Roof Maintenance Papillion NE
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Busting Your Ice Dams
Anyone living in a cold climate has seen ice dams - those sparkling bands of ice that form along a roof's edge in the heart of winter. While they may look beautiful, they can be quite destructive. Ice dams develop when snow on the upper part of the roof melts. Water runs down the roof slope under the blanket of snow and re-freezes into a band of ice at the roof's edge, creating a dam. More melted water pools against the dam and eventually leaks into the building under roofing or through roof trim. Ice dams can form when as little as 1 or 2 inches of snow accumulates on a roof surface, provided the snowfall is followed by several days of sub-freezing temperatures. Most homeowners don't understand why these ice bands form, which is too bad, because prevention of structural damage hinges on a homeowner's ability to diagnose the problem. I have investigated many ice-dam problems as a builder, researcher and consultant. Although individual cases look different and often result in different types of damage, the events leading to the failures are predictable.
Ice dams and the damage they cause are completely avoidable. There are three things required for an ice dam to form: a snow-covered roof, heat to melt the snow, and cold to re-freeze the melt-water into solid ice. The reason ice dams form along the roof's lower edge is clear. The upper part of the roof surface (toward the ridgeline) is directly above the living space. Heat lost from the house warms this section of the roof and melts the snow in this area. On the other hand, the lower part of the roof usually overhangs the building and is surrounded by outdoor air. During periods of sub-freezing temperatures, the lower regions of the roof remain at sub-freezing ambient temperatures. Roof overhangs are not warmed by indoor heat loss, and water re-freezes when it reaches this point. Deeper snow and colder temperatures increase the likelihood and size of ice dams. Every inch of snow that accumulates on the roof's surface insulates the roof a little more, trapping more indoor heat beneath it. Frigid outdoor temperatures assure a fast and deep freeze at the eaves. So the worst ice dams usually occur when a deep snow is followed by very cold weather. It's easy to understand that allowing water to leak into your house is a bad idea. Ice dams cause millions of dollars of damage every year. Immediate damage resulting from leaks is often apparent. Water-stained ceilings, dislodged roof shingles, sagging, ice-filled gutters, peeling paint and damaged plaster are all easily recognized and usually repaired when weather or budgets permit.
But some damage is not so obvious and often goes unchecked. Roof leaks can wet an attic's insulation. In the short term, wet insulation doesn't work very well; in the long term, water-soaked insulation is compressed, so that even after it dries, the insulation in the ceiling is not as thick as it was. And thinner insulation means lower R-values. It's a vicious cycle. The more heat lost, the more ice dams form. The more leakage, the more the insulation gets damaged - and so on. As a result, you pay more to heat your house. Cellulose insulation is particularly vulnerable to the hazards of wetting, but fiberglass suffers too. Water often leaks down into the wall frame, where it wets wall insulation and causes it to sag. That leaves uninsulated voids. Energy dollars are again robbed. Perhaps more important, moisture gets trapped within the wall cavity between the exterior plywood sheathing and interior vapor barrier. The result: smelly, rotting wall cavities. Structural framing members can decay. Metal fasteners may corrode. Mold and mildew can form on wall surfaces as a result of elevated humidity levels. Exterior and interior paint blisters and peels. The well-being of allergy-sensitive individuals is compromised. Peeling wall paint deserves special attention because its cause may be difficult to recognize.
It is unlikely that wall paint (interior or exterior) will blister or peel when ice dams are visible. Paint peels long after the ice and all signs of a roof leakages have evaporated. When water from ice dams infiltrates walls, it dampens building materials and raises the relative humidity within the wall frames. The moisture eventually wets interior wall coverings and exterior claddings as it tries to escape (as either liquid or vapor). As a result, interior and exterior walls shed their skins of paint. So the message here is to check your home carefully when ice dams form. Inspect your home periodically, even when there doesn't appear to be a leak. Look at the underside of the roof sheathing and roof trim to make sure they haven't gotten wet. Check the insulation for dampness. When leaks inside your home develop, be prepared. Water penetration often follows pathways that are difficult to follow. Don't just patch the roof leak. Make sure that the roof sheathing hasn't rotted or that other less obvious problems in your ceiling or walls haven't developed. Then detail a comprehensive plan to fix the damage.
Controlling the damage
The damage caused by ice dams can be controlled in two ways: Keep the entire roof surface at ambient outdoor temperatures or build a roof so that it can't leak into sensitive building materials if an ice dam forms. This second method, a barrier system, is very difficult to achieve. Cold roofs make a lot of sense. Keep the entire roof as cold as the outdoor air and you solve the ice-dam riddle. Look at the roof of an unheated shed, garage or abandoned home. Ice dams usually don't form on these unheated structures because there is no internal heat loss to drive ice dam formation. (It is possible for some ice to form on an unheated house when solar gains warm the indoor air, which subsequently rises up through a poorly or non-insulated ceiling.) It's easy to avoid the problem in new construction. Design the house to include a continuous air barrier separating the living space from the underside of the roof (most important), plenty of ceiling insulation (next most important), and an effective roof ventilation system (least important). An air barrier retards the flow of heated air to the underside of the roof. Insulation retards the conductive flow of heat from the house to the roof surface. And a good roof-ventilation system helps keep the roof sheathing cold. In an existing house this approach may be more difficult to follow. Often, however, you are stuck with less than desirable conditions.
