parking garage roof with artifical turf

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Author Message Julie Root Senior Member Username: julie_root Post Number: 24 Registered: 02-2004 Posted on Tuesday, October 18, 2005 - 01:25 am:    I am working on a garage roof with an artifical turf condition. The turf condition wants to have a minimal slope (.5 min. 1% max). In the garage we have a central plant that needs to be protected like occupied space. The rest of the areas is like a regular parking garage. Presently the job captain has provided a topping slab over a structural slab with waterproof membrane and protection drainboard in between, but the structural slab is also only sloped 1% max. In my mind a roof is a roof is a roof and my thought is that the structural slab should slope 2% min. This means that the topping slab could get deep in some areas and some others on the team are having a problem with this (curiously not my structural engineer). A few people we have consulted seem to not have a problem with a 1% slope when there is a waterproofing layer. Am I crazy for wanting to treat it like a roof with a min. 1/4"/1' even with a waterproof membrane? It is a huge area and I am also looking to break up slopes with several roof drains and trench drains along the edges of the fields. Any thoughts on experience with such a condition would be welcome. Mark Gilligan SE, CSI Senior Member Username: markgilligan Post Number: 41 Registered: 05-2005 Posted on Tuesday, October 18, 2005 - 02:06 am:    Thoughts from a Structural Engineer. Deflection of the structural slab and construction tolerances will probably create a situation where water will not drain off of the structural slab. I would expect that a rough surface like artificial turf will need a greater slope in order to drain. The protection drain board implies that water will be standing on the membrane since you cannot count on the shallow structural slab to drain. This is not good since ponding water tends to result in leaks. My recommendations: First trust your Structural Consultant regarding structural issues. This is why you hired him. Ensure that the plane of membrane and the drainage board have sufficient slope so that there is no standing water on the membrane. The topping slab need not have any slope if you can live with local bird baths. If the slab has already been poured consider a thin topping slab on top of the structural slab. This topping slab will ensure your slope under dead loads. If you install roof drains place them at the middle of the slab bay and not at the column or girders. This way the normal deflection helps you. If you place drains at the columns you just need a steeper slope to ensure positive drainage. William Wagner Senior Member Username: bill_black Post Number: 11 Registered: 07-2005 Posted on Tuesday, October 18, 2005 - 09:50 am:    What is the spec or type on the turf? Julie Root Senior Member Username: julie_root Post Number: 25 Registered: 02-2004 Posted on Tuesday, October 18, 2005 - 11:25 am:    Thanks Mark for the SE thoughts. The SE I am working with wants us to place the valleys (drain areas) at the grid lines for ease of construction and to not put stress on the center of the span. I will ask him the question with deflection in mind. It might be because it is a public school facility in earthquake country and the structure is extra stiff. Julie Root Senior Member Username: julie_root Post Number: 26 Registered: 02-2004 Posted on Tuesday, October 18, 2005 - 11:27 am:    William - For now we are writing the spec around a Sportexe system with a Brock closed cell drainage system underneath. It is public bid so no telling what we may end up with. William Wagner Senior Member Username: bill_black Post Number: 12 Registered: 07-2005 Posted on Tuesday, October 18, 2005 - 03:10 pm:    Julie- I think I have used the cell drainage system you are talking about, on a slab on-grade soccer field. At .5% it drains relatively slowly. This wasn't a big deal for the on-grade application as it still dried in a fraction of the time natural turf needs which was the point. Having seen this system work, my gut tells me you are going to see deflection and ponding. The ponding will result in the turf needing to be replaced more regularly. If you are looking for an argument against this idea that might be the way to go. -Will Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 132 Registered: 12-2002 Posted on Tuesday, October 18, 2005 - 03:24 pm:    Julie, The horizontal areas should ideally slope 3/8 to 1/2 inch per foot. 1/4 inch per foot is too low for a parking structure given the turf concept. I'm not sure what reference you'd cite in establishing that criteria except that you cannot predict the many things that could contribute to a water problem. A good thing to consider is to consult with an expert who solves parking garage water problems after-the-fact. Most of the building science /forensics firms in our area have that kind of expertise. Also, it's unclear to me how effective valleys can be 'at the grid line'. Because if this is so, your roof drain assemblies are competing with columns and beams. We usually dis-associate the valleys from the gridlines slightly, to avoid conflicts with the roof drain assemblies. This complicates the structural framing. Anonymous   Posted on Tuesday, October 18, 2005 - 03:39 pm:    Hot fluid applied rubber waterproofing membrane systems are warranted for installations over a dead flat slope. Dead flat. Same warranty as a sloped deck. I spec these systems all the time - saves a lot of messing around with slopes and takes all the worry out of it, for me. Also saves the client $. Unfortuntely, there are still a lot of "experts" and "building scientists" that insist that every single deck be sloped. Don't listen to them. Go with hot rubber and the decades of successful in-service history that these systems provide - even on a dead flat substrate. anon Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 133 Registered: 12-2002 Posted on Tuesday, October 18, 2005 - 04:05 pm:    Actually, anonymous, thou dispenseth questionable advice. Because dead flat slopes do not account for deflections of structural members, often ponds are created that have two effects: 1) water backs up 2) stuff grows where water backs up, even if the structural elements themselves remain nice and dry So, regardless of how nifty hot fluid-applied is (and I myself do not dispute) it is very poor advice indeed to "rely" upon a very low/no slope horizontal surface treatment. The building codes do not permit such a surface, nor should they - because there are other issues. That's why architects and engineers must pass these arduous exams, be registered, etc. Because if was up to you, you'd put people's lives at greater risk because you don't understand that (roofs - we're talking about a roof on a parking garage) designed to deliberately hold water tend to fail with greater probability than those that are designed with slope. Julie Root Senior Member Username: julie_root Post Number: 27 Registered: 02-2004 Posted on Tuesday, October 18, 2005 - 04:08 pm:    I totally agree with you that we should be looking at 3/8 to 1/2 per foot. My colleagues are refering to the UBC that notes a minimum 1/4 per foot and they have talked with a parking garage designer and a structural engineer who have done it with a 1/8 per foot. The project is in CDs and my colleagues are not wanting to 'rock the boat' but I will be the B*#@! if I have to make it right. A couple of manufacturers have what they call 'dead flat' systems that my colleagues are refering to. I have not had experience (neither have they) with these systems and I need to do my homework. One waterproofing consultant that my colleague has talked with believes in these 'dead flat systems'. Does any one have expeience with these systems? - JDrain and Hydrotech have them as there are probably others. Most of the research on the web seems to refer to more residental construction and then you see where they have even failed and condo associations are trying to fix the conditions. This is a school district that has not done anything like this and is the second largest district in the country. I really want to do the right thing. I appreciate your insight William. I also have a concern that the contractor cannot maintain a consistant tolerance in 180' for a 1% slope. Even if they tried very hard. I am constantly amazed at how the architects think construction has no tolerances. Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 134 Registered: 12-2002 Posted on Tuesday, October 18, 2005 - 04:10 pm:    Anonymous, check Chapter 15 of the IBC which states: "POSITIVE ROOF DRAINAGE. The drainage condition in which consideration has been made for all loading deflections on the roof deck, and additional slope has been provided to ensure drainage of the roof within 48 hours of precipitation." Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 135 Registered: 12-2002 Posted on Tuesday, October 18, 2005 - 04:27 pm:    I've got experience with Hydrotech and can attest it it being 100% capable of being installed dead-flat, but that's not the point. The most overlooked problem (beyond the deflection issue) is that stuff grows in standing water, whether it is out of sight, or a surface puddle or whatever. That 'stuff' is nature trying to take our architectural handiwork back to a stable state. And even if it doesn't affect the roof membrane itself (in our lifetime), it is not desirable and most people would consider that a failure of the design. The average guy on the street doesn't like puddles on buildings - and for good reason. Julie Root Senior Member Username: julie_root Post Number: 28 Registered: 02-2004 Posted on Tuesday, October 18, 2005 - 04:42 pm:    And standing water in lovely southern CA probably grows more stuff than