DIY House Addition: How To Properly Flash & Trim Windows

There’s nothing like having a few gaping holes in the perfectly good exterior walls of your house during rainy season to make you fully appreciate that windows in your house will either be a blessing, or the bane of your existence. Depending on how well they’re installed and the kind of flashing you use. Also, now may be a good time for you to buy stock in caulk.

You may have been around here a few weeks ago when MysteryMan thought he was going to have to call the exorcist because my head was spinning around on my shoulders with the force of my rage at the improper window installation situation we had going on.In retrospect, I’m still not sure if I would have rather risked the stability of our relationship by trying to install them ourselves, and at this point it is what it is.

So that no one else has to stumble upon the proper flashing and trimming techniques the way I did, here’s what I learned…

Flashing (not the kind that goes on at Mardi Gras)

Here’s what a window should look like that has been properly installed:


  1. Flashing at the bottom that sits under the window. Everyone tells us the roofing rubber we used was overkill, but do you want to have water in your house? Me either.
  2. House wrap should be tucked inside on the sides and bottom of the window.
  3. House wrap should be folded up at the top (and able to be folded back down after the window tape is on.
  4. For windows that come with an integrated nailing flange that has to be bent out (Marvin) they will also have corner pieces that must be properly placed and caulked.

Here’s the problem with DIYing some of your house addition… all the tradesmen we’ve worked with like to play a game called “that’s not my job”. It goes like this “I’ll install your windows and attach the corner pieces, but I won’t caulk them because… that’s not my job. The siding guy does that.”

So it’s up to you as the person who has access to the internet and therefore the wealth of the ages to be informed of these things so that you don’t accidentally trim your windows out without caulking the corner pieces. Not that I’d have any experience with that.


Ok, now that everything is caulked, it’s time for the window tape. Foil on one side, and some kind of tar based don’t-get-this-stuff-on-your-clothes stickiness on the other side. Seriously. Don’t get it on your clothes. Or anything else you don’t want stuck to the window.

Start with the sides. The tape gets applied 3″ over the top edge (enough for the top strip of tape to overlap it) and about 1/2″ up onto the side of the window. The top should start under the top flap for the house-wrap, so this image is a little misleading.

03_tape sides

The side pieces should run 4″ down over the flashing at the bottom.


The top piece runs 4″ out over the side pieces, and under the top flap of house wrap.


And then the top flap gets folded down over the window tape, and the seams are taped up with house wrap tape.


Seems like taping might be a more appropriate term for it.

That’s all there is to the flashing that needs to be done before you put on the trim. I’ll show you how our rubber flashing piece works when we install the siding. (Which god help me better be sooner rather than later.)

Let’s trim something, shall we?


First lets talk about trimming basics. Window trim is basically just four pieces of wood, no matter how you look at it, but some ways are fancier than others. Here are some things to consider.

Materials– There are lots of materials you can use – fiber-cement trim boards, extruded aluminum trim from the window manufacturer, or plain old wood. For better or worse we’re going with cedar siding on the house, and rough sawn cedar for all of the fascia and trim.

Sizes-Your trim should stick out past your siding, and since we’re using a 1″ overlaping siding we needed at to use 2x rough sawn lumber to trim in the windows. With a thinner siding you could use 1x material. Common widths are 4-6″. Honestly I’ve seen some windows with 6″ siding, and unless they are huge, they look a little overwhelmed by that thick trim. But, much like permed bangs were in the eighties, it’s a personal choice.

Corners– You can miter the corners or use a straight cut, and let me just give you an opinion on this. Don’t miter them. One, it’s a pain in the ass. Two, as the wood expands and contracts (which will be much worse on those thin corner pieces) they’re going to pull away from eachother and look like crap). Three, the miter cut is a perfect channel directing water downward towards the inside of the window, which is exactly not where it should be. I cut the first trim pieces as miters, lined the up, and realized I was a complete moron and went back to make them square cuts.

Sills– The sill can be a larger piece of wood, it can have a bevel on it, or some fancy detail, or just be a straight piece of wood like the rest of the trim. It largely depends on the style of your house and the look you’re going for. We made the trim a simple square, since our wavy-edged siding is going to steal the show. If it ever gets installed.

As for the actual installing of trim, here’s how we did it.

This is a properly flashed window:


Hey look! My first mitered board is now relegated to “spare board for measuring”.


We put the measuring board in place and then measured to the bottom of the window, which ended up being fairly accurate.

After the miter/chop saw was used to cut the side boards to length, we ran them through the table saw and took a little more than a blade width off to accommodate for the nailing flange, so that the trim would lay flat.


Which, of course, meant that the board needed to be primed again.


Then the boards were attached, using the the spare board at the top to make sure they were flush.


The fasteners are stainless steel 3-1/2″ ring-shank nails. Two at the top, middle, and bottom.

Then, the top and bottom pieces were measured…


Same process for trimming the boards down to fit flush to the wall with the nailing flange.

Interesting, on any of our “double” windows, there was a protruding piece of metal where the windows were joined.


The reason for this escapes me, other than to give me an excuse to use my chisel.




Perfect fit. Here’s our finished window.


I’ll go back and do touch up paint on the ends and over the nails.

Also, we’ll caulk any space between the window and the trim to make sure water doesn’t wick its way back inside and there will be an additional drip cap over the trim from that fun day I spent using the brake, but we’ll get into that when the siding fun begins.

