Mike Silvers, CPRC, Silvers Systems Inc. & FRSA Director of Technical Services
So, your next reroof may need tapered insulation? Here are a few things to consider.
When conscientious contractors inspect a building with a low slope roof that needs to be reroofed they will need to consider many variables. Among those considerations is how well the roof drains and what, if anything, needs to be done to improve it.
To properly do so, you will need to be familiar with current code requirements concerning roof slope, positive drainage and ponding.
SECTION 706 - EXISTING ROOFING
706.1 General.
Materials and methods of application used for recovering or replacing an existing roof covering shall comply with the requirements of Chapter 15 of the Florida Building Code, Building, or Chapter 9 of the Florida Building Code, Residential. Roof repairs to existing
roofs and roof coverings shall comply with the provisions of this code.
Exception: Reroofing shall not be required to meet the minimum design slope requirement of one-quarter unit vertical in 12 units horizontal (2-percent slope) in Section 1507 of the Florida Building Code, Building for roofs that provide positive roof drainage (High-Velocity Hurricane Zones shall comply with Sections 1515.2.2.1 and 1516.2.4 of the Florida Building Code, Building).
SECTION 1502 & SECTION R202 - DEFINITIONS POSITIVE ROOF DRAINAGE. The drainage condition in which consideration has been made for all loading deflections of the roof deck, and additional slope has been provided to ensure drainage of the roof within 48 hours of precipitation.
1507.10 & R905.9 Built-up roofs. 1507.10.1 & R905.9.1 Slope.
Built-up roofs shall have a design slope of not less than one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage, except for coal-tar built-up roofs that shall have a design slope of not less than one-eighth unit vertical in 12 units horizontal (1-percent slope).
1507.11 & R905.11 Modified bitumen roofing. 1507.11.1 & R905.11.1 Slope.
Modified bitumen membrane roofs shall have a design slope of not less than one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage.
1507.12 & R905.12 Thermoset single-ply roofing. 1507.12.1 & R905.12.1 Slope.
Thermoset single-ply membrane roofs shall have a design slope of not less than one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage.
1507.13 & R905.13 Thermoplastic single-ply roofing. 1507.13.1 & R 905.13.1 Slope.
Thermoplastic single-ply membrane roofs shall have a design slope of not less than one-fourth unit vertical in 12 units horizontal (2-percent slope).
1507.14 & R905.14 Sprayed polyurethane foam roofing. 1507.14.1 & R905.14.1 Slope.
Sprayed polyurethane foam roofs shall have a design slope of not less than one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage.
I must point out that the exception “for roofs that provide positive drainage” in 706.1 and the one for coal tar built-up are the only exceptions in the non HVHZ portion of the code that permits any low slope roof covering to be installed on slopes less than one-quarter vertical in 12 units horizontal (1/4 in 12). There are no code provisions for “ponding water specs” or roof coverings that the “manufacturer says can pond water.” That clearly doesn’t mean that all roofs on existing buildings must have a 1/4 in 12 slope. It does mean that all existing roofs must “provide positive drainage.” So, to put it another way, an 1/8 in 12 tapered insulation
system is perfectly acceptable as long as it provides positive drainage.
How do we know if the roof provides positive drainage? It may be very obvious. On a mono-slope (one-way slope), roof with parapet walls on three sides, it’s as simple as measuring the parapet height at the low side and the high side of the slope, establish the difference and divide that into the distance between the two points where the height was measured. Slope in the 1/4 in 12 range will usually be enough to compensate for deflection or construction tolerances such as improperly sized nailers.
As the slope gets less, or the roof configuration gets more complex, it starts getting harder. One way is to inspect the roof 48 hours after a heavy rain. That will give you a fairly good representation of how well the roof drains. This method can be difficult, timing is critical and a drought would obviously create problems with this approach.
Most seasoned roof inspectors can spot signs of ponding. The most obvious one is discoloration of the roof surface. Another sign can be a tide (not really) line of debris left at the outside perimeter of a persistent ponding area. Deterioration of roof coating is another
example. Observation of drain location and sighting the slope can also be helpful. Keep in mind that the presence of aggregate can reduce drying time by displacing and thereby reducing the water volume in a ponding condition. It will also increase the surface area causing quicker evaporation.
Laying out a grid on the roof and using a transit (or instrument) to plot the slope is a very accurate way to establish slope. However, it is also time consuming, making it impractical for a typical reroof assessment to provide a quote.
After making a determination that you need tapered insulation, it is very important that you realize this could be a major change in how the building and roof have functioned. You will need a detailed roof plan with an accurate foot print and dimensions. It will need to include the exact location of primary and any secondary drainage. It will also need to show the size and location of any equipment or large penetrations that could interrupt the water flow. All of these conditions will dictate how the tapered system must be designed.
The worst possible outcome to adding tapered insulation would be to cause a large volume of water to be held on the roof when your intention was to eliminate a few ponding conditions. You have to consider a clogged primary drain or, worse yet, possible flooding or storm surge that interferes with the proper function of a drainage system.
In my next column, we will evaluate several roof plans and how all these factors can enter into a good tapered insulation design and a well-functioning installation.
Michael J. Silvers, CPRC is FRSA Director of Technical Services. Mike is an FRSA Past President, Life Member and Campanella Award Recipient, and a Florida Licensed Certified Roofing Contractor, who brings over 40 years of industry knowledge and experience to FRSA’s team. Mike is available to FRSA members who have codes or technical questions and can be reached at 800-767-3772 ext. 169 or by email at silvers@floridaroof.com.
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