Foundation planning for a self-supporting porch
I am curious as to how a pro would design a basement foundation to allow for the addition of a shed roof (porch or carport). I have significant snow loads in my area (90#) and I think it makes the most sense for the structure to be self-supporting rather than be dependent on a ledger board. Regarding the adjacent basement walls and foundation, does anyone have a suggestion as to what the most prudent approach would be when pouring the foundation and walls? I know the backfill around the house will be considered “disturbed soil” so I am curious to hear what might a good approach. Would pylaster bumpouts be a cost-effective solution? Or perhaps extending the footing out further so that future sonotubes could be driven down to this footing and poured?
Thanks in advance for any details folks might be able to share.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
John,
Most pros would support it on the house. At worst the mechanical attachment of the rafter and ledger intersection might need to be engineered, but that pales in complexity compared to an independently supported structure.
To be blunt, you don't see many self-supporting porches or decks outside the comment sections of FHB and JLC. The idea gained popularity due to a cluster of structural collapses caused by poor ledger detailing. A problem which is easily remedied by following the new prescriptive code requirements for attachment and common sense flashing.
Self-supported additions bring with them the problems you are asking about plus those of differential movement from the main structure, shear-bracing, continuity of insulation and perimeter drains etc. You are still going to have to attach the self supported roof to the main structure anyway. Why not make that connection lead-bearing?
Thanks Malcolm. Those are good counterpoints. I will draw up some details in Sketchup and post them here. Here is a good picture of a ledger failure that happened in the area:
http://www.jhnewsandguide.com/jackson_hole_daily/local/laurance-s-rockefeller-preserve-center/image_7b214aba-dc33-566d-9afb-8c8ae51c38d4.html
I don't have a ton of experience detailing posts and headers inside a SIP but I need to start looking at that. Lots to consider.
John,
Malcolm has made some good points.
Here's my view: Supporting the ridge of a shed roof (as for a carport, as you propose) is entirely different from supporting a deck ledger. Many decks are designed for drainage; the same cannot be said of roofs. Problems with wall rot near deck ledgers occur because of the need for decks to drain.
Roofs must be waterproof, so the ridge of a shed roof that is attached to the wall of house can be flashed easily. The vertical leg of the flashing is lapped by the WRB above the roof, while the "horizontal" leg of the flashing (which of course isn't really horizontal) laps over the top of the roofing.
The eave of the shed roof will probably be supported by a beam and two posts. These posts will be some distance away from the house foundation; they will have independent footings, of course.
It seems from your description you are deviating from what is in the IRC for building construction. As a result it will need to be engineered. The engineering is for both the gravity loads but also the lateral loads. Your engineer or architect will be able to design the connections for the required loads.
John,
I didn't realize it was a SIPs house. That changes things considerably. I have no idea how you secure ledgers to SIPs walls. As Tim said, it's probably going to necessitate the input of an engineer whatever course you take.
Good luck with your build!