Roofing Details: Internal Roof Drains
Roof drains are provided throughout the roof to remove water from the roof surface. The National Roofing Contractors Association (NRCA) and most membrane manufacturers require the removal of ponded water from the roof surface within 48 hours after a rainfall. This is accomplished by locating internal drains at low points of the roof or at points of maximum structural deflection.
Roof Drain Design
The amount of drains required is determined by conduction a roof drain calculation that takes into account the size of the roof area and the slope of the roof among other factors.
The most important factors in roof drain design are:
• Properly slope the structural framing and deck (minimum ¼ inch per foot).
• Use an insulating fill or tapered insulation for slope.
• Properly locate roof drains, scuppers and gutters to remove water from the roof’s surface.
• Recess the drain below the roof surface.
• Design saddles or crickets to create positive drainage to the drain.
• Provide extra insulation around the drain head to prevent condensation.
Use materials consistent with the roof membrane manufacturer’s requirements.
Here are seven factors to use when installing roof drains:
1. Install flashing materials similar to the dimensions necessary to accommodate the flashing ply configurations shown on the related drawings.
2. Prior to installation, flood all drains to determine if they are in working condition. Provide a unit cost price to unplug 40 feet of drains, if and when it is needed.
3. All drains shall be covered during installation to safeguard from bitumen flow.
4. Taper roof insulation around roof drains evenly over an area 4 feet by 4 feet to aid drainage. Add tapered edge strip to create a smooth transition.
5. Install a thirty-six (36) inch square copper flashing at each sump, set on finished roofing felts in adhesive. Apply a new two-ply strip in of membrane and adhesive at the perimeter of the copper.
6. Roofing plies, copper flashing and membrane shall be secured under the clamping ring of the roof drain.
7. Install new drain strainers.