Architectural Details #1: (Part 2) Waterproofing through Design Solutions

Wash

Wash is basically a sloped surface that drains water away from all critical connections and terminations.  It is usually used on all external horizontal surfaces.  The steeper the slope of the wash, the faster it can drain water out.  A steeper slope will also hinder wind to push water back up.

Below are some examples where wash is used:

Window Sills or Door Sills

A minimum slope for this kind of wash is 1:10 or 1:12.
Chimney

The concrete cap on the chimney is given a wash to keep water away from vulnerable cracks between the clay flue and the concrete.  This wash should be at least 1:12 slope.

The cricket on the upslope side of the chimney diverts water away from the termination of the chimney and the roof.
Parapet

The coping on a building parapet usually drains towards the roof to minimize water in staining the face of the building.
Wall Panel Joints

The horizontal joints between wall panels should have a wash draining the water outside.  This will prevent water from penetrating the building even if the sealant fails. 
Roofs

The most common use of the wash are on roofs.  The slope of the roof will also depend on the type of material that will be used on it.  If shingled roofing will be used for example, the slope should be steep enough to prevent wind from driving the water up and through the gaps.  Refer to the manufacturers' standards for the recommended slope of the materials to be used.

Other applications for the wash...

Roads, Roof Decks, Parking Areas, and other horizontal external surfaces where you don't want water to retain and build-up.


Overlap

An overlap is basically any surface that extends over another surface so that water that is pulled by gravity cannot go beneath the lower surface.  For this to work, the surfaces should have a slope or are running vertically.

Roof Shingles

Roof shingles' overlapping design keeps water from entering the structure through the roof.  This only works however if the roof is steep enough so that the wind won't be able to push water back up and through the gaps of the shingles.
(Image by richwall100 on Flickr)

Wood Siding

This works like the roof shingles, only that it's running vertically. The weak spots on wood sidings are the end joints which should be caulked and flashed to prevent water penetration.
(Image by retropc on Flickr)

Flashing


Flashing is another type of overlapping.  It keeps water away from possible gaps and cracks on critical terminations.  This is usually placed on wall to roof connections, parapet to deck connections, and on window and door frames.  Other types of flashing can be used on other parts of the building where cracks and gaps need to be protected from water entry.


Overhang and Drip

Water may run down the wall of a building or structure and may enter openings in the wall.  Using the overhang and drip will clear the water by first diverting the water away from the opening by placing an extended profile or an overhang above the opening then by creating a continuous groove or drip on the soffit and letting gravity pull the adhering water away from the overhang.  Water has surface tension, a characteristic that causes water to cling to the underside of a surface.  Wind will be able to push the water back to the wall and into an opening if the drip is not present to break its path.

Fascia / Ledges / Overhangs

There are several applications of the overhang and drip that are being used on roof fascias, ledges and building overhangs.  Two of the most common applications are seen on the images above.  On the left, the fascia is extended below the soffit or the underside of the eaves so that water would drip free of the crack between the fascia and the eaves.  On the right, the drip groove, or drip mould (the term common in the Philippines), would break the path of the water going towards the wall and eventually be pulled free by gravity. The drip groove should be wide enough and deep enough so that water may not be able to bridge over it.  A width of 6mm and depth of 3mm is just about right.

Coping

A parapet coping is usually sloped towards the inside of the building to prevent staining of water on the facade.  An overhang and drip is usually placed just below the coping to keep water out of the critical joints.
Other applications for the overhang and drip...

Window sills, door sills, flashing, cantilever slab and other areas where you want water to break free from any surface through gravity.

...go back to part 1
...continue to part 3

(Image above by Johnny Jupiter Photo on Flickr)

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