Library Desk Pour

Walter leveling the desk mold.

For our next larger project working our way towards doing the ceilings we built a desk in the library. The desk is a very shallow arch, a bridge spanning between the bedroom on the left and front door partitions along the east wall. This desk needs to be very strong as it will support a 500 lb fresh water aquarium so I did not do a plaster layer on the bottom like I had done with the bathroom lower shelf. The desk is 80″ long, 24″ wide, 7.25″ deep at the ends and 2.5″ thick in the middle of the arch. The cement is a high cement mix with added PVA fiber from Nycon and 661010 WWM reinforcing. Additionally there are rebar pins all around the ends joining the desk to the partition and side wall.

Form End Supports

The support for the mold form is made of 2×4 joists with a 24″ long end plate. This sits on shims which sit on stacks of concrete blocks. The purpose of the shims is two fold: 1) to let me get the form support exactly level and 2) so that I can easily remove the shims when the concrete is hard such that the form support drops down out of the way and then I can peal the foam form mold off of the concrete.

The reason the middle joist is doubled is that neither of those pieces of wood were quite long enough. I’m loath to cut wood, or string, unnecessarily. It is a bit of a joke around here. “Just cut the string!” Holly will often say, even when we’re not talking about string, theoretical or otherwise. Well in this case I didn’t. Instead we put the two boards together so we didn’t have to cut another board shorter. It works beautifully and means one more board is saved for some future time when we’ll need the extra length. My mother says it’s a product of being a child of the Great Depression. I’m not. She is. I’m her child, the child of a child of the Depression. My father has another saying, “Buy it new, use it up, make it do or do without.” A modern similar mantra is “Reduce, Reuse, Recycle.” Of course, it could just be I’m a cheap penny pinching miser as someone once said.

Completed Frame

The completed support frame for the foam mold with the adjustable cross support pieces that will be used to hold the shape of the curve. Only the middle upright 2×4 is toe-nailed into the joists to hold it in place. The other pieces are simply set on the joists so they can be slid to match the curved arch of the 1″ foam to come. The front 2×8 board was set in place for this picture but then I removed it so I could…

Foam Mold Arch

put the 1″ pink foam insulation sheet in place. I use pink foam because it is so slick the concrete does not stick well, especially if you demold it early. Spraying with a release oil makes it even smoother and easier to release but is not absolutely necessary as long as you don’t leave it in the mold too long, say a week. Generally I’ve demolded within a few days and the foam sheet just peels right off.

In this picture the cross supports have been positioned to support the foam so it won’t lose it’s shape as I add concrete. To further help I add the concrete a little at a time starting at the ends and working toward the middle.

A note on getting the right length on the piece of pink foam that makes the curve of the arch: You could calculate it mathematically. You could calculate it geometrically. You could use a tape measure of the actual curve and then transfer that measurement to a sheet for cutting. Or, as I did, you could bend the sheet of pink foam in place next to the wall and ding it with your finger nail in the spot to be cut. Simple and effective. I then used a right angle to cut it square and it fit perfectly.

Drilling Sill Pins

Here is my son Will hammer drilling out the holes for the sill pins. I should have had him do this before I setup the forms but I forgot until later – mea culpa. In addition to three pins in the sill there were eight pins along the wall and the wall was roughed up a bit to make sure the desk would lock in. It will be heavy and be supporting a heavy aquarium so I don’t want any chance of it falling. The pin holes in the vertical walls are significantly larger than the rebar pieces so that the cement flows in around the pins and they are angled downward into the wall to lock tight.

Siliconed Front Edge

After putting the front board on I used a bead of silicone rubber to seal the foam to the wood. This creates a little bit of a curve, after I ran my finger through it, and keeps concrete from dripping. I used a wood board with no foam on the front because we will peel the wood 2×8 off early to hand smooth and round the front edge of the desk after adding another complexity to the curve – details tomorrow.

The Completed Mold

The completed foam mold ready to accept concrete. It still needs pieces of 2″ PVC pipe in strategic places along the back so that electrical wires will be able to pass up along the wall. I put those in as we added the concrete.

Poured desk

The desk is curing eight buckets of concrete later, mixed by Ben and smoothed by Will and I. The white blobs in the back are the tops of the 2″ PVC pipe w
here wiring will pass through the desk. The setup took a lot of time but the actual pour went very quickly. Unfortunately it was snowing heavily outside, where we had setup the concrete mixer. When we had set it up things were nice but the weather changed quickly. For the next steps I’m going to move the cement mixer inside again – Ben will appreciate that.

The reason the top of the desk is rough is we’ll be adding another 1″ layer, with some fancy front curves and pigmenting it. But that is tomorrow’s project.

Note: I faked the gorgeous sunny views through the windows. While we were working on the library desk the weather was almost blizzard conditions outdoors and all you could see was white much of the time. So I Photoshopped in views from another post. Much more interesting and how I would have liked the weather to have been. Wouldn’t it be nice to be able to Photoshop the real world?

Outdoors: 37°F/18°F 8″ Snow, Sleet, Hail, Rain, Windy
Farm House: 72°F/53°F two logs
Tiny Cottage: 48°F/41°F door open too much and no sun

About Walter Jeffries

Tinker, Tailor...
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6 Responses to Library Desk Pour

  1. pablo says:

    I’m impressed with your craftsmanship, in both the desk and the photo trickery. It looks to me as though the center of the desk is only going to be an inch or two thick. I suppose that will be enuf to hold an aquarium full of water, correct?


  2. Anonymous says:

    I’m just wondering how much weight (max) could your concrete desk support?

  3. Pablo, in the middle the arch is 2.5″ thick. The tank actually sits just barely (3″) on the window sill putting it’s mass along the strongest part of the middle of the arch.

    Anony, the limiting factor on the strength of the desk isn’t actually the arch but rather the buttressing strength of the concrete end wall partitions to the left and right. For the arch to go down they must shift outward. The end walls are locked into both the floor with rebar and mortar and interlocked with the blocks of the east wall (outside of the house) and there is a steel reinforced bond beam at desk height that locks it all together.

    In a month we can test the million dollar question. :)

  4. Oh, Pablo, one more thing I forgot to mention is that the aquarium tank acts as a beam across 48″ of the desk thus moving the forces out from the center to within 26″ of the ends of the arch. That right there makes a huge difference. I’m more concerned with someone standing on the middle of the arch, jumping, sitting, bobbing, etc than the weight of the tank.

  5. Betty says:

    Hi, Walter,

    Actually, I DON’T think it would be nice to be able to Photoshop life…I can think of all sorts of downside stuff, but for starters, it’s just too controlling.

    I love your desk: will it be totally taken up with the aquarium? Which will block the window? (or rather, will be a “screen” which filters the outside, see the outside through it?)What else will the library hold? Doesn’t look big enough to hold much in the way of books. CD’s? DVD’s? Will there be shelving? Your tiny house is fascinating. See you soon.

    By the way, great minds think alike, I guess: my entry for this morning, before I viewed yours was April Showers as well. We only got about eight inches, but that’s an elegant sufficiency, and maybe even more than I’d like to see, in April. Three weeks til pasture is ready seems a stretch at this point!

  6. Betty, the aquarium will occupy just the space in front of the window in about the bottom 15″ of height allowing us to see over it to the outdoors, mountain and sky. This makes it like a set of low curtains in the window and gives the tank natural light which works very well in a heavily planted tank.

    Along the partition walls will be book shelves. The desk is for Ben & Will’s school work area. We homeschool and that will provide them with a study space. The final desk will have a curved front.

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