Roof Construction

Roof Construction Problems/Solutions
Roof Construction
Roof Shingles and Deck Installation
Ventilation
Flashing

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Roof Construction Problems/Solutions

Roof Construction Systems are vulnerable to moisture from three sources: rain penetration, water vapour and ice damning.

Rain Penetration
It occurs when water finds an opening in the roof system and sufficient force is applied (wind) to drive the water through the penetration in the roof system. Sloped roofs seldom leak because gravity offsets the force of the wind if the shingles are installed correctly (over lapping of shingles). Gravity acts to move water outward and down the slope of the roof which counteracts the wind pressure differential that tends to drive the water inwards. The greater the slope, the greater gravitational force acting against the wind. Leakage occurs when the roof is not sloped enough to allow the force of gravity to counter the wind force driving the rain through an opening. The way to prevent rain penetration from penetrating the roof system is to install the roof system as the manufacture recommends. The only way to prevent water penetration in low sloped roofs is to install shingles using low slope applications and incorporating a continuous water proof membrane (ice and water shield)

Water Vapour Penetration
Condensation in the roof cavity (attic) is caused by water vapour from the inside escaping through the ceiling into the roof cavity. The interior warm humid air condenses on the roof structure if their temperature is lower then the dew point which happens mostly in winter because the inside air has a higher moisture content than the outside air. The roof system should be designed to prevent the entry of water vapour by diffusion (penetration of humid indoor air through materials) by incorporating a vapour barrier and adequate roof insulation at the eaves by incorporating raised heel trusses. The roof system should also prevent air leakage by sealing all penetrations through the vapour barrier. There will always be some warm humid air that finds its way into the roof cavity, which most be removed.
Any warm humid air that penetrates the ceiling system into the roof structure has to be removed. This is done by providing continuous ventilation in the roof cavity to the outside. Ventilation is provided at the highest point and the lowest points of the roof assembly (ridge, soffits and gable ends. This method is similar to the chimney stack effect to promote good air movement. Proper ventilation helps to replace warm humid inside air in the attic with fresh air. It is not advised to use power ventilators in winter, because they create a negative pressure in the attic which increases the flow of warm humid inside air from the interior.

Ice Damning
Ice damning is the build up of ice at the edge of the roof, which mostly occurs over the exterior walls. This is caused by heat escaping from the interior and melting the snow. This water then flows down under the snow covered roof. As it reaches the eave over the unheated soffit, it freezes, creating a ice damn.

Solution
Install the proper amount of insulation in the attic, provide a vapour barrier, seal all penetrations from inside to roof cavity, use a raised heel truss which will allow you to properly insulate to the outside of the top plates with R-40, install eave protection (ice and water shield-best, asphalt-impregnated felt or rolled roofing) from the edge of the eave (overhang) to 1'0" beyond the interior wall and at valley intersections and provide proper unobstructed ventilation at the eaves (50 %) and ridge (50 %). When installing rain gutters ensure the that outer edges of the gutters are lower than the slope line to allow snow and water to slide off the roof surface.

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Roof Construction

Truss System
Rafter System
Flat Roof

Truss System
General
A Truss system can consist of several truss designs (conventional truss, parallel cord truss, scissor truss, girder truss, etc...). All are designed by a truss manufacturer. They use a computer system to develop a truss system. If your roofing system is to complex they refer it to a structural engineer who will certify the truss system for the manufacture.

General Precaution
Do not attempt to design and build your own truss system. It is very time consuming and very technical. And you will have difficulties with your local building permit departments. Most importantly to the overall structure they will not last as long as a manufactured truss simply because they use self nailing steel galvanized gusset plates, not wood and nails.

Considerations
All Our plans are designed with trusses and rafters that incorporate a raised heel which provides maximum insulation and ventilation at the eave.

Rafter System
This roof system should be designed by your engineer and designer. This system depending on how complex, must be approved by a structural engineer before building permit application. This system can be separated into 2 sections. Loadbearing ridge design and non-loadbearing ridge board design.We can help you develop the design and structure of your new home.

Flat Roofs Slope 2:12 or less
Their built using roof joists/rafters or parallel cord trusses with built up roofing (tar and gravel) or 2 ply torched on membrane.

Ventilation
If each roof joist space is not separately vented at both ends, the roof joist spaces must be interconnected by installing 2" X 2" purlins on top and perpendicular to the roof joists.

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Roof Shingles and Deck Installation

Asphalt Shingles
Wood Shingles and Shakes
Sheet Metal Roofing

Asphalt Shingles
Regular Slope Application Slope 5:12 to 12:12
When installing asphalt shingles on sloped roofs follow the manufacturers instructions for storage, handling and installation procedures. Ideally, shingles should only be installed when the temperature ranges between 10C to 23C (50F to 75F). When installing shingles below 10C (50F) the self-sealant does not adhere properly. If you are installing shingles in these conditions seal the shingles manually with 1" spot of adhesive or roofing cement under the centre of each tab.
Apply a layer of roll roofing making sure to lap the upper layer over the one below and staple it into place. Make sure the roof sheathing is flat with no noticeable projections. Applying roll roofing extends the live of your asphalt shingles and provides extra protection to the structure if the shingles fail. Provide 3'0" of ice and water shield at all eaves making sure that it projects 1'0" beyond the interior finish and at all valleys.

