Overlay vs Stripping

We are often asked about the pros & cons of removal versus covering over existing roofs. Most residential buildings and many old commercial buildings have wood decks. Building owners are often unaware of slow leaks in their roof systems, which have been quietly rotting sections of their plywood, OSB, or pine plank wood decks. Ninety percent of the damage occurs at the outer edges of the roof in the roof overhangs, thus the leaks often do not become apparent on the interior of the buildings.

All the pictures shown in this section were taken on roofs where the owners had no idea of the extent of substrate deterioration. Before starting the roof we were told there were no leaks. Take a look at the pictures. In 50% of cases roofs require no repairs. When they do, it’s often only one or two sheets that need replacement. In some extreme cases, it might be necessary to apply a complete new roof deck as in some of the pictures shown. Plywood can sometimes warp terribly when there are moisture issues in the attic, or if there were some form of defect in the plywood. On very old buildings, the pine planks can become dry, brittle, broken, etc. Things usually look ok from inside the attic, but once the roof is off, the defects are visible.

The Case Study herein shows five pictures in sequence on the same project, where the planks were in poor condition, and as such a new layer of ½” plywood was installed over the entire surface prior to roofing. This simple procedure would have increased the life of that particular flat roof by double or triple.

As such, although it is acceptable in the Building Code and with most manufacturers of shingle products to do overlays on top of up to 2 older roof systems, we strongly recommend removal of your old roof systems. Doing so ensures you have the opportunity to check the decking, make repairs, install proper underlay, and to have the shingles lay as flat as possible.

Where insulated roof decks are involved, like in most commercial flat roof applications, building owners often wish to retain their old insulation intact or go over top of an old roof system. While this is considerably cheaper, care has to be taken in making this decision. If there have been prior leaks or wetness in the old insulation or membranes, the new roof can be ruined within one or two years. The moisture underneath can cause the new roof to blister in the heat as the moisture heats and expands into water vapour.