The Observatory Roof

The roof is basically made of two triangular side pieces separated on the “top” side by five 2-inch x3-inch cross-beams. Two other beams, one each between the
other apexes of the triangles, were added to make the structure more rigid.


The wheels running in the rails

Four wheels were positioned and marked on the bottom side of each triangle. 1-inch deep recesses were then cut into the beams, with a router, so that the aluminium wheel frames would sit into them (a router makes cutting such recesses to the same depth very easy). This meant the roof would ride a little less high off the basic observatory structure than would otherwise be the case. Combined with the grooves cut into the roof rails, the bottom of the roof now rides just over half an inch above the rails.

The wheels were then bolted into place and the basic roof frame was manhandled into position at the end of the rails. Standing upright, with the wheels facing the observatory, the roof-frame was lifted and tilted until the wheels settled into the rail grooves. This would be the first real test of the wheel-in-groove design and of the accuracy in my measurements.

For the most part, the test was successful. The roof ran relatively smoothly the length of the rails, but the wheels did have a tendency to stick when being rolled from over the observatory proper into the “rolled off” position. Evidently, the rails weren’t exactly parallel. A quick check by eye showed that the wheels were rubbing against the outer edges of the grooves, so the rails were converging slightly. Some minor work with the router widened the grooves at the appropriate positions.

With the roof frame in position, plywood was cut to fit on top of the frame and around the edges of the roof. A 3” overhang was left all the way round for water runoff. Some help was drafted in to help with laying the roofing felt. This was a real test of the entire observatory’s structural strength as it had to support two (slightly) overweight men during this phase. I wasn’t sure if the whole thing would take our weight or collapse like a house of cards beneath us. We need not have worried. The structure was sound (something of a pleasant surprise to me!)

The weight of the roof was increased quite considerably by the addition of the plywood, nails and roofing felt. I was concerned that one person alone would not be able to roll the roof off or, indeed, back again. This was something I just had to have faith in since I didn’t do any calculations on the roof’s eventual weight or how much load would need to be brought to bear to get it moving (well, who would?…except, maybe, a professional builder?)

Anyhow, my luck was in, and I could just about move the roof using just one hand. Since I didn’t want any old passing opportunist to come along, just roll off the roof and nick any equipment inside the observatory, I needed some way to lock the roof in its closed position. I though about various locks, levers and bolts, but the simplest solution was to use two G-clamps (one either side of the observatory on the inside) to clamp the roof to the rails. It’s surprisingly effective!

Finishing Touches


The roof (before the cladding was added) in its rolled-off position. East is to the right of the picture

With construction basically finished, there was one slight problem – the observatory leaked like a sieve. And the wind howled through the gaps between the roof and the observatory base.

Pine cladding was nailed in position around the outside of the observatory and the roof. The cladding was also placed on the door, hiding the hinges from view and preventing them from being unscrewed. Two wooden strips were then added to the rolling sides of the roof. These hung just outside the observatory walls so as they would not impede movement of the roof but would prevent the wind and rain from blowing through the roof-wall gap

I waited for the next rainstorm to arrive. Typically, I didn’t have long to wait. The observatory still leaked, not as badly but there were definitely some precise areas of water entry. What I found was happening was that water was running along the grooves from outside (where they were collecting rain) and trickling down the joints where the roof rails met at the top of the observatory walls. Drilling a few drainage holes through the outside sections of the rails cured that problem, but the observatory still leaked.

This time the cause was rainwater running down the grooves formed by the tongue-in-groove cladding on the roof. The water actually ran behind the wind-breaking wooden strips and dripped onto the joins between the inner plywood and outer cladding on the lower observatory walls.

Out came the silicone sealant. Every join and joint received a liberal application. A thick strip was also placed behind both wind-break strips to prevent water dripping in that way.

The next rainstorm came and the observatory was...dry. Success at last! That is, until the next time I entered the observatory (a few days later), I noticed dark splotches covered all the exposed plywood surfaces. Fungus!

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