overlap to our window opening and find the layer and next point of water entry. 

OSB under> WRB under > Flashing under> Sealant under> Nail Fin under > Flashing over WRB.

The last step is the vertical joint we are creating. If I use 6" flashing tape over the WRB to the window nail fin I will have close to 4 inch overlap to the WRB and 2 inch over the window flange. But I have to make sure that the nail fin of the window is sealed so I look to go up the side of the window to channel any water down the side just incase my flashing tape does not make a perfect seal to the flat part of the window nailing fin. 

OSB under > WRB under > Flashing under > Sealant under > Window Nail Fin under > Flashing over WRB and Window nail fin going up the vertical sides of window over the nail fin. 

So far it appears to show your windows are going to leak some place over the life of the window. I'm not going to factor in the issues of screws or nails not driven straight into the nailing fin and which create small air pockets that eventually will fill with condensation to rust fasteners and leak into your window sill.  

No matter how you flash your framing, WRB your sheathing, caulk your nailing fin you're going to get water in places you don't want water. The best design is going to be based on how you channel water that gets in behind all your WRB. If you can make water flow down your window jamb to your seal pan that has a slope that channels water out and away from your wall components or on top of your WRB to continue downward you are actually creating more of a mechanical method that doesn't place as much emphasis on your caulking and liquid sealants. 

So now it's a question of "Do we wrap the WRB into and over the side window jambs?" With our drainable WRB which has channels you would think it would drain into the side of your window. The fact is all drainable WRB is design to drain vertically in a downward direction. So unless you installed your drainable WRB  incorrectly or didn't remove the channeling guides in the WRB before wrapping into the inside of your window jamb you shouldn't have any issues. 

Window Jamb and WRB wrapped inward. 

OSB under > WRB  Wrapped inside of window jamb. 
or
OSB under > WRB > Cut at edge of OSB

In this case we see we need to think what the WRB is actually protecting. The edge of the OSB is not protected from any water intrusion. If we allow this the OSB will swell and cause the sealant or caulk to have gaps which would allow more water to swell more sheathing. 

What's the homeowners view and answer? Flash the jambs in both cases so you prevent water from actually coming in contact with any materials being protected by your WRB. If you insulate or seal for air penetration you're most likely going to flush cut the WRB so foam or your rigid insulation have no obstructions caused by your WRB.  

Final Review of the full flashing process for windows from a homeowners point of view no matter if you flush cut, cut back or wrap over your window jambs and sills. 

Step 1; Install Window Flashing:

  1. Flash the sill pan so that it drains any liquid outwards. Provide a horizontally level sill pan that has a 6 to 8 degree outward slope.
  2. Flash the window jambs by starting just below the window header down to your sill pan overlapping by 2" and overlap outward on top of your exterior wall sheathing by 2" or at least the the width of your nailing fin. 
  3. Flash the header even with the top of your window extending out either side a minimum width of your windows nailing fin. Use the same flashing material to maintain an even flush surface around all sides of your windows covering. This padding should cover the width of your nailing fins. 
  4. Install your window using your manufacturer's installation method.

Step 2 Option 1 WRB Fold, Option 2 WRB Flush Cut, Option 3 WRB Cut Back.

Option 1: Using the WRB Fold In on window jamb method :

  1. Cut the WRB from the top corners outward at 45 degree angles. Make your cuts long enough to place flashing tape over the top window nailing fin and at least 3 inches over the window head.  
  2. Wrap your WRB into the window jambs making sure not to go past the depth of the actual window. See note 1.
  3. Install your window using your manufacturer's installation method. 

Option 2: Using the WRB Flush Cut to window jamb method: 

  1. Cut the WRB from the top corners outward at 45 degree angles. Make your cuts long enough to place flashing tape over the top window nailing fin and at least 3 inches over the window head.  
  2. Cut the WRB flush with your window jamb on both sides of the window.
  3. Install your window using your manufacturer's installation method.

Option 3: Using the WRB Cut back from window jamb method: 

  1. Cut the WRB from the top corners outward at 45 degree angles. Make your cuts long enough to place flashing tape over the top window nailing fin and at least 3 inches over the window head.  
  2. Cut the WRB back outside the window jamb the with of your nailing fin. Do this on both sides of your window rough in opening.
  3. Install your window using your manufacturer's installation method.

Note (1): Some foam spray window insulation acts as a sponge and when your window leaks it will absorb the water and take time to dry. Be sure your foam does not come in contact with other porous materials like your wall framing. 

If you are installing windows in a shed or accessory building you do not have to use WRB but it is recommended that you flash the window to drain water away from the inside wall. 

Bottom line is simple, your windows are going to leak one day. It's important to mechanically channel the water outward. In a severe driving rain like what hurricanes can produce very few windows can drain fast enough so you might get window sills filling up with water. We used towels at the base of the sill to soak up the wind driven rain. The water was channeled just couldn't escape the weep holes faster enough so it backed up on the sills. Lucky for us the sills have a slope to keep the water at the window. 

Keep these points in mind at all times when designing your windows and creating your installation process. 

  • Pan flashing shall be sealed or sloped in such a manner as to direct water to the surface of the exterior wall finish or to the water resistive barrier for subsequent drainage.
  • Openings using pan flashing shall incorporate flashing or protection at the head and sides.
  • The flashing shall extend to the surface of the exterior wall finish.

Note: The Cut back method may be a better choice for those working with a fabric type of WRB or if you can not get the WRB to lay flat at all contact points around the window where flashing and nailing fins will be in contact. Wrinkles in the WRB that continue into the the window will channel water. It would be better to flush cut or cut back the WRB if this is the case. 

Resource Research: 

  • IRC 2015 Code
  • Leading window manufacturers
  • Leading WRB manufacturers 
  • Leading flashing tape manufacturers