The Floor Slabs Of Usonian Houses

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Laurie Virr
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The Floor Slabs Of Usonian Houses

Post by Laurie Virr »

For both practical and aesthetic reasons, the floor slabs of the Usonian houses were required to be troweled to a high degree of accuracy and finish. Regardless of whether the grid was formed with a grooving tool or a concrete saw, it was necessary that this operation was performed before the masonry walls were laid up.

I have seen numerous construction drawings of individual Usonian houses on which there have been clear specifications printed on exactly how it was desired that the work on the floor slab should be carried out, but nothing on how the work should be protected thru’out the remaining building operations.

If a brick fell from staging, a carpenter dropped a hammer, or some other accident occurred during the major period of construction, the surface of the floor was at peril unless some form of protection was afforded it.

What measures were taken in this regard?

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Post by KevinW » ... 1430432850
This photo taken by Bob Beharka during the construction of the Llewellen Wright house suggests the slab was after the CMU? (Photo from FLlW Foundation archives)
Regarding the grid, were any usonian slab joints saw-cut? I thought they were all tooled. I think I remember Bob telling me he covered his own house slab with Masonite to protect it, so perhaps that was used elsewhere.

Laurie Virr
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Post by Laurie Virr »

Thank you for your reply.

My understanding is that during the 1950‘s some of the grids were saw cut. My preference is for the tooled joints: the rounder edges are more appropriate.

The problem with pouring the slab after the masonry is laid up is that it is difficult to get the grooving tool hard against the wall, especially if the grid is not rectilinear.

Unless the slab is poured at the end of the construction process, the need to protect it still arises. Moreover, in that circumstance the masonry walls require masking, to protect them from splashes of concrete.

I have always specified for the slab to be poured before the masonry is laid up. For protection, I have used on occasion, thick sawdust, 6 mm plywood, or carpet underlay, but the best results have been using layers of discarded carpet. It provides protection from the base plates of the staging, not least when the masons decide to drag the latter over the floor!

Regardless, it is a perennial problem, but one it is necessary to live with thru the period of construction.

Laurie Virr
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Post by Laurie Virr »

Is it possible that the intention for the Llewellyn Wright house was to pour a screed over the structural floor slab? The image indicates there was considerable building debris on the floor.

My experience with screeds is that they have to be either very thin - 19 mm, or comparatively thick - 75 mm. For some inexplicable reason 50 mm screeds always develop cracks.

It is difficult to bond a screed to an existing slab, given the 28 day curing period for the latter. As a consequence, measures have to be taken to allow the slab and screed to move independently, necessitating the laying of a membrane to separate the two.

Furthermore, there is the cost of the additional labor, concrete, and mesh reinforcement to be accounted for if screeds are employed.

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Post by DRN »

The Sweeton house, completed in 1951, has tooled unit lines in the slab of two shapes: rounded "V" from the living room back to the bath (indoors and out) as well as the workshop; and sharp "V" shape from the first bedroom to the master bedroom. As the rounded shape is in the workshop which was poured first, I am to assume the concrete finisher changed from rounded to sharp "V" during the course of his work.

Given their edge condition, I have found that all slabs in the Sweeton house were poured prior to wall CMU being laid up. I assumed Wright favored this detail because it eliminated an unsightly joint in the floor with the wall that would be prone to chipping or opening, and it eliminated the chance of concrete splatter on the wall masonry. While digging at the exterior of the house when we ran the gas line, I found that the slab meets the exterior face of the wall with a thickness of about 1" or less, as if the top course of foundation block may have been cut or hollowed to allow the outer shell to form a pourstop of much for thermal bridge prevention.

In the correspondence, I found mention of "lay slab plank" in some notes between the Sweetons and the builder. This seemed puzzling to me until I read this thread. Possibly, wood "planks" were placed at the slab edges to protect them from the masons?

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Post by KevinW »

I will have to take some closer looks at some of the Usonian saw-cut examples. I finally talked my current client into tooled over saw-cut. I was able to supply numerous examples of chipped saw-cut edges, terrible accuracy, favor tooled.

