Correct roof installation of an asphalt shingle system
Successful roof installation requires a little knowledge and planning which can be found in this guide. It is important to learn how to correctly install asphalt shingles. Follow the steps found below and you will have a roofing system that will outperform most other roofs, easily outlasting the warranty period. If any of the roof installation measures explained below are not followed your materials may start to deteriorate only after a few years.
For a worry free roof read this guide and of course ring and speak to one of our roof installation team with any questions you have.
We generally start with the trusses “rafters” spaced at 600mm centres.
If rafters are at 900mm centres you will need to use 15mm plywood in 2700mm x 1200mm sheets as per Australian standards.
Before the plywood can be installed we need to perform a few simple things. The centre valley line needs to be nogged to stop any movement in the plywood. If the ply can move in the valley the shingle roof tiles or rubber membrane can rip and water can enter.
The ridge needs to be nogged 100mm back from the apex. This gives the plywood something strong to attach to and also provides support to the ridge ventilation and capping tiles.
Special attention is given to the leading edge of the plywood above the fascia board. This is where we achieve “Inlet Ventilation” around the entire perimeter.
The section near the barge is detailed in the picture below.
A shingle roof installation uses “Australian Made” 12mm A-Bond Tongue & Groove Structural Plywood – stamped (AS/NZS 2269).
I stress “Australian Made” because there are imported plywood’s that are stamped AS/NZS 2269 that are not seasoned in our country. Most imported plywood will equalize to our moisture conditions and may shrink, buckle and warp – when this happens you will need to replace the entire system (ply and shingles). You only make this mistake once 🙂
To install Plywood you will need to follow the manufacturers roof installation guidelines but generally speaking 70mm long shank framing nails spaced every 150mm will suffice. This is also suitable in high wind zone areas. It is important to use tongue and groove plywood as it provides the stability needed under the felt paper and shingles. The tongue and groove edge should be butted together tight with the short vertical edge needing a 2mm gap for any expansion in the wood.
Clips are not preferred to join the plywood as they don’t hold the ply together sufficiently. When walking on the roof the ply will move and both the shingles and the felt paper will tear and rip causing water to enter. If your roof has been installed with clips it does not meet Australian standards and will need to be replaced.
We then staple down #15 Asphalt Saturated Felt Paper – holding it in place. We overlap 50mm on the horizontal joins and 100mm on the vertical joins. Between 8.5 degrees and 18.5 degrees pitch we either use a double layer of felt paper or Armourguard rubberized adhesive membrane to cover the plywood.
On low pitches (between 8.5° – 15 degrees) we generally use an adhesive rubberized membrane material. This is because the water doesn’t flow down the slope as easily – wind-driven rain can flow up the slope and under the shingles.
Asphalt saturated felt underlay is a 920mm wide felt paper coated in asphalt saturant. Underlay paper is used for the following reasons.
- Protecting the asphalt roofing shingles from any resins that are released from the Ply
- Underlay provides secondary protection from wind-driven rain.
- Most manufacturers require the use of underlay in the roof installation to validate the warranty.
- The use of underlay can flatten out any irregularities in the plywood base.
- A Class A fire rating will only be obtained with the use of saturated asphalt underlay.
- The minimum required standard for the underlay is No. 15 / ASTM D226 Type 1 felt.
- See CASMA Technical Bulletin No. 3 (1992) or ARMA Technical Bulletin Underlayment for more detailed underlay roof installation information.
We then line the valleys with “IKO Armourguard” which is a thick rubberized membrane. This gives additional protection at the roofs most venerable area.
Colorbond drip edge at the fascia is installed. At the fascia (bottom edge of the plywood) this should extend at lease 110mm up the roof. This flashing is used to protect the bottom edge of the plywood from water damage – rotting. The barge flashing protects the plywood from water damage and keeps the water out.
The asphalt roof shingles are then installed with galvanised clout nails (10mm head and at least 25mm long shank) the nail should penetrate through the plywood by 2 – 3mm.
