Prepare for Success, prepping your old roof for metal roofing

The modern trend to installing new metal roofing directly over failed roofing is growing,and with good reason. The installation of longer-life, more durable materials without removing the old roof surface is proving to be a sound environmental practice, reducing tear-off cost, time, labour, haulage, and landfill environmental fees. A direct install over old roofing also offers increased sound-proofing and a second line of defense, –even if minimal, should traumatic roof failure occur due to loss of shingles or sheet roofing by wind.

A new direct-over install can be a substantial time-saver, quickly and easily accomplished if old roofing remains relatively intact and even. Trouble is, we invariably run into problem roofs where sun-damaged, uneven asphalt shingles are severely curled, bubbled and warped, presenting a ragged, uneven, and difficult working surface to the roofer. The solution? Prepare the whole roof carefully before starting.

Why waste time preparing a roof? Doesn’t new roofing just cover it up anyway?

Fastening new roofing directly onto very rough surfaces can result in a less-than quality appearance, and worse yet, an unhappy customer or homeowner. The simple over-tightening of fasteners on new roofing on a rough, uneven surface can easily result in a wavy, uneven, and unattractive appearance . An accidental misstep on new steel roofing laying over jagged substrate can also result in unattractive marks, with visible “imprinting” on the new roof. Professionals know that smoother working surfaces inevitably save time, energy, and aggravation. For an optimal appearance, prepare the substrate carefully. Think “trim and smooth” as you prepare for success.

Structurally solid?

Almost any roof can be rebuilt. The edge of an older roof usually displays the most severe deterioration, but even the worst condition may be overcome with careful preparation. Partial sheeting replacement and / or the installation of strapping may be required. Cut out decayed, failed sheeting sections and replace where necessary. Fasten supports, or even new extensions on truss tails if required, then replace the sheeting, and you’re good to go.

Is the old sheeting straight?

Old roofs may be visibly wavy and sagging a bit between the trusses or rafters, which is usually caused by weak sheeting material or excessive snow load. Consider strapping the roof if this condition appears excessive.

Right on top of the old shingles, or strap?

Steel roofing can be fastened directly on the shingles if the roof is prepped adequately . Cut away any rough, curled, or excessively elevated, curled shingles to make the surface as smooth as possible. Trim off the old asphalt shingle reveal flush to the roof edges. On gable ends, ensure it is trimmed back far enough to avoid interference with the new steel edge finishing trim. Install a waterproof membrane if the pitch of the roof is less than 4:12 or as recommended by the roofing manufacturer of the system you have chosen.

Too crooked? Use strapping.

If the old sheeting sags excessively, consider strapping the whole roof across the trusses. Use suitable 1×4″ Spruce or other suitable dimensioned lumber spaced 12″, 16″, 24″ on center, — or at intervals required for your specific steel roofing system. Generally, the more support the better, but locating strapping to accommodate the new system fasteners correctly is paramount.

  • Begin by strapping the perimeter of the whole roof completely. Ensure the perimeter strapping is placed accurately and aligned squarely. Fasten strapping securely using 2″ or longer anodized screws. Let’s try and hit the rafters or trusses with those screws, they’ll hold much better than the old plywood or chipboard sheeting will. It may be a timely idea to use a few extra screws on the strapping too, with increasingly severe winds and the potential damage it can cause.
  • Install strapping progressively up the roof, ensuring spacing is placed correctly for the new system being installed. Pre-cut a set of spacers to speed up the strapping install.
  • Cut away old shingle material that prevents straps from sitting solidly when it is fastened in position. Try out that new straightedge to determine if the top mounting surface of the strapping is acceptably and consistently straight across several straps. Find any excessively high or low spots? Loosen and cut away offending shingle material to enable installation of the strap securely, or use opposing wedges under strapping placed at the trusses to raise a strap if required. Adjusting the plane of the strapping carefully at the onset will save a lot of aggravation and result in a straighter, better and more professional-looking job.
  • Tip: There’s nothing quite as annoying as trying to place visible roofing fasteners uniformly, accurately and neatly, only to discover newly-installed strapping “runs out” off center. If that problem occurs, (remember Murphy’s law, ‘if it can happen, it will ‘ ) simply relocate the offending strap. If long vertical sheeting has already been fastened in place preventing simple relocation of the strap, little choice remains but to install a second strap adjacent to the first one to maintain the desired pattern of fasteners.
  • Leave enough space between the last two opposing rows of strapping at the ridge to allow for a ridge vent cutout; allow 2″ or more as required along the full length of the roof. Install strapping around vent locations as required if other ventilation systems are used.
  • Ensure that defective or inadequate flashing is replaced and correctly installed where required. With a long-life steel roof, new flashing is always a good idea.

Should a waterproof membrane or a roofing felt be used?

When steel roofing is installed directly over existing roofing, the installation of a waterproof membrane is recommended for flat, low-pitch roofs (3:12 pitch or less) and may also be required for some specific steel roofing types. Ice & Water Shield( ™) edging is recommended for steel shingles. As a matter of interest, historically, most roofing was installed with a suitable felt or building paper over the complete roof surface, but with the exception of the application of the quintessential edging, most roofing is now installed without.

There are pros and cons; installing a paper barrier provides not only a second line of defense against leaks, but decouples the roofing to some degree for expansion and contraction. Barrier installation under some brands of steel roofing may be recommended for the same reasons. All recommendations made by the manufacturer should be followed to optimize the performance of any roofing system.

Ready now? Install that steel roof

Now it’s easier to install your roof. Attach starter strips all around and get that first row of roofing dead straight, whether it is steel shingles, long sheets of standing-seam roofing or a modern painted-steel corrugated type of roofing. If working across the roof, measure constantly to ensure the sheeting remains square. Go for it.

In preparing your roof for success, always work safely; use appropriate equipment and safety protocol on the roof at all times. We can see that new ridge cap up there already.