Water Managed Roof – Re-roofing and Adding Insulation over a Flat Roof

Controlling rainwater is the single most important factor in the design and construction of durable roof assemblies. The fundamental principle of water management is to shed water by layering materials in such a way that water is directed downwards and outwards out of the building or away from the building. The key to this fundamental principle is drainage.

When a flat roof is retrofitted by adding rigid foam insulation above the existing roof sheathing, the new air and water control layers must be carefully integrated with the existing roof structures.

The assembly shown here is an unvented roof assembly and must meet relevant code requirements (Table R806.4 Insulation for Condensation Control of the 2009 IRC (ICC 2009a) and Table R806.5 Insulation for Condensation Control of the 2012 IRC (2012a). The ratio of air-impermeable insulation (polyisocyanurate) to air-permeable insulation (cavity fill) must be sufficient to avoid condensation problems. Assemblies with greater depth of cavity fill insulation require more rigid board insulation. Per 2012 IRC Section806.5, the interior of the roof (gypsum board or plaster) can have a Class II or Class III vapor retarder; a Class I vapor barrier (polyethylene) is prohibited as it will eliminate drying of the assembly. For further explanation, see IRC FAQ: Conditioned Attics.

Polyisocyanurate is a suitable rigid foam for installing above the roof deck. The rigid foam can be installed over new sheathing coated with an integrated water-resistive barrier as shown in Figure 1 or the insulation can be installed over the existing roof sheathing, which is first covered with a fully adhered air control membrane that extends up the parapet sides above the flat roof as shown in Figure 2. If foil-faced polyisocyanurate rigid foam insulation is used, a protective layer of insulation cover board should be installed over it prior to installing the roofing membrane water control layer, as shown in Figures 1, 2 and 3. Glass fiber-faced polyisocyanurate can be installed without an insulation cover board.

“Flat” roofs should never be flat. All low-slope roofs must be sloped to drains (as shown in Figure 4) or scuppers at a minimum slope of ¼ inch per foot (ASTM 2009). It is vital that any roof penetrations (drains, skylights, or mechanical curbs) are properly flashed to prevent water entry. The materials that form the water control layer (in this case the roof membrane) should overlap each other in shingle fashion or be sealed in a watertight manner (in the field of the roof if the slope is insufficient to rely on shingle-lapped water shedding).

Skylights, mechanical curbs, and other roof penetrations must be integrated into the roof’s drainage plane(roofing membrane) (see Figures 5 and 6). Membranes or formable flashings that line these curbed openings are all elements of the roof water control layer. These approaches work best when they are sloped toward the roof drain, so that rainwater is directed off of the building.

How to Re-Roof a Flat Roof

1. Inspect the structural integrity of the roof. Pooling water (seen in Figure below) is a sign that more slope and drains or scuppers are needed. Remove the existing roof membrane, insulation, and sheathing (where needed) and check the roof framing for any deficiencies, rot, insect damage, etc. (see Figure below). Do not proceed if any repairs need to be performed. Based on the findings, revise the roof assembly and review specific detailing as needed. Follow the minimum requirements of the current adopted building and energy codes.

   

   

2. Remove two or three roof sheathing boards near the perimeter of the roof, leaving one or two boards at the parapet (Figure below). Mechanically fasten a strip of pressure-treated plywood to the interior vertical face of the parapet to allow for attachment of the strip of fully adhered air barrier membrane. Install a strip of OSB sheathing with an integrated water-resistive barrier at the roof perimeter adjacent to the parapet on top of the remaining board sheathing (Figure below). Provide a continuous bead of sealant between the existing board sheathing and a strip of new roof sheathing.

   

   

3. Spray 2 inches of closed-cell spray foam in the roof cavity at the wall perimeter to create an air barrier connection between the wall and the roof, and to provide adequate thermal resistance to prevent condensation (Figures below). The area should be free of debris and dust prior to spraying for adequate adhesion. Install fibrous insulation (e.g., cellulose) in the rafter cavities beneath the existing board roof sheathing (Figures below). Re-install the roof sheathing boards.

   

   
   

   

    

   

    

   

4. If needed to create a slope, install sleepers (tapered wood furring strips) on the roof rafters prior to installing the new roof sheathing (Figure below). Install OSB sheathing with an integrated water-resistive barrier over the entire field of the roof (Figure below). Seal all of the sheathing seams with appropriate tape that is applied to a clean surface and pressed on with a roller to ensure good adhesion. This coated, taped OSB is the air control layer at the field of the roof. Install a strip of fully adhered air barrier membrane on top of the new roof sheathing at the roof perimeter adjacent to the parapet and extend it to the top of plywood to form an air barrier connection to the parapet wall. Seal all penetrations, such as drains, skylights, and mechanical curbs, in an airtight and durable manner to the air control layer.
   ALTERNATELY, instead of the coated OSB, install a fully adhered air barrier membrane over the existing roof sheathing and extend up to the top of the plywood at the parapet to form a continuous air control layer. Seal all penetrations, such as drains, skylights, and mechanical curbs, in an airtight and durable manner.

   

   

5. Install polyisocyanurate rigid foam insulation over the roof sheathing (Figure below). The ratio of air-impermeable insulation (polyisocyanurate) to air-permeable insulation (cavity fill) must be sufficient to avoid condensation problems, per Section R806.4 of the 2009 IRC (ICC 2009a) and Section R806 of the 2012 IRC (2012a). If tapered sleepers are not installed, provide tapered insulation to ensure the roof is sloped to drain. Fit the joints of the insulation together tightly. When installing multiple layers, offset the seams in two directions, add a continuous bead of sealant at the perimeter of the roof between each layer, and tape the seams of each layer. Install wood blocking for future photovoltaic (PV) panel installation (Figure below). The surface of the blocking must stand proud of the roof deck to reduce water penetration risks at fastener penetrations (Figure below). Install insulation cover board (typically fiberglass-faced gypsum board) over the polyisocyanurate rigid foam insulation. Ensure compatibility of the cover board with the roof assembly.

   

    

   

    

   

6. Install roofing membrane over the insulation cover board and lap it over the parapet to provide a continuous water control layer (Figure 20). Seal all penetrations, such as drains, skylights, mechanical curbs, and PV blocking against water leaks per new construction best practices (Figures 21, 22, and 23). Install metal cap flashing at the parapet over the roofing membrane. Provide drip edges on either side of the cap flashing to prevent staining of the building façade. (See Figure below and BSI-050: Parapets—Where Roofs Meet Walls.)

   

   
   

   

   
   

   

    

   

    

   

7. Install walking mats to protect the membrane surface when accessing the flat roof (Figure below).