Question: A structurally sound plywood roof deck, about 15 feet wide and 60 feet long, dead flat, forms a walking plane and terrace for housing units above.

The wood deck is to be covered by a hot-mopped fiberglass roofing system, which in turn will be protected by a concrete topping, sloped to drains, which will also be a walking surface.

The topping can vary in thickness from a minimum of 3/4 to a maximum of 1 1/2 inches and still drain properly. The questions are:

  • What concrete mix is recommended?
  • Is there any way to reinforce the slab to minimize cracking?
  • Should a bonding agent be used to try to bond the slab to the membrane?

Answers: To answer your last question first, it seems best not to try to bond the slab to the membrane.

Regarding Question 1, the ACI 302 Standard, "Recommended Practice for Concrete Floor and Slab Construction (ACI 302-69)" provides information for unbonded toppings (called Class 7 floors) for concrete. Some of this information should be adaptable to your problem. The committee recommends concrete having 28-day design compressive strength of 4000 psi. If the concrete will be exposed to freezing and thawing cycles it should be air entrained. A Portland Cement Association publication, "Resurfacing Concrete Floors," IS144.03T, suggests a mix using 3/8 inch maximum size aggregate proportioned as follows: 1 part portland cement, 1 part sand and from l l/2 to 2 parts coarse aggregate mixed to a water-cement ratio of no more than 0.44.

Regarding Question 2, the ACI 302 Standard and IS144.03T both recommend the use of welded wire fabric at the rate of approximately 30 pounds per 100 square feet. ACI 302 recommends a thickness of at least 2 1/2 inches, and IS144.03T recommends at least 2 inches. Recommendations for unbonded toppings over conerete call for a joint above each joint in the base floor. This would suggest that if there are locations in the wood deck in which large localized movement might be expected it would be wise to provide joints at those places.