Arctic Engineering Module 7b Slide notes Page 1 of 26

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Philip Williamson
5 years ago
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Another issue facing the designer of structures in cold regions is that of snow management. In regions were snow melts soon after falling, this is not normally a problem.

1 Another issue facing the designer of structures in cold regions is that of snow management. In regions were snow melts soon after falling, this is not normally a problem. In areas where temperatures stay below freezing for months at a time, the issue of what to do with the snow that accumulates during the season is a major concern. Where do you put the snow so that your facility can still function without an excess of maintenance cost? The photo here shows the accumulated snow at a turn out in the Turnagain Pass area of Alaska. This snow shown is not piled up as part of a snow removal program. The ground snow depth is actually twelve feet. The shovel in the door way is for shoveling your way out if you use the facility during a snow storm. This facility has been upgraded significantly since this picture was taken in Page 1 of 26

In designing buildings, you have basically two options for snow storage. Either design the roof to hold the snow or design the roof to shed the snow so that it can be stored somewhere else.

2 In designing buildings, you have basically two options for snow storage. Either design the roof to hold the snow or design the roof to shed the snow so that it can be stored somewhere else. Flat roofs (or nearly flat) will hold snow until the snow melts or is manually removed. Roofs with shallow slopes can be designed with roof sheathing that is sufficiently rough to keep the snow in place. Roofs with slicker surfaces can be fitted with snow fences and/or clips to hold the snow in place. When roofs are designed to shed snow, care must be taken to direct the snow to areas that will not be a hazard to people or equipment. A maintenance program may be needed to remove the snow from around the building between storms. Roof systems that shed snow do not have to be designed for the same loads as flat roofs. Designers can take advantage of the reduced loads seen by a sloped roof. Page 2 of 26

It is very difficulty to vent flat roofs so they are almost always considered to be hot roofs. This means that they need to be water tight when subjecting to standing water.

3 It is very difficulty to vent flat roofs so they are almost always considered to be hot roofs. This means that they need to be water tight when subjecting to standing water. In many regions, nearly flat roofs are sloped to the outer walls. Drainage is provided through parapets by scuppers. These systems don t work well in cold regions. As water drains out of the scuppers, it freezes, forming icicles down the side of the structure and eventually plugging the scupper. The resulting ponded water can be a serious structural hazard. If nearly flat roofs are designed for cold regions, it is best to slope them to internal drains that will not freeze. Page 3 of 26

Building code requirements require a minimum slope of ¼ per foot of slope to ensure adequate drainage. These slopes should bring the water to internal drains.

4 Building code requirements require a minimum slope of ¼ per foot of slope to ensure adequate drainage. These slopes should bring the water to internal drains. The drains then must transport the water to somewhere. The water should be transported to either a storm drainage system or some other location other than a sanitary sewer. If the system is daylighted into a creek or other surface location, care must be taken to ensure that the drain outlet does not become clogged with debris or ice. A blocked drain will cause the system to back up and cause water damage in the structure. One advantage of a flat roof system is that you make use of the waste heat of the building to remove the snow automatically. The downside of this is that flat roofs are viewed as a maintenance headache since they are difficult to keep water tight. Page 4 of 26

Sloped roofs that are designed to shed snow need to be laid out well. The upper left image shows a sloped roof system that dumps snow on the main entry way from three directions.

5 Sloped roofs that are designed to shed snow need to be laid out well. The upper left image shows a sloped roof system that dumps snow on the main entry way from three directions. This is probably not a good idea. The good news is that the asphalt shingles, cold roof design, and relatively low slope prevent large dumps of snow into the entry way area. Metal roofs do a good job of shedding snow. The lower left picture shows the results of snow shedding of a large metal roof on an elementary school. Note the fence that is supposed to keep children from playing under the eaves. The gate had to be added for access by heavy equipment. The snow is removed mechanically when it becomes apparent that the snow accumulations were sufficient to block the windows. This school requires periodic snow removal from around the building. The windows are intentionally set back under the eaves to prevent the snow from building up against the glass. The upper right photo is at the same school at an area of the building where they were unable to hold the windows back under the eves. The snow is piled up against the windows. The lower right photo shows a steep roof with a rough asphalt shingle surface. The shingles will hold the snow until the weight of snow becomes such that it discharges as a slab avalanche. The entry to the house is right under that eave. To reduce the hazard they have built a substantial roof over the porch and walkway. Page 5 of 26

6 When designing for a sloped roof system you should consider the points listed on the slide. If you don t have a place for the snow to fall, it is possible to add features to the roof that keeps the snow on the roof. Page 6 of 26

7 This is another view of the snow storage area shown on a prior slide. This image was taken after the snow pile has been removed. This arrangement is adequate for temporary snow storage as long as controls are in place to keep people away from the eaves when there is a danger of sliding snow from the roof. Page 7 of 26

8 This is an example of a situation where snow storage was not well considered. Snow sheds from the metal roof on the left. The snow fills the space between the two buildings. The resulting pressure on the walls has caused some structural distress. Page 8 of 26

Metal roofs tend to have a real problem with snow sliding in sheets. The upper image shows a slab of snow that is waiting to be released. It is hung up on a vent pipe.

