Slab on grade construction?

[MushCreek]

As an aside to my barn questions, I'm thinking about making part of my house slab-on-grade. I'm worried about termites, being in an 'extreme' area. Our existing house in FL is concrete block on a slab, but the little buggers entered in the seam between the garage and main house, and began munching on an interior (wood) wall.

The house I'm toying with is a 20 X 36 2 story, per the plans here, but with a shed roof addition- that's the part on a slab. The reason for this is simply money. Part of the addition will be a garage, and the rest will be a small den, and a foyer. What I'm worried about is where the slab connects to the foundation- termite heaven! This will be made worse by my wanting to build a raised wood floor, since the garage will be lower than the main house. I want the den to be the same level as the rest of the house, and the garage needs to be lower (I think it's code, for one thing). I also hate living on a slab.

You can see the dilemma, though. The wood floor over the slab will completely obscure termite activity between the slab and the foundation. From what I've read, and experienced, about termites, it's a matter of when, not if. The poisons that work are very dangerous, and even then, they are a control, not prevention. I'm curious how you guys would approach this?

[MushCreek]

UH-oh.........I think I've stumped the experts! Here's the first and second floor plans so you can see what I'm thinking. The basic house is a 20X36 2 story, and the garage, den, and foyer are on a slab. It's the joint between the slab and the foundation that scares me.

[MountainDon]

Why not do the whole thing on a slab?   Do a monolithic slab with footing all poured at once. The concrete guys were wizards, all one piece xncluding the garage floor that is at a lower level than the floor of the rest of the house. Twenty six years sitting on virgin desert (except for scrapping the top off) with a well known termite population as well. 

[MushCreek]

My land- almost all of it- has a pretty good slope to it. They'd have to do a lot of filling. If I go crawl space, it will be almost as tall as a foundation anyway, and I'd like to have a basement for a long list of reasons- bad weather shelter, easier utility installation and maintenance, game room, and storage. I also think you missed the line 'I hate living on a slab'. Having lived on a concrete slab for the last 30 years, I vowed 'never again'. Too hard, cold, and unforgiving. I can suspend a wood floor over the slab, but then there's no way to keep an eye on termite activity. If I put basement under the den and foyer, and a slab for the garage, I can keep an eye on the slab/foundation seam from inside the garage. Of course, that's more work and money. Maybe as a compromise, I'll go crawl space in that area. It's a bit harder to keep an eye on the little buggers, but at least it can be done. If nothing else, my bug guy can crawl in there every year and check for activity. I'll have to get comparative pricing to see how much difference there would be between the various approaches. You're right in that a monolithic slab is easier to termite-proof than this hybrid concoction of mine. I guess I'm paranoid after living in FL. You wouldn't want to stand still too long here if you had a wooden leg......

[MountainDon]

Right, I missed, overlooked, didn't remember the part about not liking slabs. It's a personal preference I guess. After 26 years of ceramic tiled slab under foot everywhere we still like it. Some rugs here and there and not cold in winter when insulated underneath with foam. I liked a basement when we had one back home. That doesn't seem so important now, though if I lived in hurricane/tornado country I'd likely think of a nice poured concrete wall safe room in a basement.   I can't think of anyway to head off the termites when there are slab/wall joints other than to pretreat the ground before construction. They do that here now as well as spray the bottom three feet or so of the framed and sheathed walls (exterior OSB) with a borate solution. They do that just before applying the exterior house wrap.

[MountainDon]

Mushcreek, some thoughts on this from PEte, the PE....    

If you have termites in your area, get the framing up off the ground, with a good sound, continuous, termite shield all the way around, under the mud sill (sill pl.). Cast concrete foundation walls, properly reinforced in both directions, provide a foundation with the fewest potential cracks. Concrete block will likely crack and then offers an interior pathway for the 'little buggers,' which you can't see. Mixing foundation types adds another problem, the potential of differential settlement and movement at the building joint between the two foundation types, another potential entry point. You're saving about (40' N-S + ~16' E-W at the foyer + ~ 20-22' around the O.H. garage door, about 76') of shallow found. length, but potentially opening yourself up to a far more expensive problem a few years down the pike, with the 'little buggers.' Put some dimensions on your plans.

You say you have a full basement under the east half: step the footings at the west wall of the basement to go around the foyer/den/garage, but continue the foundation wall which can now be thinner, and still properly reinforced at changes and corners, to prevent cracking. Then cast a slab in the west half at the same time you do the basement slab, and leave under the den open for inspection and storage. You will likely need special fire rated walls between the garage and habitable spaces., from slab to roof.

