Tag Archives: ice damming

Attic ventilation a critical design factor

 Attic ventilation and insulation is critical to reduce cooling costs, to prevent shingle damage, to minimize ice damming, and to prevent moisture buildup. Installation is not complicated.

An unvented attic is like a politician or me—–full of hot air.

If the area directly under your roof does not have adequate air movement for cooling, the sun can raise temperatures to levels damaging to asphalt shingles. Heat is also transferred to the living space below, and increases cooling costs. In the winter, heat rising through the ceiling to a poorly vented attic space can melt snow on the roof. The resulting water can run downward, and refreeze on colder areas of the roof to form ice dams. The water pooling behind the ice dams can overwhelm the roofing and leak through. This will cause severe damage to insulation, ceilings, and interior decorating. Excess humidity needs to be removed from the attic space to prevent damage to the roof and insulation from mold and rot.

Attic ventilation

Types of attic vent

In one severe case, I saw where asphalt shingles badly curled in 5 years. They normally would not have been as bad after 20 years. This was a shed type roof on an addition to the south side of a two story house. The shingles were black. The wall above was painted white, reflecting heat onto the roof, and a wall opposite was also white, reflecting heat back again. The original cedar shingle roof had been removed, and insulation had been stuffed in from the top. No attic ventilation of any kind had been provided.

Admittedly, this is an extreme example, but it serves to demonstrate the importance of attic ventilation.

Complicated roofs can complicate the equation considerably, but adequate ventilation on a simple gable roof is not difficult to achieve.

A continuous ridge vent combined with vented soffits is a low maintenance passive system that works well. Ridge vents should not be combined with other types of vents in snow areas, as snow may blow into the attic. There is much less chance of serious leaks with ridge vents.

In older houses, the 3” x 10” screened vents about every 4 feet in the soffits combined with one or two square vents near the ridge, is not really adequate.

I believe that passive attic ventilation is a better option for attic ventilation than powered fans, or systems with moving parts. It is dangerous to be on the roof to affect repairs when there is snow. Working on the roof in hot weather can damage asphalt shingles. There can be noise problems. Power outages render fans useless. Electricity consumption is a cost, and an environmental concern.

A complicated roof may require more complicated solutions. Vaulted ceilings make insulating and venting difficult, as do attic living or storage space. Of course, this is another argument for simpler roof designs.

There are minimum requirements in building codes for attic ventilation, but once again I advocate going a step further, and providing somewhat more.

Care must be taken to not block the attic ventilation at the eaves when insulating. Insulation stops will be needed in many cases.

There are a great many attic ventilation devices on the market.

Most require a hole through the roof which means they may leak, if not properly installed, or if they become damaged from wind or hail. Made of metal or plastic, they are usually obvious, and not necessarily attractive. Most will work fairly well, if there is enough of them, and they are well placed.

Gable vents seen on older houses are not that effective, due to poor air distribution.

Turbine vents are popular, but I really don’t think they are much more effective than static vents. They can be noisy, and more so if they are damaged by wind or hail.

Power vents can move a lot of air, but care must be taken to provide extra inlet vents, if they are to be any more effective. Distribution of air movement may not be that even. Their true effectiveness is arguable. Solar powered models are available to reduce energy cost and eliminate wiring.

For cost and effectiveness, I believe that ridge venting with vented soffits is the better option for attic ventilation. It works the best on a gable type roof. If you choose the type that you shingle over, it is virtually invisible from the ground. Some other roof designs will require different solutions.

Share this:
Share this page via Email Share this page via Stumble Upon Share this page via Digg this Share this page via Facebook Share this page via Twitter

Roof choices for your home

Whatever the wall type, trussed rafters are a good choice for the roof framing.

Trusses provide a clear span that makes interior design much more variable, and makes future renovation easier. Trusses can be used in floors as well to provide for a clear span in spaces below. Ease of construction tends to make truss use nearly as inexpensive as other methods. Trusses can be engineered in a great many configurations, and for many roof styles.

a common roof truss

A common gable roof truss

They can provide for vaulted ceilings using scissor trusses, studio trusses, or trusses with center bays. and can be built with “heels” to allow room for more insulation in the attic space It is quite rare to see a house built without trusses, in this part of the country.

Attic trusses can be built to allow for extra rooms under the roof in a story and a half configuration.

Spans can exceed eighty feet with timber roof trusses, although, I can’t imagine many houses requiring that wide a clear span.

You can build your own roof trusses (or as an alternative you can visit www.lamcal.com.au), but they are usually made in a factory setting where they are joined together using steel gang nail gusset plates that are pressed into place. They may include steel structural members. Trusses require careful planning and should be engineer designed or approved. Extra care must be taken here, as this is, perhaps, the most important structural element of your house. You have no doubt all heard of roof collapses, or for that matter bridge collapses. These are often the result of compromised, or under designed trusses. Because the cost is not usually great, I prefer to have trusses designed for higher loads than would normally be required. One way is to have your trusses designed for 2 ft centers, but place them on a narrower spacing.