9 Metal roofs tend to have a real problem with snow sliding in sheets. The upper image shows a slab of snow that is waiting to be released. It is hung up on a vent pipe. The left image is shows a large ice chunk that that has developed where a low roof catches snow when it slides off the roof. The ice on the low roof has successfully kept the snow from sliding on the roof above it, but notice that the snow from above is slowly pushing the obstruction out of its way. The hanging cantilevered ice chunk is a hazard to anything below it. The lower right image is the entrance to a mall in Eagle River, Alaska, that had a history of falling ice and snow that was hazardous to the patrons entering and exiting the facility. The roof was retrofitted with snow fences along the eave to keep the snow on the roof. The retro fit appears to have been successful. Page 9 of 26

This is another view of the small roof with the large block of ice on it. The lower roof was a retrofit designed to enhance access to this door. The images were taken in two separate years.

10 This is another view of the small roof with the large block of ice on it. The lower roof was a retrofit designed to enhance access to this door. The images were taken in two separate years. Note that in each case the low roof is not effective in maintaining access to the door. Door ways should not be placed in buildings where they can be blocked by sliding snow. Page 10 of 26

11 These two entries are in the same school. One of the entries is in an area where snow is not dumped. It is also recessed in the building so that when the doors swing outwards they will not be blocked by fallen snow. The second entry comes out under an eave and exits to the side. There is some question as to whether or not the exit path is too close to the adjacent eave, but other than that it is a well thought out entry scheme. Page 11 of 26

12 The building entries shown here are all well designed. In each case the snow does not slide off on the entries or their approaches. The doorways are all sufficiently set back to avoid problems with snow fall blockage. Page 12 of 26

13 Arctic entry ways are small vestibules with two sets of doors that form a sort of air lock. They are effective in reducing cold infiltration. They also provide a place for people entering the building to stomp the snow off of their shoes. These entries often are designed with their own heat sources to further reduce heat loss from the main building. This additional heat also melts snow that gets tracked in. An on going problem is to provide drainage for the resulting snow melt that will not clog with the dirt that is tracked in with the snow. Page 13 of 26

Roof penetrations are generally heat sources. They tend to melt snow around them. The resulting water refreezes as ice as it contacts cold surfaces further down the roof.

14 Roof penetrations are generally heat sources. They tend to melt snow around them. The resulting water refreezes as ice as it contacts cold surfaces further down the roof. In the picture shown, notice that these exhaust stacks have heated the roof surface around them. Note the lack of snow directly adjacent to the stacks. An effort to reduce heat buildup has been made by building the boxes around the stacks. The tops are vented to allow heat to escape. Page 14 of 26

15 Another common occurrence in cold region buildings is seasonal movement due to thermal and moisture related expansion and contraction. Truss/wall separation is a common problem in wood framed structures. This occurs when trusses bow upwards during the cold weather season, lifting the trusses off of interior partitions. The resulting gap is often disconcerting to the owners. Several details have been proposed over the years to deal with this problem. The one solution that you do not want to do is to fix the trusses to the interior partitions. This imposes loads on the trusses that they were not designed to take. The successful solutions are designed to mask movement as opposed to preventing it. The behavior is probably driven by shrinkage of the warm bottom chord as it looses moisture content during the winter months. Another situation that occurs commonly is wall bowing. This phenomena results from differential temperature and moisture through the thickness depth of wood wall studs. The wall tend to bow outwards, opening a gap between the wall and countertops or cabinets that are not attached to it. Page 15 of 26

16 There has been a lot of effort to increase the thermal efficiency of walls over the years. Most approaches have tended to thicken walls to either increase insulation and/or provide a thermal break between the inner and outer surfaces. Again care must be taken to make sure that a good vapor barrier is in place on the warm side of the wall. Page 16 of 26