In my experience, when staples are used to fasten shingles they slowly work their way out of the plywood over time. They tend to cut through the shingles above letting the water penetrate.
Asphalt roofing shingles should be fastened with wide head Galvanised clout nails. The head should be 10mm wide and the shank barbed or ribbed for superior holding strength. This is the ARMA’s (Asphalt Roofing Manufacturers Association) recommendation. IKO shingles should be installed following the tips provided below:
- Four GAL Nails are used on each roofing shingle. (always refer to individual product installation requirements)
- The clout nails are positioned just below the glue line and above the cut out strip on Marathon roofing shingles and on the Cambridge Laminated shingles follow the line.
- The clouts must be flush with the top of the roofing shingle as per the diagram above.
- The nail should pass through the plywood deck by 2 – 3mm as recommended by the ARMA.
See ARMA Form No. 221-RR-93 (1994) for more information.
In high wind zones it is advisable to install asphalt shingles with a six nail pattern. This provides additional holding power. Above 60 degree pitch each shingle tab needs to be glued down manually with asphalt mastic.
Where a sloped roof meets a vertical wall we use “step flashing”. Each course of shingles buts up against the wall and a metal step is nailed on top. The nail should be covered by the next step. This guides any water out onto the shingles and not in and underneath. The step flashing is covered by the wall covering. eg..foam, blueboard etc…
Velux skylights can be purchased with a step flashing kit to be used with asphalt shingle roofs. You will need to ask when purchasing.
Step flashing is to be used where any vertical penetration comes through a sloped section. This includes air conditioning units, evaporation units etc…
1000mm of ridge ventilation is equal to 1 x “whirly bird” ventilator. Ridge ventilation is essential for adequate exhaust ventilation. Box vents can be used but provide very little “Net Vent Area” so a standard size roof may need to 20-30 box vents which is not very practical.
Probably the most important installation consideration is proper ventilation. The amount of ventilation needed is determined by the size and design of the roof. For attic spaces above an insulated ceiling, the vent ratio is one square meter of net free ventilating area/300 square meters. For low slope roofs or cathedral ceilings the vent ratio is one square meter/150 square meters.
Airflow is maintained two ways. First, hot air naturally rises and exits out of the ridge vent, pulling in cool air from below. Second, positive airflow across the ridge of the house creates a “venturi effect”, or a negative pressure, which pulls air out of the ridge vent and brings in cool air from the Inlet air gap created. In calm or windy weather, the entire attic is vented by a constant flow of cooler, dryer outside air.
Here is an example of an asphalt shingle roof “completely sealed” No Ventilation, No Underlay and the bathroom exhaust venting into the attic space. The plywood was not tongue and groove and no drip edge was used. The shingles were fastened with what looked like household staples – As you can see it was a mess.
See CASMA Technical Bulletin No. 1 (1992) or ARMA Form No. 209-RR-86 (1994) for more information.
The best form of ventilation is a premium ridge vent. The vent we use is the Trimline premium ridge vent.
The correct use of ventilation plays a number of rolls.
- It regulates the air temperature inside the roof area keeping it the same as the outside air temperature or at least +/- 10 degrees.
- In attics which are not properly vented, moisture is also created by simple condensation. The air inside an improperly vented attic will be warmer than the air outside. When this warmer, moist air comes in contact with the colder roof sheathing condensation will occur. In effect, it can actually “rain” in your attic.
- Stops buckling of roofing shingles due to deck movement.
- Rotting of Plywood.
- Stops premature failure of the roofing including blistering.
In roofs where there is a small amount of ridge it may be necessary to install a solar-powered roof vent. These exhaust the equivalent of 12 spinning ventilators and only operate in daylight hours. This means they take the heat out during the day and leave the heat in the roof space at night.
I have provided a lot of information for you to take in however if you fail to understand or your builder doesn’t follow these steps you might find yourself without a manufacturer’s warranty and with a roof that needs replacing after only a few years.
If you have any queries please call myself on 0413 742 745 or Email to discuss any of the above installation techniques