Shedding Water from the Roof

How roofing works to shed water from the structure

This image of the problems with a gutter that is improperly discharging water onto a roof came today with the interNACHI home inspector monthly newsletter. It immediately reminded me of why we approach homes as a system and evaluate the performance of the structure starting at the top of the house.

Water is a house's biggest enemy. Whether it's moisture from cooking and showering inside or as in this image, water from the elements not being properly handled by the water removal system, water will and does destroy most structures over time. Of course wind and fire are more destructive in the short term, you can lose your house entirely in a matter of a few minutes or hours, but water will over months and years surely lead to catastrophic failure.

To properly shed water, the envelope of the house, consisting of the roof covering, flashings, chimney and other roof penetrations, gutters, siding and windows must all be correctly installed and maintained over the life of the home.

Starting at the very top of the house, the shingles are probably the single most important component of house integrity. The inspector checks the roof covering, the fasteners, and the deck sheathing underneath. Inspectors check the slope and underlayment, the ice barrier and the drip edge. We check the flashing around the penetrations in the roof and the valley flashing.

The selection of a quality shingle with a 25 or 35 year life is paramount. The shingle itself is now a highly developed building material consisting of two or three layers. The shingle, combined with the slope of the roof is the main method by which roofs shed water. Asphalt shingles are re-enforced, generally with glass fibers.

The design of the roof must take into account the underlayment and roof covering material, the slope and the type of weather encountered in the area.

Notice we are not concerned with the esthetics of roofing in any way, only that the roof is functional and in good repair. Inspectors will always look at the edge of the roof to discover how many layers of roofing are installed. In general, no more than two layers should be evident. Three or more indicates the roof was not done properly the last time, and the next time the roof is re-covered, the entire three layers will have to be removed and new underlayment and shingles installed. It may be that the decking will also require some replacement.

One more detail of roofing material is crucial to understanding the condition of the shingles and that is the granules. Granules, which are adhered to the surface of the shingles during manufacture, is to protect the shingle from sunlight, from damage by UV radiation and from the weather in general. Granules also dictate the colour of the roof. Lighter colour granules are coming into favour as they may contribute to cooler roofs and therefore longer lasting roofs and a more comfortable living situation.

The image made me think about flashing. This is the area that most homeowners overlook. Flashing with gutters catch the rain or snow melt being shed by the roof covering and directed to the ground and away from the house.

The image shows the interaction between the shingles and the catchment and points out various areas of failure or incorrect installation.

Starting at the top, the installer has left off the end cap at the back of the gutter. It is clear this was installed in this manner because there is no drainpipe that is letting water out of this gutter. Did the installer think the gutter would overflow and water just fall onto the roof below? No, the installer has left the end cap off the end of the gutter so water will flow out of the back of the gutter onto the roof and run down further to the gutter below.

This method of directing the water to the lower levels, causes excessive wear on the shingles that are receiving the water. These shingles receive much more wear per water event than the rest of the roof. In addition to the excess volume, the water will be moving faster than the water falling or melting from the remainder of the roof leading to premature wear and requiring replacement sooner than the rest of the roof.

The image also shows another possibility of water penetration into the building by leaking through the flashing at the side of the dormer. All the aspects of water flow on the shingles are also present on the flashing. Water can work it’s way between the metal sheets that form the flashing and either through flow or freeze and thaw will split the flashing away from the wall and get into the building.

Thirdly, the excess flow fills the lower gutter beyond its capacity, usually overflowing and running down the front of the house. Also in this image, there is some discharge gutter but it is discharging onto the roof repeating the flow issues mentioned above. In this case, either the downspout was not installed in the first place or perhaps the piece that was there has fallen down or been damaged and removed but never replaced.

This situation should be identified as a deficiency and remedied by the owner or the buyer. These fixes are easy and not very expensive. Fixing the drainage issues will contribute to the longevity of the roof on this house.