October 20, 2021
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A lesson from Wafflemat Tom with his bucket

By: Irene Hilsendager
June 7, 2019

Not knowing what a wafflemat is, I contacted an engineer who knew the business inside and out. Being a farm girl, I thought I knew the ins and outs of concrete slabs. No so! When he entered my office he was carrying what I thought was an extra-large gray Tupperware container. To my surprise, it is a plastic form set directly on grade to create a grid of ribs. After hearing Tom’s explanation about the pros and cons, it does make a lot of sense.

A waffle slab foundation is an above-ground type of foundation used to provide load-bearing capacity in expansive, rocky or hydro collapsible soils, as Tom calls it “bad” soil. If driving through the section where contractors are building new homes, you see miles of just flat, probably twelve inches high, of just solid cement forms. Now stop and think. If heavy rains, floods or earthquakes; which we all have experienced, where will the water seep to; under the cement slab. What will the slabs do? The dirt will rise and move the cement especially at the corners and it will crack and if the structure is already on the slab, you know that the doors, windows and cracks in the ceilings will occur. If using the waffle slab, the dirt can rise up into the plastic forms and the cracks, etc. will not happen. The wafflemat acts as a relief valve.

During the last earthquake in Napa, it was proven that the structures that were on the wafflemat stood very sound, but across the street without the wafflemat, the buildings completely collapsed.

By checking with engineers, it has been proven that the wafflemat is by far the least expensive and now around 85 percent of contractors have switched to the wafflemat for foundations. The foundation is created by placing a series of single-use plastic forms set directly on the ground to create a grid of ribs and inflexible pouring a post tensioned, rebar reinforced concrete slab, usually four to eight inches thick between the ribs. Sometimes, expanded polystyrene blocks are used instead of plastic forms, to prevent creating an air space under the slab. The pour created concrete beams running throughout the footprint and perimeter of the foundation, with voids between, in one operation. The completed slab then sits on the ground bearing on the ribs created between the forms. the void areas underneath the slab allows for soil movement.

The waffle slab foundation is very stiff with strength to resist swelling from landscaping practices, surface drainage or flooding from any source. It does not require presoaking underlying soil and there is no need for footings and since the slab section is typically 14-20 inches above grade, it will typically not require a moisture barrier.

The good news is that the plastic forms are made in San Leandro, Ca. and materials are less expensive, due to the fact they are made from recycled old milk cartons and plastic water bottles.

Tom told a story of a Pat Tang, an engineer at Apple, who contacted him. He needed 11,505 plastic forms in the Philippines to build a very large community center. One problem arose though. The monsoon season was upon them very soon and to send that many by ship would take 33 days to get to the construction site. Tom solved it by loading the forms on a 747 at the San Francisco airport and within five days the forms were in place ready to start the community center and was finished before the monsoon hit.

Wafflemat has installed 25,000,000 feet of forms across the United States and Mexico and there has not been one structural failure or callback. 

Wafflemat slabs work really well on sites that are almost flat, natural soils or a controlled fill. There is no need to haul out the bad dirt and bring in good soil. They work well on non-reactive sites and slightly reactive clay sites. Waffle slabs are not recommended on highly reactive clay sites as the requirements for good drainage is almost impossible to achieve.

As of July 8, MKM & Associates will be starting to pour foundations at the Station Ave. in Rohnert Park. North Coast Concrete is the contractor and MKM will be the sub-contractor.

If you wish to see or know more information on the wafflemat slabs, call Tom Richards at 925-683-2739 as there is now an MKM & Associates office in Rohnert Park.