Moisture Control - Roofing and Architectual Details

Claudette Reichel  |  6/20/2008 2:07:40 AM

Figure 1, Step Flashing

Architectural details should shed rainwater away from the foundation, doors, windows and walls.

Roof overhangs are not just for shade, but are an effective investment in the longevity of doors, window framing and siding. Two-foot-wide overhangs provide the best balance of water shedding and shade benefits without creating wind load uplift challenges in most wind zones (consult an engineer or code official); 18 inch overhangs can suffice.

Roof Pitch: Although roofs should be sloped, a very steep slope does not shed water away from the house as well. A roof flatter than a 3:12 slope (3 inch rise per foot of horizontal run) is at greater risk of both leaks and high wind loads; one steeper than 6:12 can increase moisture problems and wind loads. Avoid details and trim that allow puddling and collection of water. Make sure decks, balconies, trim and all other protrusions are sloped.

Gutters: Where overhangs are inadequate to shed water from walls, screened or specialized clog-free gutter and downspout systems are helpful to reduce splash back on the walls and direct water away from the foundation.

Roof leak reduction

Design: Choose a simple roof design and avoid flat roof segments. Complex “designer roofs” are more prone to leak problems than simple designs without valleys and roof/wall intersections. A simple hip roof offers several advantages (moisture control, energy savings, and wind resistance).

Drip edge: The outer edge of all roof decking should be protected with a metal or plastic drip edge to direct water away and prevent it from clinging and moving under the roof.

Flashing: All flashings should be integrated shingle fashion with the roofing as well as sealed. Where a roof segment ends against a wall, install step flashing on each course of shingle and apply an adhesive membrane strip over the step flashings along the wall sheathing. This will keep water that runs down the wall from seeping under the shingles of the roof. At the bottom of that roof-wall intersection, use a kick-out flashing to divert the water running off the roof away from the wall.  (Figure 1, Step Flashing)

Underlayment: Roofing underlayment is an important second line of defense against water that gets under the roofing. Consider using a heavier roofing felt or membrane under curved tiles (Spanish, Mediterranean tiles) and v-crimp or corrugated metal roofing.

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