Pilings in the frozen north

[Boondox]

I need to build a carport addition off the side of the garage so the car can reside outside and the tractor in the garage where she belongs. The problem is that when I started digging down with the backhoe to plant the pilings, I hit solid granite ledge only 18" down. Obviously this is not below the frost point, and while the granite won't heave, the water above (and the base of the piling) will.

After a couple of beers I was struck by an inspiration. What if I use hinges where the rafters attach to the side of the garage instead of some rigid method? That way the pilings could rise and the whole thing would flex at the hinge. i'm thinking with this approach I might want to go with 16" or even 12" OC with metal roofing at a fairly decent pitch...

Any other ideas out there?

Pete

 

[RobS]

Pete, can't you fasten the piling directly to the granite. Surely that granite is a better footing than anything you or I could fabricate. Just a thought, I really have no experience with big rocks under the soil /w3tcompact/icons/smile.gif

 

[RichZ]

Pete, although, I'm no expert, I had a thought similar to Rob. Could you secure a metal boot, like you use to secure a post to concrete, to the granite? Is there a way you could drill into the granite to secure the metal boot?

 

[Boondox]

<font color=blue>Is there a way you could drill into the granite to secure the metal boot? </font color=blue>

I'm not sure; that's why I was asking. /w3tcompact/icons/laugh.gif Has anyone done anything like that?

Pete

 

[MarkV]

Pete,

I think the other guys have the right idea. Here is what I would do if I were sure I had a sizable hunk of granite under there. I would use a hammer drill to drill a hole into the granite about 8” if possible. Then get a metal post base (the kind that allow an anchor bolt through the bottom and have sides that nail to the post) for the size post you will be using. You take a piece of threaded rod and epoxy it into the hole for the post base to bolt to. With this method your post will not heave unless the granite moves. This is a fairly common method for working on rock ledge. You may even do a net search and find pre packed capsules of epoxy that get dropped in the hole and are broken when the rod is inserted.

Of course there is also dynamite./w3tcompact/icons/eyes.gif

MarkV

 

[Carl_NH]

Pete,

I've got granite ledge too, and when faced with this either drill and attach a metal bracket made for posts as stated above, or do what I did, and put in a 6 or 8" sonatube directly on the ledge with 2-3 holes and rebar drilled into the ledge and fill it up to ground level then attach the post support into the concrete.

It sounds like you only have a few holes? so this would also work for the others if you need to go down to 4' range too.

If your main garage is a metal roof and this is going on the non gable end then the snow load coming off that roof could put a sizable load on this carport roof. If you use metal on the carport this will prevent the snow build up too, and pending the width 2x8's 16 o/c would be OK but I think 2x10's are better suited 16" o/c.

Carl

 

[DaveNH]

The ledge won't move.
Over here in the granite state of New Hampshire,
I've seen houses that had foundations that incorporated the ledge. The problem will be the footings that don't sit on the ledge. There, you'll have to install the 48" (assumed frost line) deep footings.
You surely don't want half of your building to move./w3tcompact/icons/shocked.gif
I do not recall see any special methods of attachment.
Looks as those the foundation was poured directly onto the ledge. As for the rafters, I check out the local code.
You get some snow up there don't you?

 

[Corm]

Hi Pete! As the others have said, granite ledge makes as good a footing as you can possibly get, it won't ever move! If the ledge is pretty flat and level, with 18" of fill around your posts, I don't believe you need to go through the trouble of drilling holes in it. If the ledge is not flat or level, then drilling a couple of holes and inserting re-bar would be advisable. If the Mrs were to back into a post (excuse me - if you were to back into a post/w3tcompact/icons/smile.gif), it would break before it would move on you.

As far as what to use from the ledge up to ground level, sono tubes will probably work, but with only 18" of fill around them, I'd be concerned that frost may pick them up in the spring. I'd build some 24" pyramid shaped flat topped forms out of plywood - 18" square at the bottom, and 10" - 12" square at the top. With forms of that shape, there is no way the frost can move it. Think of how frost moves a post - it squeezes it, then lifts it in the spring as the frost comes out of the ground. With a pyramid shaped form, the frost will lift away from the post as it lifts up in the spring. Through the years I've tried Sono tubes, using a pipe within a pipe, using ridgid styrofoam around the post - nothing works like the pyramid shaped forms. I've never had a post move or lift using that method, and have won a couple of bets from skeptics.

Sorry for the long post, just tying to help. If you need a hand, let me know!

Corm

 

[Boondox]

Hey Ed! Long time no see!

Nothing on my property is flat and level, including the house! My concern is that the exposed granite is so irregularly shaped (bumps and holes, though no cracks -- it appears quite solid) that water would fill the holes and lift anything not attached. Guess I need to do some drilling, though was hoping to do this without buying another tool that will rarely see use.

When are we going to get our families together?

Pete

 

[Boondox]

Thanks, Carl and others. Sounds like I really need to do some drilling, as the granite is uneven and sloped pretty steeply in this location. Bummer.

The addition will be coming off the side of the garage, pretty much under the windbreak, so the snow load will be minimal. Thanks again.

Pete