Engineer's question: adding a story on a block foundation?

djmcmath · 11-12-2008, 05:25 PM

My wife and I are in the house-buying market. Many of the houses that are in our income range are just too small, and on lots that are tiny. The obvious answer is to expand upwards. Permitting questions aside (though I know those issues can be huge), I'm concerned about the engineering issues involved in adding a second floor.

The problems, in order of my concern:
1 - If the houses have basements, they're usually showing a concrete block foundation. Given the era (pre-1950, mostly), there's very little chance of rebar. Is it possible to add a story on top of that kind of foundation? What methods are available to reinforce such a foundation (assume that no shifting is readily apparent) to meet modern code for building another story?

2 - The walls for the first floor are, most likely, 2x4s. Now, given the era, they're probably actually 2 inches by 4 inches, but that's probably still insufficient to build on top of. What methods are available for adding strength to 2x4 walls?

3 - Many places have finished attics, and some even look like they were done to code. If the permitting guys have come through and said "This place is safe to finish the attic," does that imply that the foundational issues of 1 and 2 are mitigated?

I have a few ideas of how to deal with this problem:
1 - Build new. Specifically, pick a place with a big backyard and expand the main body of the house into it. Problem: permitting is liable to be impossible.
2 - Build new foundations for new posts which support the upper floor independent of the first floor. Problem: Expensive, in a big way.
3 - Tear out the walls, replace 2x4s with 2x6s, replace the walls. Problem: time consuming, incredibly messy, though perhaps less expensive than either of the other two options.

Thoughts? Anyone else ever done this? Am I crazy to even consider this kind of thing? Is it just right out of the question?

Thanks in advance,
Dan

ckissick · 11-12-2008, 06:19 PM

The answers all depend on the house. As for 2X4s in walls, most 2-story houses have 2X4 walls, same as 1-story. Around here, people add a second floor all the time, and they don't generally have to beef up existing walls, or at least not very much. What they do, is they transfer the additional loads to existing foundations and/or add new square footings or drilled piers for point loads. The existing spread footings may need to be underpinned.

Here in California, we don't deal with unreinforced block walls. If a wall is old and unreinforced, it gets replaced.

Each house has to be evaluated individually. You would need to consult with a structural engineer who can inspect the house. You could also have a contractor inspect the houses for less detailed, but pretty good evaluations.

The biggest problem may be getting a construction loan. Most of my clients can't get them anymore.

ckissick · 11-12-2008, 06:23 PM

Oh, and as for finishing an attic, if it's just a matter of adding sheetrock and trim and such, the new loads are minimal and the rest of the house should be fine as is.

nota · 11-12-2008, 07:50 PM

jack up the house and add a new stronger 1st floor

afterburn 549 · 11-13-2008, 02:28 AM

after much thought jacking up 1st floor is empirical to adding a second floor less work cheaper

VINMAN · 11-13-2008, 03:48 AM

As a contractor, I would not do any work like that without a structural engineers approval first. While it is certainly doable, each house has its own issues.

dad911 · 11-13-2008, 04:26 AM

It is such a buyers market right now, you can certainly buy cheaper than build.

onewhippedpuppy · 11-13-2008, 05:01 AM

I would not buy a house with a block foundation expecting to be able to build up. At least not without significant foundation work. Around here block foundations don't hold up too well. Adding a second floor only increases the stress on it.

djmcmath · 11-13-2008, 02:52 PM

When you say "It would be cheaper to buy than to build," how much are you anticipating a job like this costing? I mean, just rough order of magnitude, $60K? $100K? $150K? I'm a very DIY guy, so I'm capable of a lot. At the same time, I recognize that lifting a house is beyond my comfort level. But then, the houses that we're looking at are not worth lifting to add a floor. Seriously.

Thanks for the feedback,
Dan

dennis in se pa · dennis in se pa's Garage

I have done it twice. That is - added a second floor on a 1 story house. The foundation and first floor walls are not the problem The problem is the weather. Plan properly. Move quickly. Plan for a bit of rain with lots of tarps. And just do it. Unless your foundation was poorly placed that will not be a problem. 2 x 4 walls on the first floor are the same as you find on a 2 story house most places.
AND NO YOU DON"T LIFT THE HOUSE to add a floor!!!
You don't need to listen to Rube Goldberg, you just need a contractor that knows his engineering and codes.

DanielDudley · 11-13-2008, 06:00 PM

block is laid on a footing. 2x4 are considered good for three stories. If all is good then you would be fine.

Dave L · 11-14-2008, 01:29 AM

We looked into this, we had a 1950's brink bungalo with a block foundation. We talked to a company that specifically does second floor additions that are mainly prefab and they had done a few homes in our area. We didnt go too far down the road but essentially they remove the roof and span the entire structure with laminated joists then hoist the prefab sections up and close it in. The process from removing the roof to closing it in is 1 week. The inside can be contracted out or you can do it yourself. The foundation didnt seem to phase them in the least but I am sure further into the process they would have to have an engineer do an inspection.

