roof overhangs for summer shade and moisture protection

Overhangs are a key part of any home’s design. Do not undervalue their importance. They are critical to shade windows from undesired solar heat; and also to protect exterior walls, doors and windows from rainwater, while keeping the foundation dry.

Some decades ago, along with modern design principles there came the widespread vogue of flat roofs and no roof overhangs.

Their many serious side effects and unfortunate consequences are now obvious to all.

Overhangs can help solve moisture problems

Overhangs are more than just a small architectural detail.

They are important in energy efficient homes, to control solar heat gains. But they are also instrumental for keeping water away from the walls and the foundation of the house, and to protect windows and doors from rain-water.

Keeping the water out of the walls, basements and crawlspaces

It makes sense to protect walls from the rain, in most climates.

Without properly designed roof overhangs, the walls becomes too unprotected. Though they can’t keep wind-driven rain out the walls, they help keep the walls dry during many rain bursts.

Besides, it’s important to ensure that water doesn't drip near the foundation, to keep crawl spaces and basements dry – and overhangs allow it. Keeping the eaves-drip away from the house and walls also reduces the chances of damage caused by splash-back – a typical cause of siding rot in wood-frame construction.

Recessed walls and balconiesRoof overhangs & multi-story buildings

Obviously, keeping the water out of the walls and foundations is a lot more difficult to achieve in multi-story homes.

To solve the problem, architects can consider a roof overhang above key parts of the first floor (in two-story homes) or use recessed walls and balconies. See image.

In apartment buildings, balconies and recessed walls can assume the function of roof overhangs – and be used both to control solar heat gains and to keep the water off the walls.

Note: Two-story homes may need a roof overhang for every floor; the commonest solution uses a “brow” roof, that is, a well sized (relatively narrow) shed or hipped roof at the level of the first-floor ceiling

Protecting the windows (from rainwater) with and without overhangs

Windows can be protected by roof overhangs and overhang details, but also by recessing them in thick walls, and by including architectural features like protruding ledgers, head casings and head flashings. They are common in many old buildings, and there is no reason to not follow basic architectural wisdom. They are key to help deflect rain.Moisture protection
Overhangs can help prevent water build-up and moisture: they are very effective at keeping rain water out of walls, windows, doors and foundations.

Many of these elements – including overhang details – may be only some inches wide (say, 10, 12, 14 or 16 inches wide), but are critical anyway. They keep the drip line of the edge flashing away from the plane of the bricks.

Do not minimize the importance of architectural elements like arched stonework at the window heads, and stone sill. They may seem minor, but they are pivotal to protect the windows from water and to protect he bricks and other wall elements from damage.

Modern architecture often forget basic principles of water management – one more element of stupid building construction with very unfortunate results.

Protecting the doors with and without overhangs

What’s mentioned above for windows is also valid for doors. Why keep exterior doors unroofed or unprotected?

Overhangs can protect them, preventing jamb rot – while keeping you and visitors dry until the door is open.

But there are other ways of protecting exterior doors. We can also recess them from the exterior plane of the walls, or use some of the features - and gable and crickets - mentioned above for windows.

Overhangs in wood frame, brick and masonry construction
Protecting the walls from the rainwater is not just a problem of wood-frame construction. It also affects brick and other masonry facades. If left unprotected by roof overhangs, rain will soak the bricks or other elements of the walls with obvious negative impacts.

Overhangs, Shade and Solar Heat Gains control

Overhangs are also very effective at blocking sunlight during the summer months, when the sun - in non-equatorial regions - is higher in the sky and shade.

They just have to be properly sized.

They should be designed to provide shade and to block solar heat gains in south-facing walls (or north-facing walls, in the southern hemisphere) during the summer months; but also to block sunlight during the cooling season.

That's possible, but you must consider the sun's path (and angles) across the sky during the year.

Note: in cold and moderate climates, roof overhangs are not effective in east and west-facing walls, where the sun is too low in the sky to be blocked; in these sides of the house you have to use properly located hedges and small trees, or awnings with a fairly large drop, or other shading devices. Typical roof overhangs are solid and fixed, and are largely maintenance-free and cheap to build in new construction. But overhangs can also be retractable (which can be important in cold climates). Or louvered or vegetated, in hot climates…

Overhang design for solar passive Houses

The working principle behind the overhang design in energy efficient homes is supported by two obvious facts:

1) Walls and windows located on the sunny side of a house (the south in the northern hemisphere, the North in the southern hemisphere) benefit from direct sunlight in the winter months but not in the summer.

2) To effectively design an overhang we have to size it according to the summer sun angle, which varies with the latitude and the climate.
Overhagns principle technique

Image credit: Infinite Power Organization

overhang Depth for solar heat gains and shade

The depth of roof overhangs is crucial in cold and moderate climates, where oversized overhangs will have negative side effects.

You may use wide overhangs and verandas in hot climates without a cold season, and in hot-wet climates with high levels of rainfall, but oversized overhangs in other climates will cause unwanted shade in the winter months, making homes cold, gloomy and uncomfortable.

To prevent it, you have to consider carefully its depth, as shown in the images below...

When the sun is higher in the sky (first image), a 2 feet/60 cm overhang is long enough to shade the entire wall; but the same overhang will not shade the entire window when the sun is lower (second image)... Only a 3 feet/1 meter overhang (third image) provides enough shade to protect the entire window, for that position of the sun.

Overhang, shade and depth

Image credit: Austin Energy

Sizing the overhang

Overhang disadvantages
Overhangs have obvious benefits. But we shouln't forget their possible disadvantages. Oversized overhangs will reduce solar heat gains during the heating season, and will make homes gloomy, dark and ultimately uncomfortable. We should apply basic principles of passive solar heating, when sizing them. Also remember that wide roof overhangs should be properly engineered. Roof and overhang systems should be designed to resist wind uplift, especially in hurricane-prone regions. Oversized overhangs can increase the risk of roof or truss uplift

There isn't general rules to size an overhang; the optimum depth varies with the latitude and the type of climate.

In hot climates (without heating requirements) overhangs and shading devices should be large and cover the solar facade and possible adjacent outdoor living spaces.

At high latitudes (cold climates) the summer sun is low, which also makes larger overhang projections advantageous (in the sunny side of the house).

In mid-to-high latitudes - where the summer sun is higher in the sky than in high-latitude regions - the depth of the overhang should be proportionally smaller.

The basic guidelines listed below, for three climates zones, are recommended by the EERE for Northern hemisphere climate zones:

- Cold climates (with more than 6,000 Heating Degree Days, base 65°F/18°C): use the June 21 sun angle, and locate the overhang shadow line at mid-window;

- Temperate climates (below 6,000 Heating Degree Days, base 65°F/18°C, and below 2,600 Cooling Degree Days, base 75°F/22°C): use the June 21 sun angle, and locate the overhang shadow line at the window sill;

- Hot climates (above 2,600 Cooling Degree Days, base 75°F/22°C): use the March 21 sun angle, and locate the shadow line at the window sill.

To know about your Heating Degrees Days and your Cooling Degree Days, ask your regional weather service. 

See, for technical details on how to size overhangs:
PathNet organization
Your Home, Australia
Roof overhang calculator for passive solar design

 

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