Installing a DIY Under-Deck Drainage System

Water is one of the most dangerous elements that can destroy your wooden decking. Any amount of water that is logged on your deck may cause the wood to rot, warp and crack. If the water is left to its means to drain wherever it may even cause the underside of your deck to rot and warp too. The absence of a proper water drainage system would also mean that you cannot use your deck's underside for anything.


Although, you can mitigate this problem entirely by choosing tanzanite stone deck tiles rather than wood decking. Tanzanite decks can be installed only at a minimum additional cost and since they are made of stone, they would not wither away or disintegrate over time due to the effects of weathering.


In case your deck is elevated, primarily if it is located on the second story, consider installing a DIY under-deck drainage system. It prevents water from passing the deck and pooling underneath it. Instead, the water would be redirected into the gutters so that all the area beneath the deck would stay dry and usable.


If your deck is not installed yet, you can get more out of investment with an under-deck drainage system. After you install it, you can convert the space below your deck using outdoor patio tiles to a patio or use the space as waterproof under-deck storage.


Read more: How to Lead Deck Stairs to a Patio


There are a number of counters over the joist deck drainage system, meaning when it is installed before you lay the deck boards. This type of deck drainage system would allow you to finish the underside of the deck with lights etc.


What to Consider Before Installing a Drainage and Gutter System


Deck drainage and gutter systems can easily be installed above or below the joists, however, the thing to consider is the amount of protection each provides. Some above the joist system would shield the entire substructure from water.


Under-deck gutter systems that use below the joist panels can offer no such protection because one way or the other some water will always contact the joists. As a matter of fact, the panels can accelerate decay by trapping moisture in the joist bays. Rotted wood can even fall onto these panels and impede the flow of water further exacerbating the problem.


Take into consideration that every below the joist approach, be it a retail kit or anything, it makes it impossible for you to install any kinds of lights or other electrical accessories in the ceiling below the deck.


How to waterproof your deck with a DIY drainage system


Step 1: Cut Your Downspouts and Install them along one rim joists


The funnel-like downspouts fit between joists that are spaced about 16 inches on the center and when it is turned to 90 degrees, joists 12 inches toward the center. They can also be adapted to fit oddly spaced joists.


1- At about 10 inches of height, the downspouts would fit well with about 2x10 joists. If your joists are any smaller then you should cut the outlets of the funnel so that they extend about 1 inch into the gutter. If you have about 2x12 joists, add a section of PVC downspout to the funnel outlet using self-drilling screws. Next, you should cut a crescent-shaped section from one side of the funnel using a utility knife. Just follow the indicated cut line when performing these cuts.


Step 2: Slide one end of the trough under the wall flashing


Unroll your trough panel and allow it to relax first. Then you should fold it in half lengthwise to create a crease. This will help to form a proper trough where all the water would fall into the center and flow downwards to the spout. Slip one end of the material under the ledger flashing as far as possible and staple it in place. IMPORTANT: DO NOT STAPLE THE FLASHING.


Step 3: Staple and cut the trough to length


Staple one edge of the trough panel over the top of a joist so that it is flush with the joist edge. Keep the trough taut as you would staple it. Have someone hold the other side of the trough to make the job easier. Once you have unrolled the downspout, fold the material tightly and then cut it to length with one end falling near the center of the downspout hole.


Step 4: Staple and roll out more trough


Repeat steps 1 to 3 until your entire deck is covered. Pour some water to make sure each trough would drain properly.


Step 5: Fill in the smaller areas at the angles


If there are any angles that cannot have a downspout then make your trough sections shorter and back pitch them so that the water flows to the long trough.


Step 6: Cover the Staples and Joints will butyl tape


Using a utility knife or scissors cut away excess overlapping trough material without exposing any of the bare wood. Starting at the house end and working up towards the downspouts, apply the butyl tape over the working stapled trough ends by removing the paper backing as you would unroll and apply it. Make sure when you apply the tape you keep it from wrinkling, press down firmly so that it would form a tight seal at all points.


Step 7: Install the under-deck gutter


Install a PVC or aluminum or steel gutter so that it would align under the gutter. The gutter should be slightly sloped ⅛ inch per foot so that the water would run towards the downspout of the gutter.


Step 8: Install the Decking


You should install your decking by face screwing it or rather using hidden fasteners. Using nails is not recommended as it would puncture the butyl type whereas the screws once they penetrate the tape. Do not use nails to attach the deck boards.


Make sure to consult with your local supplies store for any more information about the different systems that are available. It is better to pre-plan for such additions, make sure you head onto our deck design tool to factor in your deck drainage system.

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