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Floor Joist Spacing

by MacCookie
(Berks County, PA)

Joist spacing for 15' x15'

Joist spacing for 15' x15'

I have 16' x 16' footer in my back yard with 6" block walls on it. The northern wall is 16' outside measure long. The eastern/western walls are only 7' long with an 8 ft wide single pitch roof. The previous owner had run plywood for the southern wall and enclosed the remainder of the footer with fencing and used the structure for a kennel. My wife and I have used it for a shed since we moved in.

Now we are going to finish the block walls and put a gambrel roof on it. I want to reframe what had been the roof to be joists for 2nd story decking, full width. Since the 6" block walls are 16' x 16' on the outside, my joists are going to run 15' long on the inside. I then have to position the joists to cover a 15' width.

My question is: In doing the layout, do I position the center joist first and space centers going left and right so the ones against the outside walls have the shortened spacing, or do I start from one side and center space the joists so only the joist of the far side has odd spacing?

Thank you for considering this question. Very little seems to be out there on walls or decks that are irregular measures.

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Mar 27, 2022
On center spacing for shed joists
by: John

It's a great question! You would want to start your spacing always working from one outside edge to the other. You wouldn't want to start in the middle because when you sheet a floor or a roof, or walls for that matter, you always start from an outside edge, not the middle of the structure. This way your sheeting edges will lay in the middles of the joists, rafters, trusses, etc.

I am confused however why, if the outside edges of your block walls are 16', why you would want to frame from the insides of the block walls? (See first picture for the framing inside of block walls.)

You are going to build a gambrel roof, so why not have the trusses be 16' wide so that they rest on the blocks? (See second picture to see what I am referring to.)

I am not sure if my representations of what your block walls would look like but I hope I have come close to what you are wanting to do!

If you have any questions about this or want to discuss further, please send an email to me at

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