Pouring post footings inside an existing crawlspace
I live in Iowa and we built a single room addition on a crawlspace. I am wanting to add support underneath the floor joists in the addition to create a firmer floor.
To add support underneath the floor joists, we will need to run a beam/or LVL over a 30 ft span, which will require posts or jackposts to support.
If I pour footings to set the posts in, do the footings need to extend all the way under the frost line?
The crawlspace foundation walls are 48 inches on top of a 6-inch 2 foot wide frost footing. The total depth of the foundation is about 42 inches below grade (12 above at most places). Additionally, I poured a 3 and 1/2 inch concrete rat slab over the crawlspace floor.
The crawlspace is sealed and I am running HVAC underneath to keep the space semi climate controlled and create airflow. I have the interior walls of the crawlspace insulated from the rat slab up. I have no insulation below ground, or outside the walls- though I suppose this would be do-able in the spring time with some digging .
My first question is- can I anchor jackposts directly to the rat slab to support a beam? I am assuming that floor will shift some over time which seems to risk damaging my floor joists.
Secondly, if I need to cut through the rat slab and add footings for the posts, do they need to go below the frost line inside the space as it is constructed?
Other suggestions are welcome. Thanks in advance!
Replies
Maybe it is helpful to add that with the beam in the center, the closest footing to an exterior wall would measure about 8ft away, while the other 3 or 4 would measure roughly 14 feet from an exterior wall.
The only alternative to proper support below the frost line is a fully compliant frost-protected shallow foundation (FPSF).
This alternative works because the frost line is raised. Still needs solid undisturbed or compacted soil below the frost line.
I suspect at this point it will be less expensive to dig your support footings to below the nominal frost line in your location.
I wouldn’t recommend anchoring jackposts directly to the rat slab since it’s too thin to handle the load and could crack over time. If you’re pouring footings, it’s best to go below the frost line to avoid frost heave, even in a sealed crawlspace. If digging that deep is tough, consider using helical piers as a solid alternative. Hope that helps!