Choosing Foundation

Started by mrbeing, March 28, 2011, 01:52:39 PM

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mrbeing

I will be building a modified 20x30 1 1/2 story cottage shortly after our wedding in a few weeks. I have mixed feelings as to the foundation I will use. Houses in this area(south Louisiana) more often than not use a simple spot footing system at grade. I'd like to build the house a little higher than what precast piers offer to give adequate ventilation to the crawlspace(floor rot is an extremely common problem around here due to inadequate ventilation and standing water under homes). So either post in ground or poured concrete columns. I'm a little worried with burying wood in the ground though. Termites are rampant in these parts and I'd fear it would give them an easy access elevator of sorts to a wood buffet. Also, it rains quite a bit year round. How do posts buried in the ground typically stand up to moisture? I would use below grade rated PT posts, but I'm sure they will become an issue eventually in the lifetime of the house. I am aiming to have a crawlspace clearance of about 2-3 feet.

MountainDon

I would think a poured concrete, rebar reinforced, footing may be your best bet for longevity. If you don't want to build a full crawlspace with concrete blocks you could either make piers from concrete blocks on that footing or use poured concrete piers using sonotube type forms. These piers would all be tied to the footing with bent rebar 'els', one end set in the wet footing concrete and the other sticking up into the blocks or sonotubes.

Just because something has been done and has not failed, doesn't mean it is good design.


mrbeing

Yeah. I just didn't want to immediately rule out the buried posts as for initial cost. We receive hurricane winds at the site location often enough to plan for. Tell me if I'm wrong, but I kind of liked the idea of the buried poured sonotube forms as they would tie the house into the ground better than placing piers at grade. I'm sure it's a bit overkill, but it would ease my mind.

MountainDon

A Sonotube poured for the pier using the full perimeter poured concrete footing will provide much greater stability than individually set sonotubes. That gives a large footprint and with it all tied together with the rebar makes for a stable foundation. The only thing better would likely be the perimeter foundation with a full stem wall (poured concrete or concrete block).
Just because something has been done and has not failed, doesn't mean it is good design.

Squirl

In your location I would stay away from PT use even if it was graded for ground contact.  I read the standards for ground contact treated PT wood is still only around half the standards for PT permanent wood foundations and telephone poles.  I wouldn't stake my house on it.

I watched some of the designs for rebuilding after Katrina.  They were like don had said only 3-4 ft tall.  Don't forget the termite screens down there.


John Raabe

A warm moist climate will shorten the useful life of wood piers set in the soil. So that's probably not a good idea. The hurricane possibility makes a shallow pier block foundation too likely to be torn apart by high winds.

You need something buried in the ground that will hold the house down. Don's suggestion of concrete piers or a concrete stem wall foundation are solid. You might also consider a PT permanent wood foundation - it uses standard framing (foundation grade PT) and good drainage to get a lifetime of the house longevity in most locations. See if others have done this in your area.

I would also check with local builders and the county inspector to see what they are recommending for new house projects.
None of us are as smart as all of us.

Erin

I'm gonna echo Don and vote for a poured Sonotube, too.

In fact, I'm not even sure I'd say a poured/block perimeter wall would be preferable in your case.  Poured tubes will let you keep your house well up off the ground and give no chance for moisture issues in the crawlspace.  
The wise woman builds her own house... Proverbs 14:1

Don_P

Some form of continuous footing or grade beam around the perimeter with piers or pier and curtain wall would probably work there. I think parts of LA are requiring engineering in the wake of Katrina.

this is a grade beam;


a waffle grid;


and a braced pile;

mrbeing

Thank yall so much for your input. Yes. Many parts of LA are requiring engineering approval. The parish/county I will be building in only requires that you build to IRC '06 and lot graded for septic suitability for non flood zone areas. Flood zone areas need elevation survey and engineer approval on the plans. I'm lucky as I am not in a flood zone. I will most likely be doing spot Sonotubes. I also plan to run another set of piers on center to eliminate the 20' free span.


Don_P

IRC '06, and all other editions, requires a continuous footing and does not show less than a minimum of pier and curtain wall. Your building department may allow otherwise though. Engineering is another way to step outside of the prescriptions in the code.

mrbeing

Well let me rephrase that. Parish building code is IRC '06 with many sections omitted. I'm still waiting for my copy of the IRC book, so I can see what these section numbers reference. Looks like the ebay seller is dropping the ball on shipping it to me. I'm also gonna be sitting down with an uncle of mine, who is a contractor and carpenter that has built in the area to help me with code compliance

mrbeing

Yay. Just got off the phone with the building inspector for the area. He said that is perfectly fine. He also told me that the soil bearing capacity tests he's seen is generally in the neighborhood of around 2500 to 3000psf in the area, but I'd need to get it tested for my particular site, before plan review.