DIY Concrete Window Sills

As we mentioned last week, there’s a ton of mudding to do around here. Booo! Hiss! Most boring job ever! We decided to take a break and do something fun instead. Remember those trimless windows we started in the guest bedroom? Here’s what they look like with a coat of plaster:

That’s going to be the look for all of the windows on the second floor. Crisp, clean lines with no trim. They’ll each have a white cement sill that slides into the open space at the bottom where we didn’t put any drywall. Making those sills was our big project for the weekend.

We gathered our supplies in the garage. Here’s our mixing station:

The base of our mold:

The side walls of our mold:

Stuff for the inside of the mold:

We had to buy our white cement from Brooklyn because we couldn’t find it locally. It’s a bit pricier than the grey stuff. A 90 lb bag of white cement cost us $19, and the same amount of grey cement was only $9. We’re going for a softer look, though, so it’s totally worth it to us. We also couldn’t find a mold release spray in our area, so we used furniture wax. And the acrylic fortifier is an optional mix-in that helps make the concrete stronger by helping it bond together better.

One thing to remember when doing cement molds is that everything is upside down. The bottom of our mold is actually the top of our window sills. If we had wanted a heavily textured look for our sills, we would have used a heavily textured base bottom — a raw piece of wood maybe. To get the absolute smoothest texture possible, we would have used steel. We went with masonite because it’s much cheaper than steel and still gives us the smooth, velvety texture we want. And we used 2×2′s for the walls of our mold because we wanted a 1.5″ thick window sill.

Here’s a little hilarity for you: 2×2′s aren’t actually 2″ x 2″. They’re 1.5″ x 1.5″. And 2×4′s aren’t actually 2″ x 4″. They’re 1.5″ x 3.5″. My brain exploded when I found out.

The beauty of cement molding is that all of this stuff is totally customizable. Want a more textured top? Go with a more textured base. Want a super thick slab of cement? Build taller walls for your mold. Want black cement instead of grey or white? Go for it. What we’re showing is what worked for us.

The mold itself is really simple construction:

The particle board is on the bottom. The masonite is on top of the particle board. And the walls are on top of the masonite. Screws holds all 3 of the pieces together. Masonite is pretty flexible, so the particle board underneath is there to make the board rigid. This is really, really important — we don’t want a flimsy mold because we’ll be banging it around later.

It’s also really important to make sure the screws are long enough. The more sturdy the base, the better.

Bradley started by cutting the 2×2′s in half to make walls for our molds. We made 5 total molds: 3 sills for the Smurf room and 2 sills for the guest bedroom. And, since our masonite and particle board sheets were large enough, we put multiple molds on each base.

He made sure the wall edges were perfectly square before screwing them into the base:

Here’s what the Smurf room base looked like when Bradley finished. Each rectangle is one window sill mold, so this base has 3 molds on it:

Next, we used caulk to seal the gap between the wall and the masonite:

And once the caulk dried (about 10 minutes), we used a razor blade to scrape up any excess gunk.

After shaking out all the dust from the molds, we broke out the furniture wax:

We coated the masonite and the 4 walls so the concrete slab will easily slide out after drying. We made sure and waxed each corner really well because those are the spots that could potentially chip when we take the mold apart.

After that came the mixing:

We mixed together 50 lbs of cement with 50 lbs of sand. And that’s pretty much all we technically measured out. Bradley’s been experimenting with concrete at work and he likes to eyeball his mix, so our wet ingredients weren’t really measured.

He poured in about half a bottle of the acrylic fortifier and enough water to get the mixer moving, and then slowly added more water if he felt like the mix was too dry.

To check the mix, Bradley shoved his trowel in and mashed things around. When it got to the consistency of frozen yogurt, he started shoveling it into the molds:

Frozen yogurt or gelato. Or mousse. Or soft-serve. That’s the consistency we were going for. Nom nom nom.

