Adding lean-to to a floating slab - proper post footings

[Bolson32]

Hi All,

I'm looking to add a small lean-to to my thickened edge floating slab. I think the city required it to be 18" edges, of which about 12" is below grade. What is the proper footing detail for a post framed lean-to? I probably don't want to do 36" but I also don't want it to rip off during a windstorm.

Anyone done this recently?

 

[Bolson32]

Bump-a-lump. Anyone?

 

[dougf]

I'm glad you posted this. I have a 20x30 thickened edge floating slab and have been researching the exact same thing. The consensus is to do a stand alone lean-to structure up against your building to prevent things issues from frost heave. However, my assumption is that similar footings that are used in the structure for the lean-to would be fine as the ground would freeze and everything would heave in unison, but I could be very wrong. Looking forward to the experts chiming in.

 

[bb29510]

i did this on my back porch, I cored drill through the slab witha 12 inch core. ( sorry, I own one) then I hand auger four foot down and filled with rebar and concrete, now i took it farther than what you need. I had a eight foot , 10 inch pvc with I carred the rebar and concrete to eight foot, and that my roof cover post

 

[Firebrick43]

The footer needs to be deeper than the frost line. What is that number where you live?

Did you insulate under you monolithic slab?

If it is and provide you have a footer that is proper depth then I wouldn’t worry about it

You can use treated wood poles on a cookie or a permacolumn on a cookie as well(preferably imho)

 

[Bolson32]

I'm glad you posted this. I have a 20x30 thickened edge floating slab and have been researching the exact same thing. The consensus is to do a stand alone lean-to structure up against your building to prevent things issues from frost heave. However, my assumption is that similar footings that are used in the structure for the lean-to would be fine as the ground would freeze and everything would heave in unison, but I could be very wrong. Looking forward to the experts chiming in.

That's kind of what I'm thinking. I don't necessarily want to prohibit the posts from heaving and I don't really want it as a separate structure either. I guess I could do that in theory, might be easier than cutting into the siding and flashing a new attached roof. But then I'd have another 3 posts in the way.

The footer needs to be deeper than the fronts line. What is that number where you live?

Did you insulate under you monolithic slab?

If it iss and you provide a footer that is proper depth then I wouldn’t worry about it

You can use treated wood poles on a cookie or a permacolumn on a cookie as well(preferably imho)

My slab is not insulated and the thickened edge is about 18", code here. It's a floating slab though, not to frost depth. 42"

 

[bb29510]

i have seen up north, in unlivable bldg, you can pour a mat, usually heavy rebar, so when the slab heaves, the whole bldg moves. but its not legal for residents

 

[Firebrick43]

My slab is not insulated and the thickened edge is about 18", code here. It's a floating slab though, not to frost depth. 42"

That building is going to move. You do not want to attach anything to it

 

[dougf]

Question for the experts. We know the slab is going to move, will a lean-to with the exact same footings for the posts move with the building as the frost comes without issue?

 

[NUTTSGT]

Are they requiring a thickened slab for a post frame ? How is the building you're attaching this to built ?

 

[Firebrick43]

Question for the experts. We know the slab is going to move, will a lean-to with the exact same footings for the posts move with the building as the frost comes without issue?

They will not move together properly.

On top of it if it’s not shallow frost protected, many jurisdictions only allow a certain size building to be built this way as the thickened slab edge can take the forces. As you get bigger the ability for the edge to do its job is exceeded. It functions no different than a beam. Many places actually call them and similar “grade beams”

 

[billconner]

I don't know size of either existing building or proposed lean, nor use of either, but I'm guessing neither on frost heaving presents very significant life safety issues. I don't think it matters a lot how the posts are set, as long as they'll hold the weight. I'd simply design the lean rafters to rest and "slide" on the header atop the posts, and figure out a way to hold the lean roof on the header against wind without restraining it. There might be slight non-straightnees/lean in the future, probably most noticed in the lean eave. Probably wouldn't bother me most settings. YMMV

 

[Bolson32]

i have seen up north, in unlivable bldg, you can pour a mat, usually heavy rebar, so when the slab heaves, the whole bldg moves. but its not legal for residents

Good thing I'm not living in it. This would probably be the best solution. Essentially tie the two slabs together. Lots of concrete though.

