How To Build a Shed Foundation

This will be a multi-part series featured on this blog on how to build a shed. This particular section will cover how I build a shed or deck foundation using post / concrete construction. These types of floor joist systems are very strong and will last many many years in the hot Florida weather. In this build, I am choosing to build a lean-to style shed that will sit next to my existing cook lofted barn.

This will be the location of the new shed. Next to this existing 10x16 cook lofted barn
This will be the location of the new shed. Next to this existing 10×16 cook lofted barn
Theres Jax my super awesome helper on all of the backyard builder projects
Theres Jax my super awesome helper on all of the backyard builder projects
Location of the new shed lean-to style building.
Location of the new shed lean-to style building.

On the initial planning of this shed, I wanted to utilize one-floor skid of the existing cook barn building. You can use an existing 4×4 skid by attaching another 4×4 to the existing skid with 6″ lag bolts. After you attach a new 4×4, add a 2×6 ledger to the skid extension using lag-bolts.

A shot of the added 4x4 skid extension under the existing cook shed.
A shot of the added 4×4 skid extension under the existing cook shed.
I am using these GRK Fastners Lag Bolts for this build. THey use a T-30 HEX head for fast and slip-free driving.
I am using these GRK Fasteners Lag Bolts for this build. THey use a T-30 HEX head for fast and slip-free driving.
I am using these GRK Fasteners Lag Bolts for this build. THey use a T-30 HEX head for fast and slip-free driving.
I am using these GRK Fasteners Lag Bolts for this build. THey use a T-30 HEX head for fast and slip-free driving.
A closeup of the 4" lag bolts I am using. These bolts are excellent for fast penetration of the lumber and they really grab as they attach.
A closeup of the 4″ lag bolts I am using. These bolts are excellent for fast penetration of the lumber and they really grab as they attach.
Illustrating the length of the fastner.
Illustrating the length of the fastener.

In this step, I used some scrap 2×6 I had to attach the lower skid extension. This will act as a nice ledger base for the joists hangers to connect to. I added a 4″ lag bolt every 18 inches.

A ledger board 2x6 is added to the new 4x4 skid extension using lag bolts
A ledger board 2×6 is added to the new 4×4 skid extension using lag bolts
A closeup on the lag bolts installed every 18 inches
A closeup on the lag bolts installed every 18 inches
A 6x6 post is added every 8' on the adjacent side of the building. This gives a solid foundation for the flooring system.
A 6×6 post is added every 8′ on the adjacent side of the building. This gives a solid foundation for the flooring system.

I dig a hole deep enough to accommodate the new 6x6x2′ cutoff post. I can add and remove grade as needed in the hole and compact to create a nice level base. Once all of the leveling is complete, I add a 50lb back of concrete to the post hole to ensure the post stays where I intended it.

Who doesnt like a fun dirty shovel shot. This reminds me how much I hate digging and ground work.
Who doesn’t like a fun dirty shovel shot? This reminds me of how much I hate digging and groundwork.
Showing the deptch of the post hole. Every hole depth will vary based off of the initial base grade.
Showing the depth of the post hole. Every hole depth will vary based off of the initial base grade.
The 6x6 I am using to complete the posts in this project. These 6x6 posts are awesome when it comes to supporting a heavy load.
The 6×6 I am using to complete the posts in this project. These 6×6 posts are awesome when it comes to supporting a heavy load.
The first ridge joist is added to the post. I make sure I level this post a little proud to accept setteling as the weight increases on the building.
The first ridge joist is added to the post. I make sure I level this post a little proud to accept settling as the weight increases on the building.

4 -4″ Lag Bolts are added to the corners of all ridge posts. For added security, you can notch the 6×6 and allow the joists to rest atop of the 6×6. Because there will only be light-duty equipment in this new shed, this will be plenty of support.

A 6' level is used to ensure the 8' joist is level
A 6′ level is used to ensure the 8′ joist is level

Pro Tip: Level as you go and level everything. I always make it a point to keep my levels on my working surfaces to act as a gauge. If you skip these important steps, you will have a crooked building and wall framing and sheathing will become your worst enemy. LEVEL LEVEL LEVEL!

I bring the level outward to the edge to ensure I am a little proud on my elevel marker. The building will settle as weight is initially brought onto the structure. THis allows for a perfectly level building when the job is finished.
I bring the level outward to the edge to ensure I am a little proud of my level marker. The building will settle as weight is initially brought onto the structure. This allows for a perfectly level building when the job is finished.
Level bubble showing a slight varience to ensure settling makes it straight.
Level bubble showing a slight variance to ensure settling makes it straight.
Measure out 8' and make a mark on your ridge joists. This will be the location of your next main support.
Measure out 8′ and make a mark on your ridge joists. This will be the location of your next main support.
Leveling the next joist
Leveling the next joist
Showing the attachment of the joist to post
Showing the attachment of the joist to post
Joist progeress
Joist progress

This photo shows the other two 6×6 posts installed and leveled on the exterior side. The center post acts as a support for the two outer 2x6x8′ ridge joists. An attachment photo below shows the connection at center.

The center outer post with two ridge joists attached using 4" lag bolts
The center outer post with two ridge joists attached using 4″ lag bolts
Top-doen view of the center post and it's attachment to the joists
Top-down view of the center post and it’s attachment to the joists
Joist hangers by USP
Joist hangers by USP

I space these 2x6x8 floor joists at a maximum of 16″ on center. ANy wider than that, you will experience deck flex and nobody wants that. Don’t skimp on the joists. You can never have too many. 16″ O.C. is the minimum here.

Joist progress photo from front
Joist progress photo from front
A shot of the joists attached to the ridge joist and ledgers. I use 2" deck screws to make the attachment and for easy adjustment if needed.
A shot of the joists attached to the ridge joist and ledgers. I use 2″ deck screws to make the attachment and for easy adjustment, if needed.
A shot of the joists attached to the ridge joist and ledgers. I use 2" deck screws to make the attachment and for easy adjustment if needed.
A shot of the joists attached to the ridge joist and ledgers. I use 2″ deck screws to make the attachment and for easy adjustment, if needed.
A shot of the joists attached to the ridge joist and ledgers. I use 2" deck screws to make the attachment and for easy adjustment if needed.
A shot of the joists attached to the ridge joist and ledgers. I use 2″ deck screws to make the attachment and for easy adjustment, if needed.
Joist hanger attachment
Joist hanger attachment
Front view of the completed floor joist system Ready for plywood sheathing.
Front view of the completed floor joist system Ready for plywood sheathing.
Front view of the completed floor joist system Ready for plywood sheathing.
Front view of the completed floor joist system Ready for plywood sheathing.
Rear view of the flooring system
Rear view of the flooring system
Connection corner
Connection corner

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