Porch Foundation Design and Construction
I’m building a house with a wrap around porch on a raised slab foundation.
Simple rectangle 52′ x40′.
2×6 @ 24 OC exterior walls with ZIP sheathing and dense packed cellulose in the walls.
Slab will be 24″ above grade using a block laid stem wall. Blocks are the way its done here in NC instead of poured seems like. The slab will be insulated like this… https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2019/03/25184350/slab3-700×998.jpg
Trussed roof and I can have the porch roof integrated into the truss package.
What is the best way to build the porch foundation? Wood framed and open underneath the porch or build a raised slab for the porch as well? Aesthetically, I like the look of it being open underneath with stone or brick columns and lattice type panels in between (see attached pic). Considering the insulation detail and the thinner “shoe block” at the top of the stem wall, can I still attach a ledger board at that point on the stem wall to wood fram the porch? Or is the thinner block at the top a problem? In regards to building an additional raised slab for the porch, this would mean building a stem wall, back-filling it, and pouring the slab on top. This would completely encase the main stem wall of the foundation in back-fill, soil, dirt, etc…to the point where it could not be visually inspected for termites. Termites are an issue here. Is this a problem or am I over thinking this?
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Replies
Jason,
I'll give your post a bump. FWIW, I think pouring a raised slab just to support the porch will involve a lot of unnecessary expense.
Steve,
Thanks for the reply. One of the things I'm wondering about is the cost of each method and which is more economical, but mainly I'm wondering about the ledger board attachment considering the thinner concrete at the top of the stem wall.
P.S. - It appears that we're climate zone neighbors. :)
Hi Jason.
I am just a homeowner with a passion for green building. So keep that in mind. Frame construction is likely to be your most cost-effective option. I’ll let one of the professionals comment on the ledger board issue.
Steve
Steve,
I certainly appreciate the input. I too am just a homeowner who loves to be as involved as possible.
I should have mentioned the total size of the house. It's a 52' x 40' rectangle. So it will be a fairly large porch since it will cover a 52' side and a 40' side and be about 7' to 8' deep. With that in mind, I've gotten some contradicting info on which would actually be the most cost effective way to do it. Hence my reason for posting the question here, along with question on the potential ledger board issue. I'm sure the slab being raised plays a significant factor. Perhaps one of the professional builders will comment on this.
Anyone else have any thoughts or input on this?
Jason, I like raised slabs for conditioned space, and have designed the same for porches as well. So far I have always gotten strong pushback from contractors; they say it's too expensive to fill and compact the sub-slab area. Either way can work; I would suggest pricing it out both ways and see for yourself which makes more sense.
If you're concerned with attaching the ledger in a stick-framed floor, you could set the whole porch on piers, including the end closest to the house. Helical metal piers make this especially easy.
Michael,
Thank you for the advice. My concern regarding the ledger wasn't an attachment to a stick framed floor, but to the thinner top portion of the block foundation wall. I'm attaching an image of my planned insulated slab detail. The ledger would sit against the thinner "shoe block" at the top of the foundation wall and would be backed by rigid foam instead of concrete as it would without the slab insulation. I don't know if the thinner portion of the stem wall creates a structural issue for the ledger or not. good question for a structural engineer I suppose.
On different note, would, or have you had any concerns in the past regarding termites in a situation with the raised slab for the porch as well considering you cannot visually inspect the foundation any longer with it being completely "encased." See my second attached imaged with a raised slab for the porch. The red circled area can no longer be inspected and termites seen before they have a chance to cause major damage. Perhaps I'm concerned about a non-issue? Overthinking is major weakness of mine...
Thank you again for the input. It is sincerely appreciate.
Jason, sorry, I meant "for" the stick-framed floor--of your porch. I would have no concerns about attaching to a reduced-thickness stem wall if it's poured concrete. I don't have experience doing it with block walls, but I would be more comfortable attaching to filled cells. Or making the porch floor freestanding.
Good question about termites. We don't have them around here so I don't think about them much, though I do often spec termite shields to be safe. Maybe include a metal flashing let into the wall and the slab?
I agree with Michael in regards to anchoring to an open core in a block wall. You could have the bolts embedded into the block and core fill the top 3 courses and make it a solid connection. It takes a bit of planning to get them placed in the right spot, but so does the rest of the house. I would think that a framed porch would be cheaper than going with concrete, but maybe prices in your area are more favorable. If you do go with a concrete porch, you could have an exposed aggregate finish to have something different. As far as having a brick look with a stem wall, you could have the masons put a brick ledge at the post locations and put regular brick at those locations. Or have them bump the wall out at the post locations and install thin brick to get your look.