I don't know size of either existing building or proposed lean, nor use of either, but I'm guessing neither on frost heaving presents very significant life safety issues. I don't think it matters a lot how the posts are set, as long as they'll hold the weight. I'd simply design the lean rafters to rest and "slide" on the header atop the posts, and figure out a way to hold the lean roof on the header against wind without restraining it. There might be slight non-straightnees/lean in the future, probably most noticed in the lean eave. Probably wouldn't bother me most settings. YMMV

This is my thought, it's going to shift a bit? Sure. Is it going to matter for the tractor attachments I'm putting under it? Nope. I'm not going to sheetrock it.

 

[Bolson32]

Are they requiring a thickened slab for a post frame ? How is the building you're attaching this to built ?

It's a stick framed building. Haven't actually checked local code yet but I'm betting it's not accounted for.

 

[NUTTSGT]

First and foremost, check out what is required locally, both to code and frost depth requirements.

Once you have that information, start your planning with that information in hand.

 

[Bolson32]

I feel like you're missing the point. Frost depth is not required for outbuildings as was mentioned in my original post. I just built the garage 2 years ago.

I've since been through the code and there's no mention of depth requirements for porch posts on outbuildings on floating slabs.

 

[NUTTSGT]

I feel like you're missing the point. Frost depth is not required for outbuildings as was mentioned in my original post. I just built the garage 2 years ago.

I've since been through the code and there's no mention of depth requirements for porch posts on outbuildings on floating slabs.

You may not need to follow code for frost depths but if you're using posts to frame this, I'd pay attention to what the code is, at least as a reference.

I surely wouldn't want to put an addition on using posts only to have them heave and cause damage later on, requiring more money to remedy the situation. You can proceed however you want and spend your dollars.

 

[Bolson32]

I've done a bit more googling rephrasing as porch attached to floating slab and found some more results. Seems like consensus is to match and attach the foundations or keep them separate. Which does make sense.

Porch off a floating slab - GreenBuildingAdvisor

Hi. My house is built on an insulated floating slab in southwest Wisconsin. I’m planning to build a roofed porch off the west side of the house, and am wondering […]

www.greenbuildingadvisor.com

 

[billconner]

Bolson - I'm not sure if you're planning a slab under lean or just gravel, but if a slab, I'd do same as enclosed. If gravel (which would be my choice) just concrete as deep as your thickened edge.

One or the other or both may move a little. Frame accordingly. You could look at I'm drainage also, keeping water from ponding near the building.

In my area, the floating slab is limited to 600 sf and to accessory buildings - plain IRC. It sounds like you may be beyond that size but still permitted there.

 

[Bolson32]

I already have gravel there, was hoping to just sink a few biscuits in the ground and put posts on them and run a metal roof to the main building. Garage is 16x32 and not conditioned.

I have a lot of tractor equipment, pallet forks, blades, buckets etc that really don't need to be inside, I'd just prefer they're not in a snow bank all winter.

If it shifts...meh, I don't really care as long as nothing breaks. It's not like I need to keep sheetrock from cracking.

 

[ConCretin]

I'm kind of late to the party but I'd just pour a slab similar to the existing one and call it a day. What you want to avoid is differential movement between the existing and new structures. If the existing foundation 'floats' on the frost, the addition should too. You don't want a structure sitting on frost protected piers connected to a floating slab.

 

[billconner]

Posts 12-15' from building on floating slab? I wouldn't worry about differential movement.

 

[Bolson32]

Posts 12-15' from building on floating slab? I wouldn't worry about differential movement.

Yep, posts would probably be 10-12ft.

 

[bb29510]

need to invent some type of ball joint on the existing building so it can move, without ripping your house apart

 

[bb29510]

off topic kinda, when ever we build long building we will have two beams, ususally concrete beams ,sitting on top of each other, not connected with a teflon sheet in between, so the building can move

 

[billconner]

I don't think the differential movement over 10-12' is going to stress the lean to building joint. Movement is not at all a given. It takes specific conditions for frost heave.