Taming Trusses

Today was a rest day for Ben and Will after our long push to get the walls up. While the concrete cures Holly and I worked on the trusses and other details getting ready to do the roof. Realize that these trusses are all temporary – they are part of the form work that holds up the concrete in the roof as it cures. Think of this as an inside out mold.

This morning I recalculated the roof for the Nth time, wavering between two different curves. For the final house, of which the tiny cottage comprises a small piece, there will be buttresses formed by the smaller towers outside the house to transfer load to the ground as well as the next vault to also take horizontal loading. In the cottage there is just the internal partitions, the rebar in the loft/attic beams and the thicknesses of the block walls themselves to handle the spreading loads of the roof. As if that weren’t enough overkill…

After much debate and finally looking at a section of rebar curved upside down on the north wall in various shapes I went with the design (dotted line) that will be better for the final house although it is not optimal for the cottage (blue line). The loads on the cottage, since it will not be earth bermed yet, are minimal (mostly snow) and the strength is far better than needed for that even though it isn’t optimized. A lot of diddling over a few decimal places.

The spiky thing in the upper photo is a truss. The trusses get mirrored to create an arch and then the arch gets repeated twice more to give three truss sets connected by ribs that create a barrel vault. We’ll put this in place, probably Tuesday afternoon or Wednesday, and then cover it with 661010 Welded Wire Mesh (WWM). This is what WWM looks like:

You might remember it from when I made the chicken sunroom, greenhouses and winter farrowing hut last winter. Very handy stuff.

Over the form work of WWM we’ll lay expanded metal lath which looks like the stuff on the top of this pile:

and more WWM which will become embedded in the concrete roof. In the rib sections we’ll put the rebar for added strength. The ribs are tied across the house with additional sections of #4 (1/2″) rebar that act as tensors across the attic and the loft.

On top of all this metal work we’ll pour, pack and maybe spray the PVA fiber concrete onto to create a ferro-cement arch. See MXSteve’s excellent articles on this topic as well as discussion at FerroCement.net. Some time I’ll have to write about our Tirolessa cement sprayer – it’s one very cool tool.

The weather is supposed to take a turn for the cold this week so hopefully we’ll get that done before the cold and be able to insulate the roof so it can cure properly.

That collection of lumber and plywood is the jig upon which we bend the bars that support the roof on the tiny cottage that Walter built. Well, building. And it’s not as crooked as the house that Jack built by far! Using this rebar bending jig we bent the rebar that will go in the four ribs of our roof. Making identically bent 20′ sections of #4 rebar is very easy if you have a jig and tie the rebar together before bending. Baling twine works great for binding bars. Most of the roof will be very thin (~1″) but the ribs will be thicker areas (3″ to 8″).

After we got done making the roof rebar pieces we hung them up over bales of insulation so they would keep their shape. Hope discovered that the rebar makes an excellent musical instrument. You might note there are only three strings on her Hopesicord – the fourth stayed on the jig to act as a guide for making the trusses.

This is the spiky thing mentioned before in the diagram – a finished truss that will support the horizontal form ribs that hold up the WWM that forms the lath that catches the fiber filled concrete that makes up the roof of our tiny cottage. In just a little while we’ll be removing all of this wood from our house. As we joked the other day, it is almost like we are building a house inside a house to get the final thing cast in place.

54째F/34째F Mostly Sunny

About Walter Jeffries

Tinker, Tailor...
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3 Responses to Taming Trusses

  1. PV says:

    Im awd!!!!!!!

    Shes a rocker!

  2. karl says:

    you have probably seen this one http://photos.itsa.info/
    i you haven’t you might find some further inspiration.

    this is an interesting read for cooling ideas

  3. Mark says:

    I haven’t been following your construction, but it looks interesting.

    I am reminded of some concrete construction I saw in Mexico that did not involve significant engineering. It was a two-story, with a poured second-story floor. I went into a ground-floor room below and immeditely exited. The center of the floor/ceiling had sagged at least a foot after the pour. It had curred a long time ago, but the sight was so unnerving I couldn’t stay in the room.

    My own concrete form work was just footings, which in my part of the country are usually simply scooped out about a foot deep with a backhoe. My building site was on a slope that required lots and lots of steps in the footings as well as built-up forms at the excavated grade. The total drop from the highest point to the lowest was about 12-14 feet, as I recall. I looked at some photos recently and it was hard to believe the work that went into all the forms that were eventually torn down and burned.

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