i had a similar issue 2 yrs ago the best available space was in the attic. i was lucky the where the roof peak was a height about 10' my bench work 50" high with a 12" high backdrop the layout was roughly a 8x8x8 u shaped 16'' wide with a return loop built behind the back drop(that was rough) it turned out to be a horrible place for myself the summer it was 100 degrees from 10am to around midnite, in the winter it would average about 30 degrees it was an old house with no attic insulation then the hornets and the bats what a nitemare
now we are in a different place and the train room is a spare bedroom at the moment the height is 51 inches high, which i thought would be perfect i'm 6'4 . i have a 5yr old who is into trains and it is almost a foot higher then him so having him operate is difficult and i have a section that is about 8 feet long 16" deep that the track is operational so i did a test run operating that section after about 15-20 minutes it started to bother my back. so i'm hoping to lower the benchwork to 41" and as you mention use an office chair. the floor has a medium carpet which might make it difficult to roll around on so i will lay a piece of osb on the floor. when the height is lower i'm going to go thru and touch up on ballasting etc where it did not matter that much because of the height some of the track is behind a long building so you could not see the bald spots. luckily the bench work is all door slabs on shelf brackets so lowering it should not be that difficult, whats wierd is i'm more content with a smaller layout.
k time to go do Bible reading.
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