Not really relevent to this thread, I might start a new one reporting on a product I used on my red concrete floors to wax them, with unexpected beautiful results....and no back breaking polishing on hands and knees like with the paste wax I tried before.

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Post by KevinW »

The photo is not conclusive, I guess. I had never heard of using sawdust as a protective barrier while in construction, so there very well could be something there. Unfortunately all the other photos I have are from the exterior.
Last edited by KevinW on Mon Mar 11, 2013 1:21 pm, edited 1 time in total.

Roderick Grant
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Post by Roderick Grant »

Paul Hanna's book about his house has construction photos on pages 63 to 67 showing the concrete mat constructed first, with no protection as the rest of the structure went up. One photo is captioned, "Pressing the 1-1/2" metal strips into scored lines where walls will be erected." Fitting brick walls into the hex pattern before the fact would have resulted in a lot of grid lines not quite hitting the mark.

Education Professor
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Post by Education Professor »

I believe that Wright typically preferred to have the floor slab poured before the walls were constructed. It seems that this order would be especially important with hex/triangular designs to ensure proper alignment of the grid lines with the walls. The grid lines of the Palmer teahouse slab were very intricate and could only be aligned so well by being poured before the brick walls were constructed.

Nevertheless, I seem to recall that the beautiful stone walls of the Shavin house were constructed before the slab was poured. Perhaps this approach would work okay with square-grid designs? Of course, one would have to account for splatter issues and running the grid lines hard against the walls.

During the construction of our own home (unfortunately many years before we "discovered" FLLW), we utilized sawdust from the family lumber mill as well as discarded carpet and carpet pads to protect our basement slab with very good results.


Laurie Virr
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Post by Laurie Virr »

My experience has been to thoro'ly wet the sawdust after it has been spread over the floor slab. It forms a cake like layer as it dries out, and hence is not blown away by wind.

I always specify that no work is to take place on, or around the slab for 28 days after it is poured. This not only allows the concrete to attain its maximum strength, but also ensures that the sawdust does not adhere to it.

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Post by jmcnally »

Here at the most recent Usonian to be built (Florida Southern College), the floor was poured and contoured before the walls were erected. No protection was laid over the floors while the walls were being installed.

Check Michael Maguire's gallery for photos - especially July & August 2011.

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Post by dtc »

Laurie, thanks for bringing up this topic. Very little has been discussed about this issue.
If I could add a few comments: Here at Dobkins, Wright used a masonry/retaining base to level out the first floor elevation by projecting the structure out from the subtle sloping terrain.
Some of you are familiar with this feature which many describe like the "prow" of a ship.
Notable examples would be the Hagan, Walker, Reisley, Dobkins and others.
Masonry went up first, the pour of the concrete mat second. We have evidence of that with cement splashed on masonry walls that were not wiped in areas of closets and built in base cabinets. There are areas where the dry shake pigment can still be found in masonry bed joints.
Also the hand tooling of module joints are just short of the exterior masonry walls.
In many areas one can slip a fingernail between the masonry and the concrete mat.
In areas where the backfilled soil and or stone subsequently sunk the concrete mat sunk as well, and one can see that the pour came after the masonry.

An interesting side to all of this is every now and then we discover paw prints impressed in the floor concrete mat. It is always a pleasant surprise when we discover one. A cat, a raccoon? Perhaps a squirrel that appeared after the cement contractors left for the day? We know curious neighbors liked visiting the job site when they got wind of a home designed by Wright was being built. I guess critters were also curious.

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Post by Reidy »

Unity Temple has such prints, too, as you walk from the foyer into the west cloister.

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Post by egads »

In my own 1954 Cliff May, after removing the vat & mastic (bean-e-do!) I found footprints, boy sized, going from one end of the house to the other, in a line that must have been made before there were walls. At the end of the footprints is a small area where someone beat the almost cured concrete with a hammer.

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Post by dkottum »

I wonder what problems, if any, were encountered with the floor system of the Lowell Walter house. Each unit of the floor is separate and apparently poured individually, then assembled over a bed of broken stone containing the heating pipes. The corners of adjacent units were supported by concrete pedestals.

We visited the house some years ago but never noticed anything odd about the floor, perhaps not aware of its unusual construction at the time.

doug kottum

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