To see how we put all these steps into action (and our strategy for DIYing the trim together with only minor violence) you can check out my article and accompanying video on Old House Web:  The Best Laid Plans: DIYing Your Way Through Window Trim

As you can see, we did get into a pretty good groove.

For now, things are looking much more substantial with the trim on.



And also slightly more chaotic.


Eh, making messes is one of my natural talents.

17 Responses

  1. Bravo! YES, you SHOULD take “overkill” measures when flashing doors and windows. I have maintained private islands in the tropics (and other places) and shoddy window installations make for tons of extra work down the line. By the time you realize that your windows have been leaking for YEARS, you have a ton of costly damage to repair. When I built my house here in the islands, I even wrapped the rough opening lumber with protecto wrap and tucked the flashing into my courses of hardieplank. Wind driven rain from hurricanes here kills, but I’ve replaced windows and rotten sills, framing elsewhere too. Top notch work, as always. Thanks for sharing.

  2. Great to properly install windows. Waste of money to do it over vertical mulch (OSB). It’s used everywhere. And will be replaced everywhere within the next decade or so. We are all suckers for good marketing of planned obsolescence. This is one of the worst examples out there. Don’t be fooled. Proper sheathing matters.

    1. I completely agree! OSB subfloors in kitchens and bathrooms will swell over time. Have a pet that pees in the house? More swelling. High humidity? You guessed it. Over time, using OSB will be looked back upon as the greatest mistake any contemporary contractor is making. Plain and simple, it is NOT worth the savings.

  3. How do you install the window when you are not taking off the siding. Under our siding is black paper from the 1950’s. Do you have to piece in new house wrap?

    1. In case anyone else needs the answer to Pat’s question:

      Exactly my situation: house from the 50s, tar paper over sheathing, under aluminum siding. The answer is, you can’t unless you take down the siding. And I haven’t found any siding pros who will do it — it’s too hard to take aluminum down and put it back up and have it look like new, so they refuse. The best shops will only tear down and replace. So if you wanted to put in a new window, and wrap it well, you’d probably have to take down your siding, repair sheathing, wrap, install the window, and then cut the siding as you reinstall it around the new window.

      I took down about 6 courses under a large window that was leaking horribly. Most of the tar paper was gone, the sheathing horribly rotten, ant nests, ugh. Cleaned it all out to the studs, re-sheathed, and I’m using tyvek up to the bottom of the window and am taping it under an upper course of tar paper which is still intact. Not perfect, but should be good for a few years until we have the whole house re-sided. $200 in materials and a solid day’s work.

      Like the page, btw! Thanks for the closeups on the taping details!


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  5. nice pics on installing a window but I have always cut the window trim to overlap the sides. the sides should run past the lower trim so there is a natural rain drip edge and the lower trim is only cut the width of the window and caulked to produce the drip edge.
    as for chads comment about strand board, there is nothing wrong with strand board. been using it at least 25 years myself and we don’t have a problem with it. building a house with overhangs we always run plywood there, around tub enclosures for built in tubs, but strand on the rest of the house. you have to properly flash it, and run building paper or house wrap to keep it dry or it will rot just as fast as plywood.

  6. A lot of the things that are true of windows are also true of doors. Make sure you get them well fitted and they’ll serve you well letting you and friends enter and leave but keeping out the cold and unwanted neighbours. Poorly fitted and they’ll be a living nightmare in maintenance costs and cold draughts.

  7. Wonderful article.
    Do you have any thoughts/recommendations about trying to install wood trim around windows that are already installed in aluminum siding.
    Everything works fine now but we would like the look of wood trim. Is there a good way to do this?

  8. It looks like there are a lot more steps to installing window trim than I thought. I was under the impression the kit would basically just slide into the frame, but it looks like there is some cutting involved. I’m not particularly handy, so it might just be best for me to hire a great contractor to make sure the job gets done properly. Thanks for the awesome information!

  9. Help, re trimming exterior with Hardie Board trim, some how all trim (particle board)got moisture, nails rusted, maybe I over caulked, starting from scratch, questions- I have not seen it on other homes, but does the trim above the garage door need a drip (?, “L” shape flashing, and all windows, if so, then matter how well your paint condition is, water (moisture) gets behind siding and needs a way out or air? or is the trim left open (over garage door only) on the underside to allow air and water to escape?? need Code 3 reply, so I can do the right thing before we get our moist weather and rain! Thank You, Rick, Rescue,CA

    1. I would put z-channel over any piece of wood trim that surrounds a door or window (including garage doors.) You really don’t want any moisture getting behind a piece of trim or you’re going to have the rot and rust eventually.

      1. I see some putting over the window and when when the trim is installed, thay add a piece over it too. Two pieces of drip cap flashing now. What is your thoughts on that?

  10. My new windows are installed and old vinyl siding is removed. My contractor did not put any of the window tape on. Then he nailed rough, red cedar boards around the windows. Next he installed LP Smart siding which butts up next to the rough cedar trim. He said he will install an LP siding drip edge over the top of each window.

    Do we need the window tape?
    Doesn’t the red cedar need stained before it is installed?
    Do we caulk between the window and the red cedar?
    Do we caulk between the red cedar and pre painted siding?
    Any other suggestions? thanks

  11. I think that you bring up a very important point of flashing the exterior top trim. It is never brought up.

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