[stubits]

We are in the process of designing a 3 story addition to our DC rowhouse. The addition will add approximately 150 sq. ft., or 50 sq. ft. per floor. Our architect is just about done with the schematic design and has sent us some draft drawings. Obviously as a rowhouse, we are attached to neighbors on both sides.

In an effort to avoid encroaching on our neighbor's property, the architect has suggested we use a turned down footing. She indicated that frost protection would be required because the footer wouldn't be deep enough to meet the frost depth requirement of 30". I think the major concern, however is that my neighbor's property at that point is about 14" lower than mine.

I've attached two elevations to show what we're planning on doing, as well as how our property is relative to the neighbor.

So, what do you all think of this idea? Is it strong/stable enough? Does the neighbor's lower elevation matter? Cost wise how does a turned down footer/slab compare with a standard footer and slab? I'd imagine it is much less concrete.

 

[Scuba_Dave]

Have you had an engineer review the plans?

 

[stubits]

Not yet, the plans are still a draft. We'll definitely have the plans reviewed before we go for permits. I'm just hoping to get a feel for how this works before I meet with the architect again.

Thanks!

 

[Willie T]

I'm fairly sure a haunch footing in that multi-story application wouldn't cut it, code-wise, around here. And those stair posts on just a 4" slab might not pass either.

Need to start looking at DC codes before you get too much further down the line.

Frankly, no one better ever hand me plans with all those "as needed" and "sized to support" notes. Did the architect happen to mention just what this "frost protection" is going to be? Those plans are awfully vague... even for a "draft".

 

[stubits]

Willie- Thanks so much for your comments. I definitely appreciate your experience. Not sure if it changes anything, but the addition will bear on the existing foundation on more than 2.5 of the 4 walls. There will be just about 7 linear ft of new foundation, approximately 2 ft on the left and the full 5' on the back (or front I suppose). The problem area is that which abuts the neighbor's property, or the 2 ft. side. What would you suggest in the application, to minimize impact on the neighbor's property?

Also, I feel it necessary to defend my architect here. She's really great and comes really well recommended. The drawings I posted (even once their finalized) are not meant for contractors or for permitting. Once we work through the design details, etc., she move forward to the construction set which will have the full details. That said, if this were meant to be anything close to final I'd be in complete agreement.

Thanks!

 

[Daniel Holzman]

As a professional engineer, I would like to defend the architect's drawings. These appear to be high quality, accurate sketch plans of the building, perfectly suitable for the stage of development the project is in. Further, in my experience, the architect typically does NOT produce final drawings with structural details, they leave that for a licensed structural engineer. So it is perfectly appropriate to note items such as "frost protection as needed", with the understanding that the required frost protection, reinforcing, connections details, and other structural details will be provided by the engineer at construction drawing phase. The details are likely to show up on a set of structural drawings.

In some cases, the architect incorporates the details into the architectural plans, however this always raises the issue of where the engineer's stamp should appear. By keeping the drawings separate, it makes it easy for the architect to stamp the A (architectural) sheets, and the structural engineer to stamp the S (structural) sheets.

 

[Willie T]

Agreed. It IS a pretty picture. And it gives everyone a shot at a slice of the pie. But, as an owner, I think I would have rather seen a nice ortho rendering at this stage.

 

[Aggie67]

So, what do you all think of this idea? Is it strong/stable enough? Does the neighbor's lower elevation matter? Cost wise how does a turned down footer/slab compare with a standard footer and slab? I'd imagine it is much less concrete.

The engineer will determine if the concept is viable as drawn, in all respects, and the city building/zoning officials will have their comments. How the new foundation interacts with your neighbor's lower foundation, more than anything else, will be the big concern.

The additional materials required for doing FPSF jobs aren't terribly expensive, but the labor to install is a little more. Instead of a traditional footing, you might be doing a shallower excavation and a little less forming, but the base prep, geotextiles, foam boards, layering, fill requirements, etc, all add up. In other words, all else being equal, instead of digging a hole, pouring a foundation, insulating the wall, and back filling, you're digging a slightly shallower hole, pouring a little less concrete, but when you back fill you're actually constructing a layered cake out of synthetic and natural insulating materials. Also, I've done 4 commercial FPSF jobs, and the area of disruption is greater than a traditional foundation.

 

[stubits]

Thanks everyone! I sent a copy of the plans to our SE yesterday for a quick opinion (go/no go, so to speak). Daniel, thanks for sticking up for our architect. She's wonderful! Oh and Willie, we've got lots of great images, plans and renderings from her, these two just show the foundation the best.

Rowhouse additions are complicated in general, from what I hear, and mine is complicated by the lower elevation of my neighbor. What sort of foundation would you guys prescribe here? Any thoughts I can share with my architect?

Thanks!