Your truss supplier will need to know the spacing, span, load, eave width, slope, whether you require a heel, and roof type. If your roof is complicated in any way, they will need a set of blueprints. Load is defined as live load and dead load, where dead load is the weight of the roof itself, including any permanently installed or likely to be installed equipment. Live load is temporary loads likely to be encountered, such as workers and construction equipment, or snow accumulations. Slope is usually expressed as inches per foot such as 4.5/12 or 6/12. A heel raises the rafter portion of the truss, and allows for more insulation at the outside perimeter of the attic.

roof with many gables

Gable roof

Some basic roof types are the gable, hip, gambrel, mansard, pyramid, saltbox, shed and flat. They may be called different names, and come in many combinations, materials and slopes. They are limited only by the architects imagination, structural integrity and the ability to keep out the weather. Needless to say, the simplest   will cost the least. In my opinion, a simpler design  can be more easily blended to it’s environment. A complex roof will add interest, and can make a house stand out, if that is your goal.

Some roof types are better suited for different conditions.

A steeply sloping roof with steel cladding is a good choice in high snowfall areas. Low slope or flat roofs will interfere less with sight lines, and will keep the height of a building down. consider carefully before using a bonnet type roof in cold climates. they have a tendency towards ice damming,

A roof may be clad in many materials but the most common in North America are organic asphalt, asphalt/fiberglass shingles, wood shingles, sheet steel or tile. Tile is mostly restricted to the southern and drier areas of the continent. Wood shingles are usually of cedar and have become expensive enough that they are most often used only where appearance makes them desirable.

A flat roof consists of built up layers of tar and sheet material topped with a protective coating of aggregate, or alternately layers of bitumen or fiberglass waterproof sheet, Not common in housing , it is often used on large commercial buildings. More flat roof info is available at this link.

Roll roofing of organic asphalt is available which is inexpensive and quick to apply, and can be used if economy is the major factor.

A shingle roof, today, is placed on a base of 7/16” plywood, or OSB, with a layer of roofing felt. (I prefer a thicker sheathing.) In cold climates, a layer of self adhesive rubberized material is placed at the eaves to prevent leaks caused by ice damming. Valleys may be further reinforced with rubberized material and aluminum or steel sheet.

Modern asphalt/fiberglass shingles can give pleasing contoured visual effects, and can have lifespans of fifty years or more.This is nearly comparable to wood or steel. Wind resistance is quite good. The economical three tab shingles have a lifespan of twenty five years or more. Some shingles will have additives to reduce moss growth. An asphalt roof must be well vented to prevent excess heat build up. Important on other types, as well, for interior comfort. Proper applicaton is critical. Do your research if you plan to do your own.

man falling off roof cartoon, roof choices for your home

a roof can be dangerous

Steel and tile roofing usually rests on a framework of furring strips. Both may have a better resistance to UV light and heat than asphalt shingles. It may be more difficult to seal any perforations through the roof. Steel has the added advantages of shedding snow easily and of making a lighter roof. Expansion and contraction at differing temperatures is a factor with steel that can loosen fasteners. Steel is very slippery, and venturing onto a steel roof to remove excess snow can be very dangerous. Tiles tend to be fragile, and although walking on a tile roof is possible, it should be avoided altogether if you can.

There are quite a few color choices with asphalt or steel with the most vibrant tones being available with steel. Tile colors are more limited. Dark colors will absorb more heat, and lighter colors will reflect it. There is a considerable difference, and it should be considered when choosing colors. Extra heat can affect the lifespan of some roofing.

Bolder contours can be achieved with tile or steel, but there is considerable variety in asphalt/fiberglass shingles.

Organic asphalt shingles are made with asphalt impregnated paper and a ceramic aggregate. Asphalt/fiberglass shingles are asphalt and aggregate coating on a fiberglass base. They may be laminated (layered) for architectural appeal, added strength ,and and wind resistance. Both have an adhesive strip to bond them together after they are applied.

Roof tiles may be made of differing materials but traditionally are of ceramic or concrete.

From an environmental point of view, there may not be a lot of difference between the choices, when everything is considered. I believe the best thing to do may be to choose a product with a longer life span, particularly where the cost differential is not great. I would like to see someone do a detailed analysis in this regard.

This is a large subject for one blog post. If you would like more information, please use the links I have provided.

For an interesting look at how things are done down under, check out this link

Bill said he couldn’t make any money building a roof——-Too much overhead.

It is snowing heavily outside my window right now, and I am pretty happy to have a roof over my head.

Share this:
Share this page via Email Share this page via Stumble Upon Share this page via Digg this Share this page via Facebook Share this page via Twitter