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Simple Estimating Instructions for UnderDeck

  • If you have a non-standard deck or installation, you can still use UnderDeck. Just follow the simple estimating instructions below. If you have a standard deck, we encourage you to order a UnderDeck Kit
  • When calculating, remember that all parts can be cut to size when installing. You can use our simple estimating calculator to help verify your estimates

UnderDeck Parts Illustration
 

Determining the Number of Joist Rails
Joist Rails
  • Measure the length of all inside joists in inches and multiply this number by 2
  • Measure the length of all edge/rim joists in inches
  • Add these two numbers and divide by 37 -- this is the total number of Joist Rails needed
  • You will need 3 nails for every Joist Rail or 1 Box of 200 for every 67 Joist Rails
Order Joist Rails

Determining the Number of Collector Panels
Collector Panel
  • Collector panels come in 12' and 16' foot lengths
  • You will need a collector panel for each joist space
  • For each joist space, measure the length in inches and round up to the nearest available Collector Panel length. Collector Panels are easily cut down during installation for uneven and unusual joist spacing.
  • To determine the width of each Collector Panel, use the chart below
Inside Joist Spacing On-Center Joist Spacing Width to Order*
Less than 10" 12" For 12" OC Joists
10 1/8" - 14 ½" 16" For 16" OC Joists
14 5/8" - 22 ½" 24" For 24" OC Joists

*This chart takes into account that the collector panel will bow upward when installed.
Order Collector Panels

Determining the Number of Joist Gutters and Double Joist Gutters
Boundary Gutters
  • Joist gutters come in 12' and 16' foot lengths
  • You will need a joist gutter for each inside joist (see boundary gutters below for edge/rim joists)
  • For each joist, measure the length in feet and round up to the nearest available joist gutter length.
  • If you have a double or triple joist, order Double Joist Gutters
Order Joist Gutters and Double Joist Gutters

Determining the Number of Boundary Gutters
Boundary Gutters
  • Boundary Gutters come in 12' and 16' foot lengths
  • You will need a Boundary Gutter for each edge/rim joist. On a typical deck you will use two
  • For both the left and right edge/rim joist, measure the length in feet and round up to the nearest available boundary gutter length
Order Boundary Gutters

Determining the Number of Water Diverters
Water Diverter
  • Water Diverters are available for Joist Spacings of 12", 16" and 24" On-Center
  • You will need one Water Diverter for each joist space
  • Water Diverters are easily cut for uncommon joist spacing. For example, for an 18" On-Center joist spacing, you would order a 24" On-Center Water Diverter and cut it down to fit the 18" space
Order Water Diverters

Determining the Number of Foam Weather Stripping Pieces
Weather Stripping
  • You will need one piece of Foam Weather Stripping for each Joist and Boundary Gutter.
Order Foam Weather Stripping

To Determine the Amount of Vinyl Flashing
Flashing
  • Vinyl Flashing is used for installations that require additional flashing (triangle framing, cantilever sections, etc...)
  • Vinyl Flashing is sold in 50' rolls.
Order Vinyl Flashing

Rain Gutters (Optional)

  • Rain gutters are optionally installed on the edge/rim joist perpindicular to your joists. Rain gutters can be purchased at your local hardware store or home center.