[Don_P]

If the termite pressure is high, the safe assumption is that you cannot exclude them. The purpose of a termite shield is primarily to provide an inspection plane, when they tunnel out and around it you see the tunnel. If you can't see the entire shield or if there are any gaps in it over 1/32", it's safe to assume that is where they will breach the shield.

The next method is to poison the food. HI, NZ and AUS have switched to this approach mostly using borate treated lumber. Borate has low mammalian toxicity but kills insects that consume the wood. Recently I have read that ethylene glycol is a possible contributor to childhood asthma. Products like Bora Care do contain the glycol, as do most of this generation of "safe" low voc paints. I'm pretty sure most of the commercially sold borate treated lumber is treated with Timbor, pure DOT, with no ethylene glycol. Treating dry wood with this or Solubor in the framed structure will leave a borate coat on the wood but won't penetrate deeply, that is what the glycols help with. I'm not sure if the other glycols are suspected. Basically they dry very slowly and borate diffuses into wet wood, so multiple wet on wet coats or another wetting agent would work. The concern in the article I read was regarding paints and the common practice of painting a bedroom in preparation for a newborn. The ethylene glycol, drying slowly, is offgassing into the bedroom for some time while the child is developing and has come under suspicion.

[MushCreek]

The main house is 20X36; the garage/den/foyer is 14X40. If I follow you, you are suggesting a crawl space under the foyer and den. The garage has to be slab on grade, unless I want to build a $u$pended garage floor over a basement. I think this covers all the bases- the garage slab seam is visible, so long as I keep it open and uncluttered, and don't cover that portion of the wall. It's that seam where the termites got into our current house- behind a shelving unit. Under the den and foyer would be a crawl space for visual inspection and access, and the main house still sits on a basement. The crawl space walls would be fairly short, as that is the uphill side. The ground slopes away to the left in the orientation shown. My wife has specifically requested an attached garage for her car- the dark woods scare her a bit. Here in FL, attached garages are the norm. You do have to use 5/8" drywall as a firestop- no biggie.

I had originally thought to go ICF for the foundation, but I'm starting to think it's overkill for the climate, and there are issues with termites tunneling in the foam undetected. I know there is treated foam available, but even the pros are still arguing about the efficacy. I have a friend who lives close to our property, and his poured, uninsulated basement is 55 in the winter, and 75 in the summer. I could certainly live with that.

One of the reasons I'm trying to keep the square footage down is for property taxes. One day, we'll be on a (small) fixed income, and I worry about taxes escalating, as they always do. I'd hate to work my whole life to be mortgage-free, only to be taxed off of my land.

[ajbremer]

I don't get it, all the termite fear.

So all the pier and beam builds on this site are going to get termite infestation eventually?

I'm realizing that, because of finances, (and I love the way pier and beam look in the woods) it's best for me to do the pier and beam/pay-as-you-go method. I'm building the 20x30 with 1 1/2 story on pier and beam. Can it be said, "Oh no, don't do a pier and beam - you'll get termites!". So many here have built pier and beam, have any of them had termite damage and if so, after how many years?

[MushCreek]

Like anything else in real estate, it all comes down to location. I once did an experiment here in FL. I put a pine board on the ground in a shady area of our suburban yard. It took three weeks for termites to find it! FL is rated as 'extreme', but so is SC, where I'm building. They used to really set up subdivisions here for termite trouble. Instead of removing trees and stumps, they would bury them. The termite colonies would feed on all this buried wood, and grow and grow. 5 to 10 years later, the wood would be exhausted, and the termites go looking for the next best source. My termite guy told me that whole subdivisions would suddenly become hot beds of activity. This practice is no longer legal (duh!) In reality, with proper ground prep, removing all of the wood and pre-treating, the risk is probably smaller than I make it out to be. But our property has been wooded for about 75 years, so you can be sure there are colonies of termites feeding on fallen trees. I'd just like to do the best job I can of staying ahead of them with careful design and vigilance. It's no different than building for wind resistance or snow load; it's just a different kind of threat.

[MushCreek]

I just realized that I didn't really answer ajbremer's question about termites and pier and beam construction. I think code requires at least 8" between any wood and the soil. So if you have concrete piers coming out of the ground, you have a point to inspect for mud tubes, which subterranean termites need to travel across open areas. You should also have termite shields where the post meets the concrete pier. These are just made of flashing bent downward at 30 degrees. I wouldn't put wood directly in the ground for a house anyway, although stuff rated for ground contact is supposed to be pretty good.

[ScottA]

The easiest way to avoid termites is to raise your house up off the ground. The old rule of thumb is 2 steps high, which translates to about 15-16".

[umtallguy]

I have to say, if you are paying for the slab and roof for it all, and already making parts two stories, make it all two stories and a smaller footprint.