Let's look more closely at all of the issues that will guide your prevention strategy. Air leakage A special effort must be made to block the flow of warm indoor air (convection) into the attic or roof area. Small holes allow significant volumes of warm indoor air to pass into attic spaces. In new construction, avoid making penetrations through the ceiling whenever possible. Recessed lights are bad news! Track lights are better. Here's what to do when you can't avoid making penetrations or when you need to make an existing home airtight. Go into the attic; lift or remove the existing layer of insulation so that you can clearly see the entire ceiling surface; then seal all cracks, seams and potential air leaks with urethane spray-foam that comes in a can, caulking, packed cellulose and/or weather-stripping. Typical ceiling leakage points include:
Ceiling light fixtures
Bathroom exhaust fans
Ductwork in attic (be sure to seal ducts)
Intersection of interior partitions and ceiling Insulation Houses in cold regions should be equipped with ceiling insulation of at least R-38, about 12 inches of fiberglass or cellulose. The insulation should be continuous and consistently deep. Be sure it is carefully installed with no voids. A notable problem area is where rafters sit directly on top of the exterior wall.
There are two reasons for concern at this point. First, there is little room for insulation, resulting in low R-values between the top of the wall and underside of the roof sheathing. Second, builders are not particularly fussy when it comes to air-sealing the connection between the wall and ceiling. A leaky connection allows warm indoor air to rise up and heat the underside of the roof surface. Air can leak through wire and plumbing penetrations in the top plate. Warm indoor air can also leak into exterior wall cavities through electrical outlets, switches and other penetrations in the wall, rise upward inside the cavity, and pass into the attic through the small cracks and seams that exist between the wall top-plate and drywall. Raised-heel trusses or roof-framing details that allow for R-38 above the exterior wall should be used in new construction.
In existing structures where the space between the wall's top plate and underside of the roof sheathing is restricted, install high-R/inch insulating foam (R-7/inch). Be sure to seal all seams around the insulation at this point to prevent warm-air leakage into the attic from the living space. You may have to remove the first row of plywood and roof shingles and work from the outside of the structure to add the foam insulation over the exterior wall of an existing home. Ventilation Attic ventilation is the least powerful approach to minimizing ice-dam development. However, be sure to include ventilation as part of your overall strategy. It makes a difference. A soffit-to-ridge ventilation system is the most effective ventilation scheme you can use to cool roof sheathing. Power vents, turbines, roof vents and gable louvers just aren't as good. Soffit-and-ridge vents should run continuously along the length of the house. A baffled-ridge vent is best because it will exhaust attic air regardless of wind direction. The exhaust pressure created by wind jumping over baffles on the ridge vent sucks cold replacement air into the attic through the soffit vents. A 1- or 2-inch space, or "air chute," should be provided between the top of the insulation and the underside of the roof sheathing in all areas of the roof. The incoming soffit air will wash the underside of the roof sheathing with a continuous flow of cold air. Be sure to install insulation baffles above the exterior wall to protect the edge of the insulation from the air that blows in through the soffit vents.
The list of other solutions is long. The problem I have with many of these efforts to prevent ice dams is that they don't deal with the root cause, which is heat loss. They merely treat the symptom. Unfortunately, sometimes that is all you can do. Metal roofs are common in snow country, so they must work, right? Steeply pitched metal roofs in a sense thumb their nose at ice dams, since they are slippery enough to shed snow before it causes an ice problem. However, metal roofs are very expensive and do not substitute for adequate levels of insulation, though they do reduce the likelihood of ice formation. Many people install self-sticking rubberized sheets under roof shingles above the eaves, in valleys, and around chimneys, skylights and vent stacks. These products adhere directly to clean roof decking. Roof shingles are nailed to the deck through the membrane. The membrane is self-healing and seals nail penetrations automatically. The theory is that if any water leaks through the roof covering, the waterproof membrane will provide a second line of defense. The material is sold in 3- by 75-foot rolls. W.R. Grace (Ice and Water Shield), Domtar (Eaveshield) and Bird all make competitive products. This solution is a reasonable alternative for many existing structures where real cures are not possible or cost effective. These products also serve as a redundant layer of protection. Sometimes even well constructed roofs can have ice dams. Very deep snow can insulate the roof deck making it warm enough to melt snow over the living area. You might consider using sheet-metal ice belts if you don't mind the look of a shiny 2-foot-wide metal strip strung along the edge of your roof. Ice/snow belts are reasonable choices for some patch and fix jobs on existing houses. This type of eave-flashing system tries to do what metal roofing does: shed snow and ice before it causes a problem. It works - sometimes. The problem with ice belts is that they don't work reliably. Ice dams can develop on the roof just above the top edge of the metal strip. Ice/snow belts are sold as 32- by 36-inch pieces and come with additional fastening hardware. Contrary to popular belief, gutters do not cause ice dams.
However, they do help concentrate ice and water at a very vulnerable area. As gutters fill with ice, they often bend and rip away from the house bringing fascia, fasteners and downspouts in tow. And what about those heat tapes? In my entire life, I have never seen a zigzag arrangement of electrically heated cable work to fix an ice dam problem. The cable is heated by electric power, so you throw away heat energy twice (keep in mind that ice dams are a heat-loss problem). Over time the heat tape can make shingles brittle and create a fire risk; it's expensive to install and use; and it leaks through loose fasteners. In fact, take a good look at roofs that are equipped with heat tape. The electric cables do little to remove the ice dams. My advice is don't waste your time or money. Whatever plan you decide to follow, focus on the cause. The heat lost from the house creates ice dams. Develop a strategy that is centered on this fact whenever possible. Ventilate, insulate and block as many air leaks as practical. There are no excuses not to address this issue with new construction. At times, the cure for existing structures is elusive and can be costly. In some cases you have to treat the symptom, but the payback is well worth the effort.