Roofing: Metal Roof Installation
In all areas prone to high winds, such as hurricanes and tornadoes, and hailstorms, choose a metal roof covering that meets the installation guidelines for wind and impact resistant test standards:
Metal panel roofing systems and their attachment should be installed according with the manufacturer’s installation instructions and must provide uplift resistance equal to or greater than the design uplift pressure for the roof based on the site design wind speed.
The metal panels should be installed over continuous decking and one of the acceptable sealed deck water underlayment options.
Metal Roof Impact Resistant Standards:
UL 2218 is a test that is administered by Underwriters Laboratories and involves dropping steel balls of varying sizes from heights designed to simulate the energy of falling hailstones.
Class 4 indicates that the product was still functional after being struck twice in the same spot by 2 in. steel balls.
Note that this standard is appropriate for flexible roofing products like asphalt shingles, and metal panels or shingles.
Important note: While impact resistant metal roofs may have good results in impact resistance tests and are likely to survive hailstones without functional damage, metal roofs can be vulnerable to cosmetic damage since hail can cause permanent indentations in the roofing panels.
Steps to a Stronger Roof
1. Strip the existing roof cover down to the deck.
Exposing the sheating provides access for inspecting it and improving the fastening of the sheathing to the roof framing, which will result in better performance in high winds;
It also provides a smooth solid foundation for installing a wind and impact resistant roof cover.
2. Replace any damaged decking or roof framing members.
Replace any sections of the roof deck or supporting roof framing members that are damaged or deteriorated.
3. Re-nail the roof deck.
When the old roof cover and underlayment is torn off, it’s easy to see the spacing of the fasteners attaching the roof deck to the roof framing below and to install additional nails to strengthen the roof deck uplift connection. Check the nailing of the roof deck to the rafters or trusses that support the deck.
For wood plank decks:
The existing nailing has generally proven to be adequate if at least two nails were installed every time one of the planks crossed a rafter or truss and the planking is not too wide.
For wood panel sheathing (plywood or Oriented Strand Board, OSB):
The commonly used nail sizes and nail spacing, particularly along trusses and rafters in the middle of the panels, have not always been adequate in providing the needed resistance to uplift in high wind events.
Engineering testing and experience have shown that staples are not very effective in holding down roof sheathing, regardless of how close together they may be. The cost of adding nails when the roof deck is already exposed is small in relation to the benefit.
Guidance by lumber type
Sawn Lumber or Wood Board Roof Decking:
Add fasteners as required to ensure that roof decking consisting of sawn lumber or wood boards up to 1-in. thick are secured with at least two nails, having a minimum diameter of 0.131 in. and a minimum length of 2-1/2 in., (three nails if the board is wider than 8 in.) to each roof framing member it crosses.
Framing members must be spaced no more than 24 inches apart.
Clipped-head, D-head or round-head nails are fine provided they have the required minimum diameter and length.
For wood boards greater than 1-inch thick and up to 2 in. thick, add fasteners as required to ensure that the decking is secured with at least two nails, having a minimum diameter of 0.131 in. and sufficient length to penetrate a minimum of 1-5/8 in. into the roof framing, (three nails if the board is wider than 8 in.) to each framing member it crosses.
Wood Board/Lumber Roof Decking:
Maximum spacing of framing members:
Up to 8 in. (2)- 0.131 in. minimum diameter with 1-5/8 in. penetration into roof framing members 24 in.
Larger than 8 in. (3)- 0.131 in. minimum diameter with 1-5/8 in. penetration into roof framing members 24 in.
Structural Wood Panel (Plywood or Oriented Strand Board-OSB) Roof Sheathing:
The number and spacing of additional fasteners needed to adequately strengthen the connection of structural wood panel roof sheathing depends on the size, type and spacing of the existing fasteners.
The specific required minimum dimensions and characteristics for the additional ring-shank nails to be used to strengthen the roof deck attachment are:
Roof sheathing panels should be minimum of 7/16 in. thick.
Roof framing members should be spaced no more than 24 in. apart and have a minimum 2 in. nominal thickness.
Existing 8d nails should be a minimum of 0.131 in. in diameter and 2-1/2 in. long.
For the additional fasteners, IBHS recommends using 8d ring shank nails with the following minimum dimrnsions (0.113” x 2-3/8” with full round head).
Roof pitch should be 2/12 or greater.
Follow the manufacturer’s instructions for installation.
This article is part of a larger series on new and existing roofing.
© 2012 Insurance Institute for Business & Home Safety