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I searched the archives for answers to the following questions, but came up empty, so here goes…
Background info:
Part of my single story house sits on a slab foundation. The original portion of the house sits on a raised foundation with a crawl space. Construction is stick frame, circa 1940, and located in S. Calif. The slab portion used to be a detached garage but has been converted to a living area by enclosing a breezeway. I think the slab is ~4″ thick with a 12″x12″ perimeter footing. I’m thinking about adding a second story above the slab portion and this added floor would consist of two bedrooms and a bathroom.
Question:
What is the typical way to reinforce the existing walls and foundation to support the added floor? For the walls, I would imagine exposing the existing studs and adding 4×10’s (or whatever is deemed sufficient)at a certain spacing. But how do you reinforce and existing slab foundation?
Any insights or references are appreciated.
Replies
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Hi Arty,
Structural integrity depends on the strenght of the weakest component.
Look to the soil on which your footing is founded on.
If its on rock, or hardpan, a 2 inch thick footing would do the job.
If the soil is organic or unstable in nature, then you may have to rethink your plan.
It would be wise to call in a local soil engineer to give an opinion on your soils bearing capacity to base your decision on.
Gabe
*arty.. arn't you in the seismic zone of california...if so, won't you need engineered plans to go up another storey ?
*Soil bearing is seldom an issue. 1000lb/if is allowed on untested soil. Your footing size is an issue. 12x12 is not adequate for 2 stories. Gravity loads on walls are seldom an issue since 2x4 studs @ 16 cc is fine for 2 stories, but your header sizes may have to be increased. You will have to account for significantly increased shear loads, both from the real loads you will be inuring and increased code requirements. Adding shear paneling to the walls is easy, but you will need some hefty hold-downs. That brings us back to the foundation issue.If your zoning setbacks allow the easiest thing might be to build new footings and walls outside the existing ones. An alternative is underlining the existing footings, which can be a complicated and expensive process. A third alternative will be some sort of post and beam structure inside the existing walls to carry the gravity portion of the loads, and increased shear/hold-downs on the existing walls.You will need an engineer.
*Arty and Mike,Your problem stems from the fact that you are stupid enough to live in California, but I guess that's why I make the big bucks and you don't. As everyone who doesn't live there knows, Calif. will sooner or later fall into the ocean. Hopefully, sooner. The usual method of building with concrete footers is dangerous since concrete sinks, although there are people on this forum who will tell you otherwise. They even claim boats can be built of it. So you should build your houses on pontoons instead of footers. People will tell you that foam is a good material for this, but don't listen to them. Foam has never been proven suitable for pontoon houses. If you don't believe this put some foam in a jar with water and wait 50 years or so. But since Calif. will fall into the ocean long before 50 years, you'll have to take my word for it and use balsa wood for the pontoons. The suitablilty of this material was proven by Thor Hieroglyphic when he built the Con Tweaker and sailed around the world. My good friend Noah successfully built an ark out of balsa wood that was many cubic cubits in volume. It survived a flood of biblical proprotions. Unfortunately, until the earthquake floats your houses, the balsa wood should not have too much weight on it. In order to keep weight off of your pontoons you will need to reduce the air pressure inside the house. You can do this with a whole house ventilator in the attic. If you keep all the doors closed pressure in the house will be reduced causing it to tend to rise in order to equalize the pressure inside with the pressure outside. This should be enough to keep the weight off of the pontoons. You will however need to be carefull that the house doesn't lift off the ground entirely. You will probably need to hire an arkitect.If all this is too much trouble move to the east.
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Yes, I know I'll need engineered drawings, but I was just curious to know the common way(I) one would reinforce the walls and foundation. I was kind thinking one acceptable way to reinforce the existing foundation would be to dig under the footing directly under each 4x10 (or whatever is necessary)post and pour a bigger and keeper footing. Essentially creating a post and beam structure to carry the second floor within the existing walls.
Adding second stories to existing slab foundations is done all the time, I just don't know how it's done.
Thanks for the replies.
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Arty,
As Mike Mahan said, and your imagination hasn't told you, digging out under your current footings and enlarging them involves putting a bigger footing below, and is a very expensive "undertaking". Even if the house was built on stilts this would be a huge job, involving everything from backhoes to formwork to relandscaping the entire perimeter of the building, and possibly watersealing problems.
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Hi ARTY, I too am planning to add a second story to my home in LOS ANGELES. The info I've been given was to use some type of post and beam construction within the existing 1st floor walls using either wood or steel beams.I want to add 3 bedrooms ,a bathroom and a library upstairs and from all indications, I've been informed this is the way to go. Still in the planning stages but I'm ready to go ahead with it if BUILDING & SAFETY gives the o.k. GOOD LUCK !
*Al,Sounds similar to what I have planned. What kind of foundation do you have and how do you plan to reinforce it for the 2nd story?Thanks, Arty
*ARTY, I have a concrete foundation and it looks like it's in pretty good shape. Still having some drawings made of the different floor plans for the second story.By the way, are you going to have to move out while construction is taking place? My contractor says it won't be necessary since most of the work is going to be from the outside. What do your guys say about this? I think I'll move out anyway just to be sure, strange things happen during construction.
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I searched the archives for answers to the following questions, but came up empty, so here goes...
Background info:
Part of my single story house sits on a slab foundation. The original portion of the house sits on a raised foundation with a crawl space. Construction is stick frame, circa 1940, and located in S. Calif. The slab portion used to be a detached garage but has been converted to a living area by enclosing a breezeway. I think the slab is ~4" thick with a 12"x12" perimeter footing. I'm thinking about adding a second story above the slab portion and this added floor would consist of two bedrooms and a bathroom.
Question:
What is the typical way to reinforce the existing walls and foundation to support the added floor? For the walls, I would imagine exposing the existing studs and adding 4x10's (or whatever is deemed sufficient)at a certain spacing. But how do you reinforce and existing slab foundation?
Any insights or references are appreciated.