Don Plumley · Don Plumley's Garage

I've seen three houses near here that jacked up the first floor and built a new floor underneath. It's wild to watch. I think one compelling reason to do this instead of building on top is so you don't have to make a new roof.

Porsche-O-Phile · Porsche-O-Phile's Garage

Agree with pretty much everything ckissick said.

How is the sill plate anchored to the block wall - are the cells all grouted or did they put a bond beam across the top and sink the anchor bolts into that? Or (worst of all) did they just sit the sill plates on the foundation with no anchors or holddowns (*shudder*)?

Unreinforced masonry walls are problematic in seismic zones although the addition of reinforcing is generally just to give the masonry some tensile strength (masonry has good compressive strength but lousy tensile strength on its own). In a situation where the masonry wall is underground and acts only in compression, the fact that it has no reinforcing wouldn't typically be a problem BUT...

In reality the wall IS probably acting partly in tension. Hydrostatic pressure surcharges against the wall would create a uniformly distributed load on the foundation causing bending and tension on the side of the wall closest the surcharge. Also, if there ARE anchors, any wind loads on the house walls would be transferred via the anchors to the foundation, creating a bending moment at the top (and again, bending with tension on one side).

So to answer your question, yes there could potentially be a few issues if you really do have an unreinforced CMU wall below grade. Realistically, it would probably be okay, but I'd get a structural engineer's okay on it. Alternatively you could devise a system where the second story loads are handled by a separate system, although this is tricky and can get expensive (You essentially build a second house with its own structural system on top of the first). Or you can laterally brace the existing foundation wall (more likely & realistic solution, and cheaper typically)...

Long story short, it depends.

tcar · 11-14-2008, 08:32 AM

Get a structural engineer to look at it.

Do not do anything until you do.

It's probably illegal to do this without one, anyway. (if it's not, it should be.)

too many variables to do online.

Seriously. Cheap. Few hundred bucks to have them look.

afterburn 549 · 11-14-2008, 08:44 AM

illgal ?Who need a stinking badge from some high class welfare office to say its ok? Dwn with the town hall !!!
( a enginmneer might be Ok if you are in doubt),,,.......and if they have a stinking code to collect your $$$$ they might notice a house lifting proj. LOL

Danimal16 · 11-14-2008, 12:14 PM

I am an engineer. Trained in California. You are well advised to get an engineer that is familiar with not only the codes in you area but also the conditions. I have been to VB (love that place) and I am sure the foundations are significantly different designs that here in California or Kansas (previous poster). Remember that the soil conditions will also give significant strength to the recommended foundations. The need for shear reinforcement is more earthquake related and the expansive nature of the local soil may also play a part in the design.

The best advice is get a good structural engineer and he can walk you through it. There may be no issue at all but check with a local expert anyway.

911Rob · 911Rob's Garage

I'm an engineer, builder and marketing expert.

Are you nuts?

I'm also a huge renovator, but anything like this is a ticket for losses.
Buy the work already done, especially in this market!
Engineering an existing building is very difficult without the specific factors and components; if there are no plans or engineering specifications from the original design, the assumptions could be too great? I would excavate a portion of the foundation from the exterior to check the footing under the block wall. If the wall is sitting on a good strip footing and concrete filled, level and plumb you're likely good to go. Portions of the load bearing super structure (2x4 walls) will also be required to expose and examine from an engineering point; dont rule out doing this also from the exterior? (remove siding and plywood).

My advise to you is forget about it.

afterburn 549 · 11-15-2008, 07:55 AM

I think you all went to school to long. Want to use your slide rule and the such.
If the freakin house is hanging in the air how much trouble can it be to put in a new foundation !
Jacking up a house is not that hard ( usually ) and the first inch is the hardest , after that it A a exercise in a cribbing throwing party !
The 1 st story would be done out of 2x6, the second story would never knew anything happened.
Unless it is one giant trophy house the weight added to the foundation is nothing more then an other run of joist , plywood 2x6 for the walls, some sheet rock .
Like I said if the house is floating in the air...concrete is still cheap. poor a whole slab !
In this case the 1st floor has been carrying the load, put that on the second floor with apprpiate wall/ structure under it = no diff.............unless built on quick sand LOL

djmcmath · 11-16-2008, 03:38 AM

We cannot, as it turns out, afford to buy a 2-story place right now. Even with the depressed market, the second story seems to add a huge amount to the list price.

Which brings me back to the costing question: how much does it typically cost for these things? Assume a best case, where the structural engineer says the foundation is fine for building on, and the first floor is still sound, and the roof needs to be replaced anyway. Or a worst case, where I'd have to build a "second house around the first," with the shiny new footings and all that. What kind of gross numbers are we talking? Dave -- those guys who did the prefab thing, how painful was that? And what kind of names to companies like that have?

There will definitely be a structural engineer, perhaps involved as early as the PPI, but definitely before anything like construction happens. The Gaithersburg permitting office is going to want to talk to my engineer and architect before they issue a permit.

Thanks again for the opinions and experience, all.

Dan