Design & DIY

How to prevent water damage when you’re building outdoors

Rainy porch

This article was originally published in the Fall 2015 issue of Cottage Life West magazine. 

Building for outdoor durability requires more than just choosing the right materials or applying a weatherproof finish. Smart design techniques will help any outdoor building project shed water and prevent the damage it can cause.


Sitting water wants to soak in, especially when it’s on horizontal surfaces. Add a physical, waterproof barrier to protect the wood beneath.

  • Deck beams and joists are prone to failure where water collects on top, in the gaps between deck boards. Apply a strip of adhesive waterproofing roof membrane before the deck boards go on, an easy step that will add years of durability.
  • The tops of fence or railing posts have vulnerable end grain exposed to the weather. A wood, aluminum, or copper cap will help keep moisture out.
  • Don’t forget flashing in places where water can flow down a vertical surface to meet a horizontal one—at the top of a window, for example. Flashing will direct the water away from the vulnerable seam, over the flat wood, and on its way.

Control drips

Capillary action and surface tension allow rainwater to move uphill or across the underside of a horizontal surface. Saw a drip groove to break the water’s path so that it falls away.

Illustration by Jacques Perrault
  • A drip groove is especially useful on the underside of a windowsill—it helps prevent water from staining the siding below.


Where two exterior surfaces meet, moisture collects. Add ventilation spaces to encourage airflow.

Illustration by Jacques Perrault
  • Raise the bottoms of posts on feet (2), where possible, to allow air to flow underneath.
Illustration by Jacques Perrault
  • A wall-mounted object, such as a deck ledger, will last longer if air can get behind it. Use water-resistant spacers for 1/4″ or so of clearance.

Keep your feet dry

Water may fall from above, but it also seeps up from below. A barrier can protect the base of anything that touches the ground.

  • The bottoms of the legs rot first on almost every picnic table. Add “shoes” of brass bar stock or plastic pads, attached with stainless steel or brass screws.

Rooftops everywhere

When building outdoors, look to your roof for guidance and incorporate rain-shedding slopes and angles in other places.

  • Bevel the top of a horizontal handrail, so water doesn’t sit.
  • If you don’t want to cap the top of a fence or a railing post, saw a slope.
  • A deck or patio should not be perfectly level—slope it very slightly away from the building.