17 Three common wood framed wall sections are shown here. The upper one is a typical stud wall with insulation and a vapor barrier on the interior. Using deeper studs allows the use of more insulation, increasing the thermal efficiency of the wall. The studs still transmit a lot of heat through the wall. The lower left drawing is a stud wall with 2x2 furring strips laid horizontally across the studs. This eliminates the majority of the thermal conductance of the studs. It is also an easy retrofit upgrade to existing walls. If retrofitting an existing wall, be sure to remove the old vapor barrier and add a new one over the new insulation and furring strips. The third drawing shows a staggered stud approach. This approach completely eliminates the through wall conductance of the wall studs. This approach may result in twice as many studs in a wall. The studs on a given side are spaced based on the strength of the sheathing they support and supported tributary area of wall exposed to wind loads. Only half the studs are effective in resisting wind loads. Page 17 of 26

This is a room in an older Fairbanks home being retrofitted with 2x2 furring strips and new insulation over the old. This retrofit was dictated when contents of the room froze to the wall one winter.

18 This is a room in an older Fairbanks home being retrofitted with 2x2 furring strips and new insulation over the old. This retrofit was dictated when contents of the room froze to the wall one winter. By increasing the efficiency of the walls, no further problems have been experienced. There a number of details that need to be considered with this type of retrofit. Window casings need to be extended, electrical boxes must be moved out, and base board heating systems often need to be moved out from the walls. Hanging pictures is also fun. Your interior studs are now running horizontal instead of vertical. Some people have a difficult time figuring this out! Note the old foil backed insulation. The foil was intended to act as a vapor barrier but is not very effective. In this case the existing foil was perforated to keep it from acting as vapor barrier in the middle of the wall. Reports from the homeowner indicate that the system has been very effective in eliminating the problems with interior contents freezing to the wall. Page 18 of 26

The floor system is also part of the building envelope and most of the same consideration for walls and ceilings also apply to floors. Vapor migration also occurs through floor systems.

19 The floor system is also part of the building envelope and most of the same consideration for walls and ceilings also apply to floors. Vapor migration also occurs through floor systems. If there is a possibility of the temperature in the floor insulation system to drop below the dew point, then a vapor barrier will be required. This is common in raised buildings built on piling systems. For a raised building, the vapor barrier should be over the joists and below the floor sheathing on the bottom floor. The insulation should be up against the vapor barrier. Page 19 of 26

20 The design of utility distribution systems and building connections in cold regions requires careful planning to avoid damage. The location relative to snow and ice needs to be carefully considered so that the utilities can be accessed for maintenance year round. Utility connections to buildings should be flexible if there is to be differential movement at the interface Exhaust vents should be placed in areas where produced heat can be removed from the roof spaces if a cold roof is to be used. Plumbing vents should pass through as little cold space as possible to prevent freeze up. Page 20 of 26

21 Here are two examples of utility connections that have caused distress as the result of differential movement. In the one of the left, the building to the left has settled relative to the one on the right. The difference is shown with the two red lines. Those two locations were originally at the same elevation. Repairs have been made to the brackets in the areas enclosed by the yellow boxes. In the image to the right, the building has settled, while the conduit has not. The conduit has torn through the building and has caused distress to the interior connections. Page 21 of 26

22 In the case shown here, the conduit is flexible enough to avoid distress of the connection for moderate differential movement. Page 22 of 26

23 The left two images show the same electrical panel located directly below the eave of a roof. It is subject to potential damage from falling ice and snow and is inaccessible until the maintenance crew removes the snow. The upper image is a set of transformers that are set inside of a containment structure. The containment structure is filled with snow and ice. If the containment is needed, hopefully there is enough capacity so that the fluids can all be contained. The bottom right is a series of electrical trays that are located such that snow removed from the adjacent road was thrown on top of them in a year of extreme snow. The trays failed as did many of their supports. To prevent future damage, the road snow removal plan should be altered. Page 23 of 26

24 These two images show some of the specific damage to the cable tray supports shown in the prior slide. On the right is a case of shear rupture of the column wall that resulted from excessive loads on the cantilevered cable tray support. The image on the left is a tube column that filled with water and froze. The expanding ice split the column over its full height. The split section is no longer as stable as the original section. This condition occurred because a plug in the top plate was not in place and the weep holes at the base were plugged by debris. Page 24 of 26

25 Covers have been added to these utility connections to protect them from falling snow and Ice. The one on the right provides better access since it shed to snow to either side. Page 25 of 26

26 This publication contains excellent information from one of the pioneers in arctic building research. It should be in the library of every arctic building design architect and engineer. Page 26 of 26

26 May 2011

26 May 2011 2 May 2011 www.professionalroofing.net Complications with cathedral ceilings Guidelines for minimizing ice dams with cathedral ceilings by Joan P. Crowe, AIA Homeowners and designers often desire cathedral