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Considerations for Arched Window Bucks in Airform Augments

Image: Side view of augment showing height difference of window buck — Left Image: Poor profile shape due to size of buck and interior air pressure.  Curves exaggerated for illustration.  The buck was made to fit a hypothetical window.
Right Image: Augment appears more square from the side view due to the increased height of the buck.  In this case the larger buck has been made to fit tight in the Airform augment just inside the window seaming.

Side view of augment showing height difference of window buck — Left Image: Poor profile shape due to size of buck and interior air pressure. Curves exaggerated for illustration. The buck was made to fit a hypothetical window.

Right Image: Augment appears more square from the side view due to the increased height of the buck. In this case the larger buck has been made to fit tight in the Airform augment just inside the window seaming.


Image: Head-on view of inflated augment showing seaming and (interior) buck placement — Left Image:  Multiple wrinkles and valleys pull into the augment crown due to buck placement well inside of the seaming and interior air pressure.  Once foamed these become hardend channels that direct rain run-off behind your window trim.  The buck was made 1.5" larger than the hypothetical window on all sides.
Right Image:  The buck should be sized to fit just inside of the vertical plane and Airform seaming of the augment.  Hence, crowns, valleys and wrinkles are minimized.  The shape is taut.  The window framers will have ample room to install or retro-fit new window technology years later.  Rain run-off is also improved to roll off the sides of the augment.

Finally ready?

So after all the back-slapping, hand-shaking and fan fair during the Airform inflation, you’re finally ready to get down to the business of interior construction. From inside, you’re admiring the eye-catching, organic shape of the inflated Airform and the ethereal translucence as the sunlight filters through fabric, when a contractor derails your train of thought.

“Hey Mac, how big do you want the framing on your windows to be? The longer you ponder, the more I make.”

“Well, what’s normal,” you ask?

Stop there!

This is a dome. No matter how crazy-efficient, a Monolithic Dome is not normal construction. Bucks are built and used rather than standard window framing, and the lessons below are based on our building experience.

Normal window framing turned out to be 1-1/2" larger than the window on each side, top and bottom for a total opening 3" taller and 3" wider than our window measurements.

Although we thought through and planned multiple events before our dome build, this one caught me off guard. Other factors unique to our situation included all of our windows were pillowed out on handsomely planned augments, and each was double-hung with attached half-rounds on top. These beautifully arched windows would match the dome’s profile and garner many favorable comments.

All was set to continue.

I haphazardly gave the go ahead of having normal sized bucks manufactured on site. Each backing board was patterned from our window measurements plus 3", then the buck boards were ripped to form the arch. Once the arched buck box was formed, they were fastened to the Airform, hoisted by dome inflation and then held square with two guy wires. Before we knew it, we were happily off to priming, foaming and enjoying the shell construction experience.

Fast forward to the day we’re setting windows in our manufactured bucks. The 4000 lb/square inch shotcrete isn’t giving us any leeway at this point. And during the re-inflation and foaming of the dome, unnoticed by the crew or me, some bucks shifted slightly so they fell just out of plum, out of true, or out of level. We even had one window that was out on all three axis!

We eventually ended up with what sounds like an enticing advertisement on a TV sales channel; each arch-over double hung window has been painstakingly and individually hand fitted into treated wood bucks by ever-patient and highly skilled American workmen. Thankfully we had a bang-up framing crew, and only one window couldn’t be set in the existing buck. After gaining a few more gray hairs, removing some buck material and several artisan touches, it now looks ever the innocent and visually appealing window that it was meant to be.

Exterior also proves challenging.

Despite the extra attention to set the windows using a normal allowance, the exterior would also prove challenging. The original plan was to use wood or vinyl for the exterior window trim. This couldn’t be reasonably accomplished due to Airform wrinkles and curves on what was to be the flat augment face. After custom setting the windows in the bucks, we now had to utilize an over budget material that could adhere to these curves and buckles.

Don’t get me wrong – we love the way the stucco trim looks – outside and inside! And the flexibility of the material was the answer we needed to tackle double arcs and deep wrinkles.

But, we did not love the higher cost of the craftsman, materials, nor the extra time spent chasing water leaks. Remember, a shotcrete dome won’t rot, but other construction materials will, such as wood fill, wood trim, steel nails and staples. Eventually, after extending our budget, we managed to get the water intrusion abated, and the trim looks like it was always meant to.

Please – don’t be put off by our experience.

Instead, learn from us. Our trial is certainly a minority among the 2000+ domes constructed by now. And, we got through the episode with visually appealing, professional looking results. If you want the best construction and most efficient structure, build your Monolithic Dome. We were prepared and remained flexible for excess time, materials and specialized labor, but we could have avoided the trouble if I, Chris, had thought a little further forward.

To avoid our window buck pitfalls be sure to discuss your situation ahead of time with your General Contractor, an experienced member of the Monolithic Dome Institute staff or a recommended source on the Monolithic Dome Builders Professionals List before you’re at the crossroads of decision. Half a day of research could save you a couple thousand dollars.

We offer these considerations for augmented window bucks constructed on your site:

1. Have your window bucks manufactured to fit just inside of the augment upright face stitching, not built to the standard 3" larger than your window width and height, especially if you have arched windows. This will allow ample room to frame your window later.

2. Calculate and construct your window buck height so the horizontal treated wood base will rest on (or just above) your floor. If your floor has not been poured, or you do not want the buck sitting directly on the floor, be sure to leave room for foaming and shotcrete beneath this horizontal member with strong blocking. The final product should remind you of a door buck due to the upright height. You will also want to consider cross member braces between the uprights to ensure foaming doesn’t squeeze your buck. Cut the base of the buck uprights 1/4" short on the inner length and match the horizontal member to this angle. Though barely visible, the slight down angle will encourage moisture to shed outward later during construction. Shim under the horizontal for added stability.

3. You can further secure your bucks tight to the Airform with 1 × 4 lumber and treated wood screws (to decrease rust). With air pressure supporting the inside of your Airform, consider using four guy wires to ensure positive control close to each corner of your window buck. Consider a 5th wire for the top of your arch if there’s any sway in the buck arch due to ripping the wood. You absolutely want the buck straight, square, true and as level as possible after final Airform inflation and before priming and foaming begin.

4. With the buck set properly, and with the larger than normal window allowance, your framers will be thankful. Any upgrades or window replacements years from now should be easily accommodated with plenty of room, and you can still have the superior insulation properties of closed cell foam integrated in your augment once your windows are framed and set.

For this and all dome construction, investing yourself in the Monolithic Dome Institute Builder’s Workshop is highly recommended by the authors. The 1999 MDI article Installing Augment Frames is just as applicable today as when it was written, and should be used for doors and windows. Take advantage of all the information available at www.monolithic.com. You’ll be glad you did!