Once it was full, we slid a piece of pipe under the mold and made a see-saw:

The is the part where the mold gets banged around a lot and could potentially fall apart if it’s flimsy. The basic idea is to whack the mold against the floor repeatedly so the air bubbles rise to the top. Little pockets of air in your cement mix could weaken your concrete slab. They could also make the top of your sill or counter look like crap, so whack the mold like you mean business.

The easiest way is to position the metal pipe in the center of the mold and stomp on one end of the mold with one foot.

The entire thing see-saws back and forth and smacks itself against the ground. It’s incredibly loud, and we’re pretty sure all of our neighbors hate us now, but it works like a charm. We did about 10 minutes of whacking per mold base.

While I did that, Bradley cut some wire and got the rebar ready:

The rebar is to strengthen the cement even more. We tied a length of wire to each end of the rebar, and gently set it into the cement:

We then twisted the wire around scrap pieces of wood that were long enough to sit across the walls of each mold. This way the rebar is sort of floating inside the cement instead of sinking to the bottom.

Here’s what it looked like when we finished setting the rebar:

Then we plopped more cement into the mold and covered up the rebar:

With a little more banging and whacking, the cement settled and filled in the spots under the scrap wood. We gave it an hour to firm up a little and then went back to remove the scrap wood.

Bradley snipped the wires:

Then he carefully lifted the scrap wood up, leaving behind the wire and the rebar floating in our cement mix:

He gently smoothed over the cement, being really careful not to tough the wires:

That whole taking-the-scrap-wood-off step is completely unnecessary. What we’re looking at in the above photo is actually the bottom of the sill, so it doesn’t really matter what it looks like. Bradley just felt like experimenting to see if he could get the bottom to be a little more even. Just in case he needs a more smooth bottom for another project.

We let the molds dry overnight, and came back the next day to crack ‘em open. We started by unscrewing all of the screws:

And then gently tapping the walls to loosen the molds. Emphasis on gently — the concrete isn’t 100% dry and tapping too hard could cause chips or cracks.

One by one, the molds released and out came our concrete window sills.

We gently moved them to the corner of our garage to let them continue drying.

The color ended up a lovely bone-white (lighter than they look in the photos) and the texture is super smooth and velvety. Exactly what we were going for.

One of the molds had a little bubble issue on the side:

Luckily it was on the side of the sill and not on the top — we can face the dimpled side to the wall so nobody will ever see it.

After we finished oohing and aahing over our new sills, Bradley grabbed a bucket of water and doused the concrete slabs:

This is totally counter intuitive — or at least it was to me — but you don’t really want your slabs to dry quickly. The longer cement takes to cure, the stronger the bond. So a hot, humid day is actually ideal for drying concrete because it slows down the dry time.

A few hours later we grabbed a couple of sills to see how they fit. Bradley slid one into place in the guest bedroom:

Perfect fit!

The sill is actually not done. We still need to sand them so the edges are smooth, and then polish them so they have a slight sheen. We also have to finish plastering the windows all the way to the edges of the sills so there are no gaps. We’re already loving the way they look, though.

What we learned from this project:

• The wet mix will look way darker than the final product.
  We had a slight meltdown when our wet cement mix looked more khaki than bone, but it dried several shades lighter. Whew!
• Mix smaller batches.
  We mixed a huge batch of cement — 100lbs of dry ingredients plus 5 gallons of water — and broke our paint mixer attachment. We ended up having to hand mix with trowels. Oops. Next time, we’re getting a more heavy-duty mixer attachment and we’re only mixing 2 or 3 sills per batch.
• Use bathroom + kitchen silicone and not painter’s silicone.
  We used painter’s silicone because it’s what we had on hand, and we think it may have dissolved a little in the wet cement mix. We think it’s the reason our edges weren’t as crisp as we had hoped for. Bath + kitchen silicone will hold up to water better, and we’ll try that out next time.
• Concrete is cheap.
  Even though we splurged on the fancy white concrete and the totally optional acrylic fortifier, we still only used about $18 worth of supplies to make these 5 window sills. We’re not sure what trim costs these days, but we’re willing to bet we’ve spent less on our trimless windows with custom sills than we would have on standard window sills with trim.

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