Roof Deck
The quality and durability of a asphalt shingle roof is directly related to the quality of the roof sheathing on which it is placed. The deck most be smooth, level and strong enough to resist loads imposed on it during construction and installation of shingles. When installing the roof sheathing lay sheathing with the grain on the face perpendicular to the roof structure. All edges to be nailed at 6" on centre and rafter/truss nail at 12" on centre. Also, incorporate H-clips to secure the edge of the sheathing between rafters/trusses. The thickness of the sheathing is a function of the span between rafter /truss members and the type of sheathing used (plywood-best or OSB).

Low Slope Application Slope 4:12 or less
Apply a layer of roll roofing (or ice and water shield-better)making sure to lap the upper layer over the one below and staple it into place. Make sure the roof sheathing is flat with no noticeable projections. Applying roll roofing extends the live of your asphalt shingles and provides extra protection to the structure if the shingles fail.
Except for the first two rows of shingles, three thickness’ of shingles are used on the entire roof surface including hips and ridges. This is achieved by overlapping the shingles by 2/3 exposing only 1/3 of the shingles to the weather.
A starter stripe is installed it can be either a solid strip or reversed shingle. The first course of shingles is then cemented to the strip with a continuous band of adhesive that is 4" wider than the shingle exposure (example 6" exposure would require 10" band of adhesive). The next course is laid in a band of adhesive 2" wider then the shingle exposure.

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Wood Shingles and Shakes

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Metal Roofs

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Ventilation
Where insulation is installed between a ceiling and the underside of the roof sheathing, space must be provided between the insulation and under side sheathing and vents must be installed to permit the movement of air from the attic to the exterior.
The unobstructed vent area shall be a min. of 1/300 of the insulated ceiling. Where the roof slope is less than 2:12 or in roofs that are constructed with roof joists, the unobstructed vent area is doubled to a min. 1/150 of insulated ceiling area.
Required vents can be roof type, eave type, ridge type and gable-end type. A combination of these is usually required provide adequate air movement. A general rule is to locate 50 % of your ventilation requirements near the top of the roof using roof ventilators or ridge vents and 50 % at the bottom of the roof using vented soffits/eaves. And where each roof joist space is not separately vented (rafter system) roof joist spaces shall be interconnected by installing 2" X 2" purlins on top and perpendicular to the roof joists. This allows gable end ventilation.
The min. space allowed between the insulation and the sheathing when using roof joists is 2 1/2". The min. space allowed between insulation and the sheathing at the eaves when using trusses is 1". The more space you have the better the roof system will breath (raised heel trusses). The insulation should never obstruct the ventilation path and should never touch the underside of the sheathing. The air space provides a separation from humid interior air and cold exterior air. If the insulation is in contact with the underside of the sheathing this creates a bridging effect for the interior warm humid air to come into contact with the cold exterior sheathing causing the snow to melt in the winter on the roof creating ice damns. Poor ventilation causes the same effects. The interior warm humid air must be replaced with cold outside air and the ventilation requirements provide for that so that the underside of the sheathing is always cold which prevents the snow from melting and causing your shingles to fail.

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Flashing

Roof flashing is used to prevent water leakage or wind driven rain through joints between materials. Flashing is installed at wall and roof intersections, at eaves, in roof valleys and around roof penetrations (chimneys, plumbing stacks, etc...).
A good roof building technique is to provide a cricket or chimney saddle where the roof slope abuts the chimney. Flash this area and the perimeter of the chimney roof connection properly. This area is a common problem for rain penetration in homes.
Use sheet metal flashing that is a min. .33mm galvanized steel, .46mm thick copper, .46mm thick zinc, .48mm pre-finished aluminum (most popular) or 1.73mm thick lead for flashing valleys, wall roof intersections, chimney roof intersections and at eaves (optional).
Some manufacturers recommend to shingle the valleys this can create a problem as the shingles age. The shingles tend to dry and stiffen creating a gap between the roof deck and the underside of the shingle. To minimize this gap a cant strip of wood should be installed in all valleys to reduce the bending of the shingles. Do not walk on or close to the valley when this procedure is used you can tear the shingles.
Plumbing stacks incorporate preformed metal neoprene flashing. Fit the flashing over the vent pipe. Metal neoprene flashing allows for differential movement between the roof deck and vent pipe.
When installing roof vents make sure to buy roof vents with roof flashing built into the vent units. Follow the manufacturers installation recommendations.

Send Your Questions to : Bill@ResidentialDesignandConsultingGroup.com