Step 4. Eave Overhang

 

Roof eave overhang on the north side of the Greeny Flat.

Roof eave overhang on the north side of the Greeny Flat.

The correct amount of eave overhang along the north side of the building is what makes passive solar design work in both winter and summer. Referring back to the sunpath diagrams we see that in summer the sun is high in the sky at noon so with a sufficient eave overhang on the north side the sun can be prevented from entering the building at all during the hottest times. But in winter the sun is low in the sky at noon so, as long as there is not too much eave overhang on the north side, the sun will angle in below the eave and enter the building as far as possible to warm it up.

So the right amount of eave overhang on the north side is critical. Too little overhang and the building will get too much sun in the summer. Too much overhang and it won’t get enough sun in the winter.

So how do we determine the correct eave overhang?

Manual method of determining the right amount of eave overhang base on noon sun angles on the 21st of each month.

Manual method of determining the right amount of eave overhang base on noon sun angles on the 21st of each month.

One way is to draw a scaled cross-section of the building showing the height of the walls and windows and the width and height of the eave overhang. Find the building site on Google Earth and record the latitude, longitude, and elevation figures from the bottom left hand corner. Enter the location figures into the Sun Angle Calculator on susdesign.com/sunangle. Find the sun angles for the site at noon on the 21st  of each month (this will include the summer solstice (June 21st when the sun is highest in the sky at noon), winter solstice (December 21st when the sun is lowest in the sky at noon), and the equinoxes (March 21st  and September 21st). Draw the calculated sun angles on the cross section to determine the desired amount of eave overhang so that the sun comes in when you want it and stays out when you don’t.

The above method works well and is relatively easy for considering the noon sun angles at different times of the year. But it can be very difficult to accurately draw the sun angles at different times of the day. A better way, which we highly recommend, is to make a 3D model using the free SketchUp® software. This software has a built in tool that allows you to perform sun studies for any time of the day on any day of the year. You can look at all the different faces of the building, from both inside and outside. This will give you a very good idea of how the sun will affect your building throughout the year, and it’s free!

3D computer model of the Greeny Flat showing the sun shading on the Summer Solstice.

3D computer model of the Greeny Flat showing the sun shading on the Summer Solstice.

Above is a picture of the SketchUp® model made during the design of the Greeny Flat. The blue box shows the “Shadow Settings”  and you can see that the sun angle is set for noon on the summer solstice (December 21st). You can also see that the model predicts that, with the right amount of eave overhang on the north side, the sun will not enter the north-facing windows at all in the middle of summer. This is why passive solar design works for keeping buildings cool in summer as well as warm in winter.

Below is the exact same model from the same viewpoint and with the same amount of eave overhang. The only thing that has changed is that the “Shadow Settings” have been changed to noon on the winter solstice (June 21st). Now you can see that the model predicts that there will be full sun in the north-facing windows in the middle of winter.

3D computer model of the Greeny Flat showing the sun shading on the Winter Solstice.

3D computer model of the Greeny Flat showing the sun shading on the Winter Solstice.

 

Actual photo of the Greeny Flat taken on the Winter Solstice.

Actual photo of the Greeny Flat taken on the Winter Solstice.

And this is an actual photo taken on the winter solstice, June 22nd, 2014. You can see that the model very accurately predicted how the sun would affect the building.

So why does the Greeny Flat have a verandah along the north side with part of the roof clear and part solid?

The Greeny Flat is built in a Heritage Conservation area and the verandah along the north side was a request from our local council’s Heritage Advisor. We like it because it provides a covered walkway and a protected entrance to the front door of the flat. But if we had made the whole roof solid it would have created too much shade in the winter time. The horizontal line about two-thirds of the way up the windows is the shadow of the verandah frame and gutter. This shows how much of the windows would be shaded in winter if the entire roof was solid. So we made enough of the roof solid to provide shading to the wall and windows in the summer and made the rest clear to give us more sun in the winter.

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