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UnderDeck Deck Drainage System
UnderDecking Deck Roof System
Simple Estimating Instructions
- 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
Joist Rails
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To Determine the
Number of Joist Rails:
- Measure the length of all joists in inches and multiply this number by 2
- Measure the length of all edge joists in inches.
- Add these two numbers and divide by 37.
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Order Joist Rails |
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Collector Panels
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To Determine the
Number of Collector Panels:
- Collector panels come in 10', 12', 14' 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. To determine the width of each Collector Panel, use the chart below:
| Joist Spacing |
On-Center
Joist Spacing |
Width to Order* |
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Less than 10" |
12" |
10.5" |
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10 1/8" - 14 ½" |
16" |
15" |
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14 5/8" - 22 ½" |
24" |
24" |
*This chart takes into account that the collector panel will bow upward when installed.
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Order Collector Panels |
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Joist Gutters
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To determine the number of
Joist Gutters:
- Joist gutters come in 10', 12', 14, and 16' foot lengths.
- You will need a joist gutter for each joist (see boundary gutters below for edge 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
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Order Joist Gutters and Double Joist Gutters |
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Boundary Gutters
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To determine the number of
Boundary Gutters:
- Boundary gutters come in 10', 12', 14, and 16' foot lengths.
- You will need a boundary gutter for each edge joist. On a typical deck you will use two of these.
- For both the left and right edge joist, measure the length in feet and round up to the nearest available boundary gutter length.
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Order Boundary Gutters
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Water Diverters
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To determine the number of
Water Diverters:
- You will need one water diverter for each joist space.
- If your installation requires that you stop and restart your joist gutters due to a beam that runs perpendicular to your joists, you will need two ADDITIONAL water diverters for each joist space. See Installation Diagram 'Beam at Center of Deck' for more info
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Order Water Diverters
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Foam Weather Stripping
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To determine the number of
Foam Weather Stripping Pieces:
- You will need one piece of Foam Weather Stripping for each Joist and Boundary Gutter.
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Order Foam Weather Stripping
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Vinyl Flashing
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To determine the amount of
Vinyl Flashing:
- Measure the length of the deck (in feet) where it is attached to your home.
That is the length of flashing you will need.
- Vinyl Flashing is sold in 50' rolls.
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Order Vinyl Flashing
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Rain Gutters (Optional)
- Rain gutters go on the edge joist parallel to your home. Rain gutters can be purchased at your local hardware store.
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