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paula_builds | Posted in General Questions on

I’m about to order the hardie trim for my siding.  Lap siding will butt up to the trim.  I’m good with the look shown below, EXCEPT the sill.  I had always scoffed at this sill treatment, until I realized that hardie doesn’t offer a good solution for the sills of my windows.

Question: How bad is it to do this?

Should use either plastic, or wood, or hardie, and rip a bevel, and a drip edge?  I did this with cedar, but it seems a little more frustrating to do with a hardie product (not to mention yucky dust).

I think I’d need to find a thicker stock than the 3/4×2.5 inch batten boards I’m using for trim.  Last time, I did a bevel rip on some 2×6 cedar, then cut a neat groove for the drip edge.  It looked great. (2nd picture)

I’m installing the siding and trim over a 1/2″ thick rainscreen gap/furring strips.  I have 2 foot overhangs, but some walls are more exposed than others, since I have a shed roof style building.

I look forward to hearing your thoughts.  thanks!

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  1. Expert Member
    Michael Maines | | #1

    Paula, I have made "historic sill extensions" many times and often include them in the homes I design now. I agree that simply wrapping the entire window with flat trim is not the nicest look. The only thing I don't like about the thick sill you show is the apron board under it; I prefer to rabbet the underside of the sill extension for the siding to tuck up into. Actually I also think the drip kerf is a good idea but it should be back farther from the front edge, where it is vulnerable to breaking off.

    I usually make the sill extensions out of wood (anywhere from 1 1/8" to 2" thick), but I also use wood siding. I would not try to mill fiber cement into this shape. Boral TruExterior would be a good choice.

  2. bigred | | #2

    Consider using Azek. It comes in different widths (unlike hardi trim). I used 5/4 stock and made moldings similar to your 2nd photo. You can glue up the azek, router, saw, joint and plane the material. Almost as easy as working with wood and will never, never rot.

  3. gusfhb | | #3

    I look at the wood trim pictured and see two paths for water to try to get into the wall that don't exist in the simpler trim system. Face it you are pretending it is something it is not, an old window frame, and taking the weaknesses with it. If you really need that look, do it on the face. You could even get fancy because plastic trim is flexy and rabbet it into the face of the trim, face screw and plug it.

  4. tallpinescabin | | #4

    I'm replacing rotting wood trim on my 100 year old house now, and using Boral TrueExterior boards....Replacing wood water table with it, too. Interesting stuff to work with.
    Contemplating the same issue for a cabin I'm building (as soon as the siding and trim on the main house is done!), and thinking I'll likely rip sills from glued up Boral...... PVC trim is nice in some ways, but the fact we can't paint it a dark color (black!) leaves us with few, non-wood options.....

  5. paula_builds | | #5

    Thank you for the suggestions!
    Some follow up thoughts:
    -I can see that my execution in the 2nd photo could have been better.
    -I don't actually have a problem with how picture #1 looks. I just had a snobbery about it because I thought the drip edge was the "right way".
    -I will have a rain screen behind the trim, so how important is it to have a drip edge? If water goes behind the trim & siding, it has a path out.
    -Milling or cutting Azek will create a lot of fine plastic waste, which I dont' really want on my property. I guess I could set myself up with a collection system. I know the dust from Boral is probably similarly toxic, but maybe not as visibly ugly as bits of plastic.
    -my paint color is an olive - so not black, but not very light either.

    My other dilemma in working out what to order - is the interior corners.

    Since that product won't be milled on site, I may see if the Boral has a 2x2 product for inside corners.

    Let me know if you have further thoughts on this! :)

    1. Expert Member
      Michael Maines | | #6

      Paula, "drip edge" and "drip kerf" have different, specific meanings. Your sill extension has a drip kerf. With a well-detailed rain screen it's probably not necessary, but without it, wind will blow a lot more rain back into the rain screen than without it, so I would include it if possible.

      I have not worked with Boral myself but I have cut a lot of Azek (and other cellular PVC) and the dust is atrocious. Microplastics are a growing problem, visible or not. A dust collection system is a great idea, but even that won't capture everything. Work inside if possible, and wear PPE.

      Siding is usually installed with inside corner boards of some sort. In some cases the siding is coped instead, but that's a lot slower.

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