Footing depth for a shed (1 Viewer)

[Betteron2wheels]

BLUF – Does a footing for a shed need to be below frost line (30”) in the DC area?

I thought I would ask the Mud community for advice before finish digging.

I’m planning a small add-on to the side of my garage to house an air compressor and provide storage. The exterior will be 146”x80” using 8x8x16 block.

That side of the garage where the shed is going is on a slope going uphill from front to back, the compressor room will end up about 3’ underground in the back with the front being level with the ground and front of garage. The garage and shed will share a wall.

The bank is dug out most of the way, I’ll probably dig out an extra 12” or so for clearance to work.

My thoughts were to dig a trench and pour a footing, then lay 5 courses of block to bring the rear above ground, then frame in the walls on top of the block.

I don’t know sh!t about pouring a footing, the many youtube videos I watched helped but no one seems to dig their trenches very deep.

I looked it up and local frost line is 30”, does it really need to be 30” deep?

 

[iwashmycar]

Yes, bottom of footing needs to be 30" below finish grade.

Id probably throw in some rebar to connect at least the rear footing up through the block to hold the dirt with that much unbalanced fill

Also make sure you regrade it so water goes around your new portion and doesnt just dam up behind it.

 

[MrMikeyG]

I know nothing about your local codes, but there may be a certain size of accessory structure that doesn't even require permits. As in; 200 sqft and up = permit while 199 and under does not. Something like that. That being said, it sounds like you are on a slope. I have seen some very questionable things out here even on a "flat" yard. I agree with the above in that you should go the extra mile for the low side for sure. You don't necessarily have to do a full length footing and grade beam for a shed though.
A typical footing for a light dead load is a block of (poured) concrete 18x18x12" min. with a 2 or 3 cross grid of 1/2" rebar. in your case I would dig down (your hole needs to be 42" now to get the 12" depth footing below the 30" frost line) Then get a section of sonotube (stiff cardboard tube 8" diameter) cut that so that top of tube is maybe 3-4" above prevailing grade. You need to tie some "L" sections of 1/2" rebar into your footing grid elevated off of the bottom by a couple inches (2 "L" per should suffice here) The sonotube slide down over the "L" and you pour everything at once to avoid a cold joint between footing and post (sonotube portion). Trying to type this out makes it seem really complicated, but the digging is likely the worst part. When the concrete is all set, you end up with 4 8" (or whatever you decide based upon floor system needs) concrete "post" top to anchor your floor system to and build up from there.

This picture shows a really well built system. Since you will be down deep, your footing will be "earth formed" it also looks like they have a rebar "box" inside the footing instead of just a grid. I'm guessing this is going to support some serious weight, and is on a job where a footing form inspection was required.

 

[iptman]

Paging @Tikicruiser ....

I think the frost line in our area is only 18 inches. 2 feet for safe measure. Jeff would know for sure though.

 

[iptman]

@Tikicruiser says 24" in our area.

 

[MrMikeyG]

@Betteron2wheels I didn't see your picture when I posted earlier. Will the door to your new space be on the downhill side (where the shovels are) or somewhere else. Also, will it have its own door or will you cut a door opening into the cmu(concrete masonry unit/cinder block) wall in the photo?
sorry my reading comprehension isn't what it used to be apparently. It seems like your biggest area of potential problems is that uphill side; I say drive on with what you were thinking (footing and cinderblocks) to retain the earth on that end.

 

[bkfj40]

30” in dc. 24” in lots of other areas.

Definitely Use some vertical #4 rebar from the footing up into the CMU. An easy way to do this is to pour the footing and then drill and epoxy the vertical into place. This allows you to verify where the cells of the CMU will be. Typically, you would also run standard joint reinforcement every other course in the CMU. Ideally, topping off the CMU with a bond beam and a #4 running horizontally. If you need a sketch, let me know

 

[Betteron2wheels]

@Betteron2wheels I didn't see your picture when I posted earlier. Will the door to your new space...

The door will be where the shovels are. I might put a small window on the existing wall between the 2 spaces if time and patience permit.

 

[MrMikeyG]

@bkfj40 I knew there was "stuff" going on inside cmu walls. I have zero experience building with them, so I didn't want to start commenting way outside my wheelhouse.
What do think regarding moisture on the OP's uphill side? Is dimple fabric necessary, or just some roll on asphalt (Henry or equivalent)?

 

[Betteron2wheels]

Regarding water, I already have water intrusion on that side and around the back corner of the garage. It seeps in where the block wall meets the slab floor. I'll pick a good sealant and plan to run a french drain along the outer parameter of the addition and fill in a decent amount with gravel.

 

[Betteron2wheels]

bkfj40 if you are willing to provide a sketch, that would be great!

 

[fjman]

I think I remember @Felix doing a shed build thread here a long time ago. That would be Alexandria though

 

[ace10]

Without getting too deep into your project....

Solve your current and future water/drainage problem first.
Then figure out how you are going to properly tie back into the existing structure.

Properly setting CMU block below grade, in a very narrow trench, is an absolute bitch.

 

[Felix]

I think I remember @Felix doing a shed build thread here a long time ago. That would be Alexandria though

It was about 13 years ago. And I'm no help - my shed was so small, we just did a foundation of 2 6x6 skids set in compressed gravel.

I will add, just for posterity in case any Alexandrians are building sheds in the future, there's an 8 foot height limit under which you do not need a city permit. That 8 feet is measured by the INTERIOR wall height. Not the exterior wall and certainly not the roof's peak. Not all city inspectors know this, so don't be surprised when you get a stop work order.