I built a loft ladder for my little barn this fall. 5/4 rough white pine
(very soft). Both the treads an stringer were just over 4" in width.

The steps were attached with twin through-tennons, wedged and housed in a
shallow dado. The ladder is about 18" wide, including the stringers and
about 7' high. It's overbuilt and rock solid. Based on my experience with
pine, 1.25 x 5 with oak is way more than enough.

Whatever joinery method you use, I think wou want to carry the weight of the
tread (person) in a shallow dado. A simpler alternative would be consealing
screws beneath a plug, Just insert a wide dowel in the underside of the
tread to offer some side grain for the screw to bite into.







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