we could ever dream. Thanks Doug. The SE notes that we would off set the valleys to one side of the colum grid or the other. I really appreciate your thoughts on the deflection and in the CBC section 1611 specifically addresses the deflection issue so I have a call into my SE No offense to you gentlemen and I am not one to ever really think this way, but I am afraid in this situation it is because the info is coming from a woman. William C. Pegues, FCSI, CCS Senior Member Username: wpegues Post Number: 502 Registered: 10-2002 Posted on Tuesday, October 18, 2005 - 04:48 pm:    I have to agree 100% with Anon above here. 30 years of experience with dead flat roofs, plaza decks, etc. with hot fluid applie systems, specifically American Hydrotech, Henry (formery Monsey Bakor) and Barraett. No roof problems. Major projects that have been up for more than 20 years include the M Street Headquarters building of the National Geographic Society. Ponding is easily prevented by coodinating with the structural engineer to locate roof drains at what will become natural sag points in the slab. Its not hard to do. The membrane is meant for water retention systems, they are the prime membranes for vegetated protected membranes systems. The quote of positive drainage with the code is not a requirement for these systems. By their nature, they are considered a waterproofing system offering a roofing guarantee. Perhaps this is a strange way around the code requirement, but all the manufacturers do if they are questioned on this is provide a standard statement to that effect, and the issue disappears. It comes up ever couple years or so, then goes away again. But that does not mean that you can't slope the slab if you want - sloping the slab is always the best, it is just that with these membranes you don't have to. William Ron Beard CCS Senior Member Username: rm_beard_ccs Post Number: 85 Registered: 10-2002 Posted on Tuesday, October 18, 2005 - 05:45 pm:    Julie: Having dealt with this scenario many times in the past - for both plaza decks with a 2' to 3' earth cover and with thick and thin landscaped green roof systems - I would suggest you consider the following recommendations for any concrete frame structure: 1. Develop a drainage plan and slope the structural concrete slabs to those drains. Follow Doug’s recommendation of 3/8 to 1/2 inch per foot. 2. Waterproof with a high-quality, reinforced, hot fluid-applied rubberized asphalt membrane waterproofing system to a thickness of not less than 200-mils. There are many superior systems to choose from. 3. Install a prefabricated, composite molded drainage panel with a permeable geotextile facing. 4. Cover the drainage panels with a heavy-duty protection course. I use a semi-rigid sheet of fiberglass or mineral-reinforced-asphaltic core, pressure laminated between two asphalt-saturated fibrous liners; 3/8 to 1/2 inch thick. 5. Install the earth cover or use on of the several green landscape systems 6. Don’t forget to address the perimeter and penetration flashings and any contraction and expansion joints. There is no need for a concrete topping slab. I have consulted with the estimating departments [something I regularly do - I never fail to get the “real scope” on products/systems and never fail to be educated] of several major concrete installers and they all same the same thing: it is less expensive and easier to provide the slopes to the structural pour than to come back and install a topping later. Not only do you save space but the reduce the total load on the structural system. It has already been mentioned about the additional weight ponded water adds to the structural system. Note Re Anon’s Comments: I obviously agree with Anon’s comment about using the hot-applied systems and their manufacturer’s do provide warranties for dead level applications. But I am a firm believer in Murphy’s Law - if it’s possible to happen, it will happen. And, there is the common sense factor: water that is not on a roof, will not leak through a roof. But being "building scientists" is not bad - aren’t all specifiers building scientists? We should always consider the factors beyond the immediate. While there are waterproofing systems capable of dead level applications, is it good for the structure? If your project is a large one, there are many other factors to consider. Ron PS: This is a rare day indeed - William Pegues agreeing with an Anon!!! Wonders never cease. <G> Tom Heineman RA, FCSI, SCIP Senior Member Username: tom_heineman Post Number: 63 Registered: 06-2002 Posted on Wednesday, October 19, 2005 - 10:26 pm:    The 20th century was full of 3-word sayings that - right or not - caught everyone's eye: "Less is more", "Machine for living", "Form follows function", "Ornament is criminal", etc. Harry Weese of Chicago was up there with the best of the motto-makers. Once he caused a pause in in discussion llke this by softly interjecting, "Flat roofs leak."