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With all sorts of weird ideas about the necessity for 180° hinges floating about in my mind, I cycled the 10 hilly miles to a DIY outlet. Where I found some nicely sturdy hinges of ample thickness, width and girth and then rode home again.
It was only on arriving home that I discovered my lack of CSK head machine screws of suitable length and diameter. Otherwise I had made a sound choice in the hinges. Having clamped a board to the hinge and the hinge to the side board, I was delighted with the action. Which laid the [simulated] open hatch completely flat on the bare obs. floor to the south without actually hitting anything. Thin rubber pads might be useful if air pressure alone does not cushion the fall. I proceeded to hack out a rebate to bring the first hinge flush with the board running alongside the stairwell. Round head machine screws were then used to temporarily hold the hinge in place.
Four 77cm [30"] lengths of near 12cm wide, larch floorboard were slightly too wide to fill the 42cm deep gap. Which allowed me to experiment with a 'tongue' projecting between the ladder handrails. I had decided to run the boards across the gap at right angles to the obs. floor boarding. My fuzzy, engineering logic suggested that four full boards would be stiffer than only two straps holding four short boards together. As would have occurred had I followed the E-W line of the rest of the obs. floor boarding.
Making the hatch/hole any narrower would have prevented my hands from safely following the handrails of the 60cm [24"] wide ladder. 77cm is quite an unsupported span but the larch floorboards are of ample thickness at 27.5mm. [1.1"] I shall proceed to complete the hatch and fit the second of the three hinges purchased. The third hinge may prove useful if two seem too flexible in practice.Gluing the boards edge to edge and then gluing and screwing the two straps seemed like a good idea for extra strength. The hatch has been left clamped up overnight under a tarpaulin in case it rains. [It did!]
Tuesday: I was just trimming the ends of the hatch boards with the router when it started raining hard. Earlier I had centered the ladder in the obs. floor cut-out as it seemed to have drifted 3cm to the left. The hatch is now pretty weighty to carry 'upstairs' with nearly 14' of 1"x 5" larch boards all stuck together!
The lower images show the hatch trimmed and the hinges temporarily screwed in place. The hatch is remarkably easy to manage with the hinges taking most of the weight and alignment duties. It certainly drops with a bang when released from vertical but that can be overcome. It feels very solid when I walk back and forth. I temporarily pushed a couple of lengths of 2x6s in near the pier posts as extra support joists. Just to spread the load between more of the planks while maintaining isolation clearance. Once the hinges are flush only the slight bulge around the hing pin will protrude from the obs. floor. Being low and well rounded it should not present a serious trip hazard. Particularly once the hinges are sunk to their correct depth. [Flush.]
The overall dimensions of the hatch ended up being quite modest. I had imagined there was a far larger hole in the obs. floor. Notice how little trimming was required to get the board nearest the ladder to close off some of the gap while retaining a full width of floor board. I used the jigsaw, with a 30° angle on the base plate, to undercut the notches to clear the ladder's handrails. Thankfully, my fears that a large and very heavy hatch would need mechanical assistance to open and close were completely unfounded.
Work has been interrupted at irregular intervals by the rain today.
Click on any image for an enlargement.
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