Canteliever beam/girder over piers (pier and beam foundation)?
I’m building a guest dry cabin / outbuilding on my property (area with no building code/inspections). Plan was a 24×14′ shed roof on a pier and beam foundation with a small loft I started digging my holes for the piers and footers (36″ diameter hole with 30″ footer with 12″ sonotube) and ran into a problem. I planned to have 8 piers total, 4 per side. The holes for the front and back right most piers contain a lot of wood. I think someone buried stumps there at some point before I bought it and those end piers fell right on the edge of the stump dump.
I was thinking about shrinking the cabin and having it use only the 6 piers that have no wood. Center of first pier to center of end pier I’d have 16′ which is a lot smaller than I want. I was thinking about cantilevering the beams over the end piers by no more than 2′ to gain back some space. But this only really seems to be commonly done with decks. Thoughts on doing this with a cabin? The beams are going to be made from 3 2x12s perhaps with some osb or something sandwiched between planks (~ 5″ x 11.25″ beam). Being a shed roof cabin, the rafter tips and tails will land on the beams, so those two load bearing walls would be on the cantilever. I’ve read that it’s possible for a beam to cantilever a distance equal to 1/4 the beam span, but I’m not sure what span this refers to. The span between piers or the total length of beam?
Unfortunately I don’t have many good building sites on the property – I have another spot cleared already but it is earmarked for an eventual garage/shop. So this general area is where I’d like the cabin to go. It’s the perfect spot, except for the darn buried stumps.
I’ve been trying to manually dig down to below the stump dump to virgin soil cutting wood out with a sawsall, but it is deep. I’m at least 10’+ deep and still encountering wood and it’s starting to feel awfully sketchy. Although I think I’m getting close. I’m exploring other options like shifting the cabin to the opposite side and adding piers opposite the stump dump but that would require digging very close to some large trees. Considering just renting an excavator to remove them as well. But I’m wondering if the cantilever idea is feasible and if it is how much past the pier can a beam made with 3 2x12s and some osb overhang? Other info that is probably relevant: Plan was to use 2×12 floor joists 16″ oc sitting on top of the beam/girder. I’m in Alaska so I’ll need to account for snow load on roof.
Thanks a lot!
Replies
In the original post I talked about the beam being made of 3 2x12s. But I don't see why I couldn't make it even larger if that helps make the cantilever more structurally sound. 4 or even 5 2x12s?
It’s great that you’re building a guest dry cabin/outbuilding on your property. The first step in building a pier and beam foundation is to excavate the area where the piers will be placed. The hole for each pier must be at least 2 feet wide and 3 feet deep. Once the holes are excavated, footings (a solid base for the pier) are created using gravel and concrete1.
If you have encountered a problem with your piers, you can try to remove the stumps or roots that are in the way. If that is not possible, you can move the pier location slightly so that it is not on top of the stump.
It is possible to cantilever the beams over the end piers by no more than 2 feet to gain back some space. This is commonly done with decks but can also be done with a cabin. The span table’s rule of thumb without an engineered design is about an inch of depth of wood per foot of open span. 6 (actual 5 1/2) inch wood should span 6 feet safely for a house. You can also squeeze the piers together a bit more and cantilever the ends of the beams and joists. Having 6 feet between piers and 2 feet overhanging at each end works just fine to support
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The cantilevered beam and girder system spans over piers, commonly used in pier-and-beam foundation construction, distributing loads and providing support.