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DIY House Addition: How To Properly Flash & Trim Windows

DIY diva

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:
01_installed

Note:

  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.

02_caulk_corners

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.

04_sides_on_flashing

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

05_tape_top

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

06_tape_down_wrap

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?

Trimming

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:

01_before_trim

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

02_measure_sides

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.

03_saw

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

05_prime

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

06_attach_sides

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…

07_measure_top

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.

08_special_notch

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

10_chisel

11_notch

12_fit

Perfect fit. Here’s our finished window.

13_finish

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.

finished_trim2

finished_trim1

And also slightly more chaotic.

14_aftermath

Eh, making messes is one of my natural talents.

DIY diva

    Comments

  • White Eyelashes


    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.

  • chad


    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.

  • Pat


    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?

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