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Old 06-26-2013, 02:24 PM   #1
Join Date: Jun 2013
Location: Seattle, WA
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Post/beam floor and foundation

I'm planning to convert a small bedroom and the garden shed below it into a functional staircase, and I need some advice on how to deal with the floor.

A 2-bedroom addition was built on the upper level of my 1-story-plus-daylight-basement house years ago before I bought it. The floor joists underneath the rooms span from the original exterior wall to a 4x10 beam supported by three posts which make up the new exterior. The area between the posts was infilled in with 2x4s, plywood, and siding to create an unconditioned garden shed. A single concrete slab goes from the original foundation past the three posts, becoming the floor for the shed as well as an outside patio.

Since our current "staircase" would better be described as a ladder, I want to convert one of these bedrooms and the shed space below into a decent set of stairs down to the basement.

However, the shed has some moisture issues I'll need to deal with. The non-pressure-treated bottom plate for the infill sections of the exterior wall is sitting on a not-terribly-flat slab which runs continuously from inside to out and isn't really sloped away from the house. I also would be surprised if there is a moisture barrier under the slab.

I'm not sure what the best course of action is here. I haven't been around post-and-beam exterior walls to see how this is normally done, but I imagine the inside-to-outside slab isn't ideal. I would imagine the best way would be to have separate interior and exterior slabs with good drainage between them, but I'm not sure if I can do that without having to take down the siding. But maybe it would be perfectly fine to leave the walls/floor alone and build a staircase with a good vapor barrier on the underside? What do you think? What more info do you need? Thanks!

- current foundation
- section view of shed and bedroom addition
- section sketch of proposed stairs
- photo of floor and wall in shed
Attached Thumbnails
Post/beam floor and foundation-img_3903-resize.jpg   Post/beam floor and foundation-img_3900-resize.jpg   Post/beam floor and foundation-img_3901-resize.jpg   Post/beam floor and foundation-img_3899-resize.jpg  


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