Dome build: Lowering the hemline.

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I have discovered there are much wider versions of my plastic DPC roll which I used as a mini skirt on the dome's bare midriff. 30cm would be more than enough. Then my wife suggested I cut strips from some heavy, rubber pond liner. This would be much stiffer and heavier than the thin plastic strip.

Which would help to keep it in place compared with the plastic. Which blows inwards far too easily in windy weather. Being much too short does not help because there is nothing to resist it. First I need to add some heavy sloping mouldings to the bottom of the dome to push the skirt out to clear the octagon top ring. This would reduce potential friction issues with the skirt on the roller brackets. Though quite what it will look like is another matter!

If full clearance is achieved then plywood could be substituted for the softer 'curtains.' An earlier plan was to have an upstand to foil the wind leaking under the skirt. This would be critically dependent on avoiding water ingress.

An alternative would be to use aluminium  windowsill flashings. These are available in considerable depths. Though 90mm seems about the maximum off the shelf from local DIY outlets.

Aluminium flashings are lightweight, long lived and weatherproof compared to diagonally ripped 2x4s on the table saw. The short upstands could be tucked under the bottom of the ply cladding for perfect drainage. But how would I cut them on the diagonal? You can't just go at them with a pair of metal shears because of all the angles. They'd need to be cut on the diagonal to match the next one along. Plus a suitable tab on the edge for pop riveting the sloping joints together.

I was given some joining plates too which might be useful. The pond liner, rubber skirt would hang down from the underside of the bottom of the profile. Probably using washers to spread the loads on the pop-rivet fixings. Though nuts and bolts would offer greater control. I'd have an outward gain in footprint of about 90mm. That's 3.5" increase in radius or 7" on diameter. The present, plastic skirt drags on the roller bases in places. So freeing them up should improve rotation.

I chose a fine toothed, arc-shaped blade for my DeWalt multi-cutter to cut the diagonals on the flashings. I now have 16 x 1m long flashings and must measure the angles required. Though not immediately as it is blowing a gale all day today.

From my computer desk I have been watching the tarpaulin which I spring clamped over both shutters. Removing hundreds more staples didn't appeal. First it rose at the top and then became more baggy lower down as the day wore on. After earlier lifting I faced it directly into the wind and this seemed to work best. It lifts, but then falls straight back into place. There are only light showers so I'll leave it to do what it will.

The mounting is well covered inside the dome. If the dome was better sealed around the base the shutter tarpaulin might not lift so readily. It could be internal inflation or it could be aerodynamic lift over the dome. At least the veranda doors are keeping some of the wind out. I also bought some sturdy bolts to keep them shut.

Today's wind and afternoon rain had a much lower impact than past episodes. The observatory floor was completely dry, but then, there was no cloudburst today.  If I can solve the problem of sealing the joints properly it could be worth continuing. After carving the joints flat I shall have to be sure they are completely dry before applying more sealant and trying to work it in. The Sikaflex 291/591really doesn't like filling its own holes.

Meanwhile I can get on with extending the skirt. Ideally I need a jig to guide the saw diagonally across the flashings. Not easily done with a Z-shaped folded length of thin metal.

Click on any image for an enlargement.

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