Building the Octagon. Pt.23 More joists and boards

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I decided to go with 1"x5" larch boards for appearance and natural weather resistance. A far more expensive option than plywood but my wife preferred boards to plain, featureless plywood. So now we know who to blame. With 6m /19' lengths to play with I shall have to measure first and then cut on the miter saw down below.

Due to the large trapdoor there won't be many full length boards. Hopefully I shall be able to reduce wastage by careful recycling off-cuts. I bought a load of 3" stainless steel, Torx decking screws while I was there. The small heads avoid splitting the boards.

Having cut most of the boards to length I screwed a few down around the trapdoor and pier apertures. Then discovered by the time I reached the posts that I was badly out of parallel. The 2x8 can be seen sunk at one end and proud the other where it is resting in the gap. I shall have to withdraw the screws and true the boards up again. The short, loose board is lying over the pier aperture to remind me not to step just there. The larger hole, with the ladder, is for the trapdoor access to the observatory.

A check with the laser rangefinder showed a near 25mm [1"] difference between each of the N and S posts at just above floor level. I intended to space the boards slightly to improve drainage. Perhaps I can use the gaps to hide the difference. Spread over a number of boards the difference is only a mm per gap. I spent an hour of two cutting more boards but not fixing them. Finally, rain stopped play.

As soon as I removed the boards I noticed problems with the alignment of the joists. So all the boards came back up and I spent Sunday morning realigning the joists to make them all parallel. It was hardly surprising that an inexperienced, amateur carpenter would fail to make the octagon posts perfectly even. Making the joists non-parallel was less forgivable.

The West and South facing verandas would have been completely out of square. Every other face of an octagon is at 90°. Or, rather, should be. Each new face is at 45°, or 135° if you prefer obtuse angles.

Here I have the floor boards up on the top rim joists to allow me to work unhindered by their weight on the joists below.There will be no veranda on the right nearest the shed. Though I have arranged for the veranda to follow the angle of the walls before stopping.

I found an easier way to measure up for the veranda, boundary rim joists. I simply placed a block of 4x4x2" on top of the joist, to a marked line and measured from the veranda on the far side of the octagon with the laser rangefinder. After that I could use a long builder's straight edge to mark the joists accurately.

Rangefinders work best in shade as the red spot is very hard to see in bright sunshine. The considerable "leverage" of the beam is also hard to overcome as the small, handheld device needs very little movement to throw the spot yards off at any great distance. That said, they manage remarkably repeatable measurements under fair conditions. Though a clear straight line is obviously vital for the beam to reach the target surface. It is best to be able to rest the device on something solid. If only to stop violent wobbles as the measurement button is pressed. This is easier where the base of the device is pressed against a flat, immovable surface. Handy for indoor rooms. Less so, for outdoor octagonal frameworks.

Getting the "corners" right on the octagonal veranda was rather more difficult. There was a lot of dangling from bare joists while I found a satisfactory compromise. I wanted to preserve the outer crossings of the joists and their supporting beams. Rather than leave corners unsupported. So I included all of them even if it meant adding an extra inch in overall width to the veranda.

There is a lot of effort invested in the veranda to break up what would have been a rather bland looking, octagonal, plywood tower. As any image search will attest an octagonal building need not be plain and boring. The veranda is also supposed to be a safe surface for working on the upper structure. If I recorded one of those "sped up" videos you would not believe how many times I climb and descend ladders in a day. Standing, sitting and kneeling, not always on a secure surface, requires constant care to avoid a fall.

I fell 10' off the house roof once but fortunately I landed on my head on a concrete slab. Otherwise I could have been badly injured. The conventional ladder I had resting on the roof rotated around the ropes at gutter level as I came down with arms full of tools. There was no way to arrange my hooked roof ladder due to an inconveniently placed dormer. The one I had just finished building. Let's be careful out there, people!

Click on any image for an enlargement.

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