Foundation question for two story addition to old home

snowblowerNovember 26, 2006

Having a second floor 14 x 18 room added over an existing single story room on our balloon framed 1870's house by a well established contractor. The existing room's hip roof would be removed, and the new second floor floor and flat roof would be supported as follows:

Contractor plans to place four "mushrooms" below frost line supporting cement columns which then would support 4 X 4 wood beams going through the existing first floor wall. The beams would then rise to carry the load for a 14 x 18 room and it's flat, slighly pitched roof.

Support for the new floor and roof would totally be on ledger boards attached to the house (one for the new room's floor and the other for the roof) at one end, and the four 4 x 4's at the other.

There is not an acceptable foundation for the single story room. Currently the wall that would receive the 4 x 4's in the single story area is supported by four lally columns resting on cement "pancakes" 16" round and 10" deep. It remains perfectly level.

Is this pier type foundation with 4 x 4's tried and proven?

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mightyanvil

It's difficult to understand your description (beams are horizontal, posts vertical, ledgers support loads against walls) but no, it does not sound like a tried and proven construction method; it sounds more than a little crazy. Don't proceed without an engineer or architect to design the structure.

    Bookmark   November 27, 2006 at 7:56AM
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snoonyb

Without an engineers sign-off, I would be hesitant.
Engineered, the method would probably work. However, tying the existing to the new support post will transfer loads causing the entire structure to act as one, and installing isolators is not practical.

    Bookmark   November 27, 2006 at 12:50PM
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early1

I'm not sure I follow exactly, and it does sound like you mean 4x4 "posts" or "columns" rather than "beams". Overall, it sounds like the intent is to carry the 2nd story loads to an independent foundation. If that is the case, it can be done, though there would be a few things to watch. First, the obvious - make sure there are enough posts to carry the loads. Many will be required if they're 4x4's. Fewer if you go to 6x6, but I assume their thickness will not fit in the existing walls. Then there is the bracing issue. The 4x4's need to be locked into the sheathing or some other lateral brace system so that they don't sway. Then there is the foundation itself. As the independent foundation is loaded, it will have its own settlement characteristics. They sit on virgin soil whereas the existing foundation has been there and has already settled. I guess you might be able to see now that it would be worthwhile to get an engineer involved. There might not be a need to go through rigorous load calculations, but there should definitely be serious attention paid to ensuring a stable structural system and a competent foundation.

    Bookmark   November 28, 2006 at 10:40PM
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wet_in_seattle

We were looking at putting in a second story addition and had to delay our plans as the structural engineer is requiring a new foundation. The reason is that we are in earthquake country and post and beams apparently don't handle the horizontal loads. Depending on where you are, this could also be a factor.

    Bookmark   November 30, 2006 at 1:29AM
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julieann_grow

Your description sounds a bit "off" for several reasons.

There are NUMEROUS variables to engineer what you have with your project and only a good, residential, structural engineer is competent enough to account for all of the variables.

I've stated this numerous times here on this forum, but I'll say it again. Get the contractor and/or framing contrator to meet with the engineer so that they can discuss this together. Don't let the engineer design this in a vacuum b/c they tend to overengineer (more money) and contractor's tend to overlook engineering details. The two minds together should be able to come up with a cost effective, adequate solution.

You also will need to have an engineer's set of plans for permitting, construction and liability reasons. Get drawings of the structural elements and get them stamped.

    Bookmark   December 1, 2006 at 10:46AM
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mightyanvil

Buildng a structure over another inadequate one is so expensive it is usually cheaper to tear everything down and start from scratch. Even jacking the structure up and building a new foundation would probably be cheaper.

In what jurisdiction would such a structure not need to be designed by a professional and why would you think a contractor had the capacity to do it?

    Bookmark   December 1, 2006 at 12:03PM
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snowblower

As a clarification, our house is 120 years old and the existing foundation is showing it's age. I doubt it's going anywhere, but adding to it is questionable at best. The idea of the mushroom footings and columns is to avoid adding stress to the existing house. The new piers would replace existing piers along the back wall of a one story addition which started out as a porch.

My initial description is incorrect. Thanks mightanvil for pointing that out. There would be four mushroom cement blobs buried 4 feet under supporting typical 10" cement columns along a 14 foot wall. The columns would support 4' lengths of lally (exposed to the elements), then 4X4 wood columns would run through the first floor wall and carry a beam for the second floor 2x12x18 24" o.c. joists. The 4X4's would continue to the roof line and support another beam for the flat roof 2x10x18 16" o.c. joists. Maybe those are called rafters, but it's a flat roof so they might be joists.

The structural engineer designing this is the architect, designer, and owns the contracting company. It's a design/build outfit that's been around a long time, and the city inspector relies on his stamp of approval.

I'm concerned about minimal requirements. For instance, we had some options to upgrade from 2X10's to 2X12's for an 18 foot span. I thought that was a good thing. So as part of a $3,000 work order change I was surprised to see the plans had the 2 X 12's at 24" o.c. After he explained that his plan met Massachusetts building code, he wants another $300 to make them 16 o.c. and thinks it's a needless change. If he didn't have our 1/3 deposit ($8k) and a ($3K) change order to beef things up, I'd move on.

I've only seen piers supporting decks, and some houses along oceanfront properties. Been searching the building codes and haven't seen much written about this design. Code is linked at end of posting.

Any assistance or words of wisdom appreciated.

I will pay attention to the bracing within the walls of the 4x4' columns as suggested above by early1.

Here is a link that might be useful: Mass Building Code online

    Bookmark   December 5, 2006 at 10:04PM
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mightyanvil

To tell you if the framing meets the MA code I would need to know the species and grade of the lumber but I can tell you that for typical framing lumber 2x12's @ 24" o.c. will not meet the MA code for an 18-0 span, so 16" o.c. would be required.

Even if the designer is planning on using a higher strength species or grade, 24" o.c. would most likely produce a bouncy floor.

I will bet that the designer is not really an engineer or an architect in MA. I would be looking for an engineer license/registration number. You can look it up on the state professional registration website.

Frankly, I would never trust an engineer who was working for a contractor instead of me.

Here is a link that might be useful: MA registration search

    Bookmark   December 5, 2006 at 11:43PM
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snowblower

See page 507 in the Mass Building Code linked below. Sorry I can't link to the specific page. Can also find it by googling "table 3605.2.3.1b"

The elasticity factor for the #2 construction grade SPF wood is 1.4, and it appears that one can span 18' 4" with a 2 x 12 at 24 o.c.

Am I reading the chart wrong? Hope so....

    Bookmark   December 6, 2006 at 9:06AM
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snowblower

Link isn't posting. It's the same as posted three comments above entitled "Mass Building Code Online". Please see page 507 (the link is to a section, not a huge document) for the floor span chart.

    Bookmark   December 6, 2006 at 9:19AM
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pjb999

I'm still confused by the description, to me it sounds as if the thing's cantilevered off a new structure, then supported internally by the 4x4s?

You'll need a number of them, with all they're holding up.

I would speak directly with the city building people if you have any questions to ensure it's going to comply, cheaper now than later.

Some 'contractors' don't know anything, I worked briefly for an (unqualified) guy who graduated himself from kitchens, floors etc to building extensions, and he failed to see why I was freaking out when I discovered a column supporting a 12x12 corner of the house roof, over a deck, was sitting on TOP of the decking and resting only on one edge of a floor joist - he'd quite happily given me the go-ahead to demolish all the deck floor, I stopped close to the post when I noticed how it was dishing. But that was an extreme case.

    Bookmark   December 7, 2006 at 1:36PM
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