Good roofing practice dictates that you install new materials upon a sound foundation. For older buildings, this may mean stripping away existing shingles, replacing compromised plywood sheeting, and applying under sheathing to provide protection against future water damage prior to installing a new metal roof. If your roof is structurally sound and able to withstand the added weight, metal roofing may be installed over existing shingles without the need for a reroof. New construction always requires under sheathing.
New Home Construction
On a new roof, plywood sheeting should be covered with a synthetic felt underlay, a self-adhered membrane, or a polymer modified bitumen membrane. The under sheathing of choice is best applied horizontally starting from the roof bottom and working upward. The rows of sheathing must be overlapped to guard against water intrusion. Preparing your roof in this manner will provide a waterproof base upon which to lay your metal shingling material.
Newer Existing Homes
If your home's roof has only one layer of asphalt shingles and the plywood beneath is uncorrupted by water damage, you may be able to install a metal roof directly over the existing material. A home inspector will be able to tell you if your roof beams and trusses will hold the added weight that an additional layer of roofing material will place upon the structure. Avoiding the cost of reroofing will more than pay for the home inspector's fee.
If your home has been re-shingled more than once without a tear off of previous materials, it may be wise to remove all existing shingles in order to perform a thorough examination of the roof structure. Older homes often require the removal and replacement of wood panels that have become unstable due to water, mold or insect damage. Once you have ensured that your basic wooden roofing platform is sound, you can proceed with under sheathing before installing new metal shingles.
Seeking Professional Advice and Assistance
Installing a metal roof is one home improvement job that may be difficult for the neophyte roofer to tackle without expert advice and professional assistance. Besides using the services of a home inspector, seek out the expertise of roofers who have experience with metal roof installation. Ask for references and speak to homeowners who have dealt with the roofers in the past. The dedicated do-it-yourselfer should seek out a local home improvement center offering a how-to seminar on metal roof installation.
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