Keystones undoubtedly add some character to your window trim, but some care does need to be taken when selecting which one to have installed. A keystone that is too small won’t fit properly into the trim, leaving a portion of it exposed. A keystone that is too large (as seen below) on the other hand, can be disproportionately large and also look out of place.

A keystone that is obviously too large for the trim that "contains" it.

The key question (get it?) then is, how do you select the right size? As a rule of thumb, keystones should be 2-3″ larger than the trim they’re being fit into. This becomes difficult with smaller window trims that are only 4″ in size like Trim 001, and in such cases I have seen contractors actually cut the bottom 3″ off a keystone so that it is height proportionate. Don’t forget to re-apply the cementitious coating after doing this.

Jul 132011

While windows are typically trimmed with the same profile around the entire window (or top + 2 sides + sill), another popular option is to simply add a piece of trim to the top of a window, otherwise known as a “Window Header”. Window headers are less expensive than trimming an entire window because only 1/4th of the moldings are required, but still make a big difference in the overall appearance of a building.

Window headers usually incorporate some sort of keystone into the middle so as to avoid looking overly “plain”. They typically extend slightly past the width of the window, and side-trim if present.

Any of Decoramould’s Window Trims, Window Sills and Wall Moldings are usable as a window header. Size needs to be determined carefully so that the header is not too large for a window. Generally:

  • Windows under 4′ width shouldn’t use any piece of molding larger than 6″ in height
  • Windows above 4′ width look good with moldings between 6″ and 9″ in height
  • Very large windows can get away with moldings upwards of 12″

Using a Decoramould product with a large, flat top to it is generally not recommended for Window Headers, as rain/snow and other precipitation tends to pool on top, causing damage over time.

Jun 142011

One thing that always needs to be considered when selecting a window trim and window sill is the proportions – more specifically, the depth. The depth of the trim is measured from the backside of the moulding (where it is attached to the wall), to the furthest point it projects from the wall. Aesthetically, having thinner trims than sills standard, so that a larger “lip” is created at the base of a window. When your contractor (or you, if it’s a DIY installation) isn’t paying attention, it is possible to select a trim that is too large for a sill. Why is this a problem?


It’s not a pretty sight. The trim overhangs the sills instead of sitting nicely on the sills upper surface. Most of the time, the edge of the trim isn’t coated, in which case the styrofoam is left exposed to UV Rays (which cause it to degrade) and is susceptible to insects looking for a place to nest. While the styrofoam trim doesn’t provide any nutrients on which insects, rot and mould can feast, like any building products, it can provide shelter for them.

One solution should you ever purchase a sill with less depth than the trim, is simply to build it up with a flat band. Adding a flat band with 1″ or 2″ thickness in behind the sill effective pushes it further from the wall, creating a larger top surface for the trim to finish into.

© 2013 Exterior Insulation Finishing System Suffusion theme by Sayontan Sinha