How To Correctly Detail Window and Door Trim

For this episode we are zooming in to look specifically at the joint between the vertical and horizontal trim pieces around a window or door. A lot of houses have a 45 degree angled miter joint as their trim detail but we don’t think this is a good idea. With wood frame construction, there is movement to be expected in the frame of the house over time, and this can result in splits or cracks appearing in the trim if the joint is mitered.

Window and door trim does not like to be mitered. Instead, trim wants to be framed in solid pieces with an expressed expansion joint. With either wood or painted MDF type trim, the trick is to have the carpenter slightly ease the edges between the individual trim pieces to make an intentional looking joint. If there is any movement, then the shifts will occur within this joint and will not look like the paint is cracking or the trim is splitting.

Today’s Slides:

  • Steve in Van

    Prior to renovations, my mid-century condo had no window trim other than a simple wood sill – the drywall return on the sides performed perfectly for 35 years, clean and unadorned.  Unfortunately the strata-mandated window replacement reduced the expense of drywall repair by adding 3.5” of trim on all sides (pic).  I suppose this is a standard ‘fast home’ practice that, when added to the features list with wide baseboards, crown molding, and six-panel doors, many consumers consider classy.
    It seems to me, however, that modern design minimizes or eliminates trim wherever possible.  Karl Ulrich explores the options ( ), and BUILD uses a very tidy solution for doors ( 
    Where applied trim is necessary, I think your solution looks great.  On the downside, the expressed vertical joint interrupts the ‘horizontality’ of the datum lines you discussed a few days ago.  On the other hand, where the joint is above or below eye-level, it conceals the butt ends of the trim pieces.  Was that your primary consideration when choosing a vertical rather than horizontal joint?

  • BradW

    I do not agree with J+M regarding the use of mitre joints in casing. I think this technique can be very successful over a long time. It really depends on the skill of the craftsmen and the quality of the material. Unfortunately, a good trim carpenter is hard to find, good quality materials are very expensive and paintable silicone caulking is cheap. I really think what J+M are trying to do with their casing design is to make it as forgiving as possible within a modern context.

    Regardless of how you decide to trim out a residential build, your life will be considerably easier if the trades that proceed this step have paid attention to their work. Framers, window guys and door guys should know how to use a tape measure, a level and a square. Many do not. Drywall is less of a problem for casing but baseboard inside and outside corners can be adversely affected. All too often trim is used to coverup poor workmanship.

  • BradW

    I should add that when trim is used to cover up poor workmanship it makes it doubling difficult to do a good job with the trim regardless of the style.

  • Terri

    It seems that almost all the details in my old house are not up to par!
    Yes, we have the mitered trim around doors and windows, solid fir, installed by good carpenter. This place is 27 yrs old, and after reading this Design Minute, I checked those corners just to see if any separation has occurred.Nope. And this in a two-storey house that has had a few small shake-ups from earth tremors (enough to make the some wood in the walls or ceiling make cracking sounds).

  • Matthew North

    Hi Steve – sorry it has taken me so long to reply – we are going through a really busy spell just before the holidays! I love your story about your condo window replacement and the addition of “fancy” trim. I also have all drywall returns in y place and I love it…..I have commercial windows however which is another episode unto itself. We use the vertical joint exclusively because after years of trial and error the vertical joint in the trim works best from an installation point of view and works best when we are ganging the trim together on adjacent doors or windows. In short, it is neat and easy to seam with the minimum number of reveals. Also, the reveals are so minimal that they do not distract from any horizontal detailing.

  • Matthew North

    Hi Brad – you are correct – a skilled craftsmen can execute a mitre joint in a high quality way. We have been very fortunate to have worked with some of the best. My major issue with the mitre joint is the use with MDF where the joint is lacquered (usually primed with filler and sanded first) and then cracks through the paint two weeks later. In Calgary at least (where our very dry climate does not help out at all) I have yet to see a lacquered mitre joint look good in the fullness of time. I think a natural wood trim would be a different story if the carpenter took due care……and the framer and drywaller before him or her also took due care. Your point is well taken.

  • Matthew North

    Hi Terri…….your humid BC climate is not as hard on your joinery as in dry and dusty Alberta! Based on your comments and Brad comments I should re-issue our position on trim……not be so hard on the mitre it seems …..and we should just complain about poor workmanship and a dry climate! I am happy to hear your house is standing up to the test of time – even with some earthquakes! Thanks for the feedback.

  • Matthew North

    Brad………and as I said when we met in Toronto….I think its great when we have a difference of opinion on the site – nothing like some good discussion and some re-consideration of position!

  • Old West Windows and Doors

    Mitered door frame joints are very likely to separate over time, but this isn’t a major issue. Slight gaps are barely noticeable and can be covered with painting.