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Architectural Projections: 7 Minutes of BS

August 08, 2017
Read time: 6 mins

Old Building Details That Work


In this episode, we are joined by Sarah GrayM.Sc., P.Eng., CAHP, an engineer with RDH Building Science Inc. to talk about using simple physics to outthink simple physics. 



Today, we're gonna be talking about "Architectural Projections" and I'd like to add a subtitle called "Old Building Details That Work."

And I'm super jazzed about this topic, because this is my heart and soul, and the way I look at buildings, and how I make some amount of money.

I guess the best way to do this, is imagine you're standing on a street corner in an old city like Boston, like New York,

Like San Francisco?

probably San Francisco,

Like Chicago?

Definitely Chicago, and look around at the old brick, stone, and terracotta buildings, and you'll see a lot of architectural features on most or all of those buildings, that break up the water line, we'll call it, and that shed water away from a building.

We have to understand that water is public enemy number one, so the goal is to keep water away from a building, so that it doesn't seep into the roofs, into the walls, into the basements.

It is true. Look at Liquid Water’s LinkedIn page: its job description is to turn houses to mush. Only you can prevent this.

Using design elements is a great way to start

So some of those architectural elements could be, if I'm starting from the top and working down, first of all, a lovely cornice, which is an overhanging projection at the top of a building, that in the late 1800s and early 1900s was decorative. It was the doodad at the top of the building, that made it look really nice, but it really served an important function, in terms of shedding water away from the brick or stone below.

These projections range anywhere from 6 inches, 12 inches, sometimes even 3 or 4 feet out, cantilevered away from the main vertical wall face below.

The blue-collar cousin of the fancy cornice is the capstone.

If there wasn't money or the decorative effort put into a cornice, you would often see a capstone detail at the top of the building, where you'd have a very durable stone, that can withstand the elements sitting on top of the brick masonry below, to serve as a more durable feature, to stop water seeping into the brick.

Brick is porous, which means there are lots of holes in it. To rain, brick looks like a whole bunch of baby birds with their beaks wide open looking for worms. I don’t want to say that water is wormy, but an impermeable capstone could stop your walls from getting wormy.

Those capstones will often have a drip edge or a kerf in them, that helps break the surface tension of water, and again, shed water away from the wall.

Further down the building, as we take our journey from the top to the bottom, you'll often see continuous horizontal bands of stone or sheet metal, again, that help capture water, that's sheeting down the face of the building, and wash it away, or shed it away from the building itself.

At windowsills, you'll see sloped stone pieces. Sloping, again, helps g et the water away from the window, away from the wall. Those sills may have drip edges, or drip slots, or kerfs cut into them, again, to break the surface tension, and get the water away from the wall. 

At the bottom of the building, particularly if the building is brick masonry, or a soft sandstone, you'll see a band of granite that's between 2 to 3 feet, maybe even 4 feet tall off of the sidewalk, or grade level, and that's called a water table.

The water table is made from impervious material that won’t wick nearly as much.

… the more dense and durable granite, actually slows the wicking, or the rising damp, up through the masonry.

And even if water does wick into that water table through capillarity, or just water absorption, those stones are often more durable, and can withstand the freezing and thawing of water, that tends to tear those softer materials apart.

So really, from the top to the bottom, we're looking at

  • Breaking up the facade
  • Shedding water away
  • Capping vulnerable details...
  • ...and using more durable materials to stand up to the forces of nature.

Keeping water out of the masonry keeps it out of the structural wood members like joists and beams. If water gets to them, water kicks the door open to more problems:

... Mold, rot, bugs, and all the other nasties.

OK, so water is bad and design is good. Let's put this information put into practical use

Let's visualize a window, looking at a window head on. The sill of the window, typically, has a piece of wood, a fairly substantial piece of wood, or a fairly substantial piece of stone on which the window sits.

The window sill

We'd like to have that stone or wood sill project out from the face of the wall below, by, I'd say, at least an inch, 2 inches is better,

Three inches?

3 inches is getting a little out to lunch, so let's say between 1 and 2 inches. That moves the plane of water shedding out away from the brick. 

But because of surface tension, which helps water to stick to surfaces by sticking to itself, shedding water is not as easy as pushing it away. 

Because of the clinginess of water molecules, the water rolls down onto the outer vertical face of the sill, and then it can actually roll to the underside of the sill, sort of gravity defying in nature's greatest way.

Without some kind of interruption, the water will keep on rolling back to the surface of the wall.

A saw kerf will do the trick.

The water traveling under the sill moves at a steady clip until it gets to the saw kerf. Like a line of marching people who come to a river, if the people in front stop, all the people behind bump into them and wind up pushing a pile of peeps into the stream.

Because the water cannot march up the kerf, it stops. 
And then all the water behind it bumps into it to form a drop. 

When the drop is big enough, gravity becomes stronger than surface tension...

...and the water can fall away from the sill. And because we have that sill projecting outward from the wall below, the water sheds away from the wall surface, and never touches it. That's the magic of a drip slot, or a drip kerf, on a sill, or horizontal band detail.

For those of you keeping track, it is water: bad, design: good, saw kerf: good.

And slope is good, too.

Well, any slope is better than none, as long as it's sloping away from the building. Some people say a quarter of an inch to a foot,

Which is about ½ degree of slope

some say 2%, 3%, 5%.

A five percent slope is about three degrees.

I would say anything greater than an eighth of an inch per foot, or greater than 1%, works well enough, perhaps a little more slope at the top cornice or capstone at the very top of the building, because that's what sees the brunt.

More slope is better, but if you get it too sloped and too angled, sometimes the architects get a knot in their knickers and don't like the look of that.

And naughty knickers are no good.


—7 Minutes of BS is a production of the SGC Horizon Media Network. We’d like to thank RDH Building Science for unfettered access to the army of engineers.

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