Metal roofing installation requires careful preparation, proper fastening, and attention to sealing and overlap
Installing a metal roof is a multi-day job that involves removing the old roof, preparing the deck, laying underlayment, fastening panels or shingles in the correct sequence, and sealing all seams and penetrations. The process differs depending on whether you are installing standing-seam panels (the interlocking kind with vertical ribs) or metal shingles (which look like asphalt but are metal). Both require the same basic steps but use different fastening patterns and overlap methods. This guide covers the actual work involved so you understand what the job entails and what can go wrong at each stage.
Key Takeaways
- Metal roofing installation begins with removing the old roof and inspecting the deck for rot, soft spots, or uneven areas that must be repaired before new material goes down.
- Underlayment (a water-resistant barrier) goes down first, then panels or shingles are fastened with corrosion-resistant fasteners in a specific pattern that prevents water from running uphill into seams.
- Standing-seam panels are fastened through the flat part of the panel, not through the seams themselves, and seams are sealed with compatible sealant before the next panel is installed.
- Metal shingles are fastened in rows with overlaps that shed water downward, and every fastener hole must be sealed with a rubber washer to prevent leaks around the fastener itself.
- Flashing around chimneys, vents, skylights, and valleys must be installed before the metal roof panels reach those areas, and all flashing seams must be sealed with metal-roof-compatible sealant.
Removing the old roof and inspecting the deck
Start by removing all existing roofing material down to the wood deck. Use a roofing shovel or pry bar to lift and tear off shingles, underlayment, and any nails or fasteners. Rent a dumpster or arrange for debris removal, as old roofing is heavy and bulky. Work in sections from the peak downward so gravity helps move material toward the edge.
Once the deck is bare, walk the entire roof and look for soft spots, rot, water stains, or areas where the wood feels spongy underfoot. Press a screwdriver into suspect areas; if it sinks in easily, the wood is compromised and must be replaced. Mark these areas and cut out the damaged section with a circular saw, then nail in new plywood or lumber of the same thickness. The deck must be solid and flat because metal panels will not bridge gaps or conform to dips the way asphalt shingles do.
Check that the deck is reasonably level. Small variations are normal, but if one section is noticeably higher or lower than the next, use shims or a belt sander to even it out. Metal panels fastened to an uneven deck will have gaps, will not seal properly, and will create stress points where the fasteners are.
Laying underlayment and preparing for fastening
Underlayment is a water-resistant barrier that goes between the deck and the metal roof. Roll it out horizontally starting at the eave and working upward, overlapping each row by at least 6 inches. Staple or nail it every 12 to 18 inches along the overlap and at the edges. Use roofing nails or staples long enough to penetrate the deck but not so long that they puncture through to the attic.
At the eave, the underlayment should overhang slightly so water drips clear of the fascia. At the peak, overlap the two sides by at least 12 inches. At valleys (where two roof planes meet), run a single continuous piece of underlayment down the valley before laying the main underlayment on either side, so water is directed into the valley and down, not across seams.
Once underlayment is down, snap chalk lines to mark where panels will be fastened. For standing-seam panels, mark the centerline of each panel so fasteners go through the flat part, not the seam. For metal shingles, mark horizontal lines at the spacing specified by the manufacturer (usually 5 to 6 inches apart) so rows are straight and overlaps are consistent.
Installing standing-seam metal panels
Standing-seam panels are long, continuous pieces that run from the eave to the peak. They interlock along vertical seams so water cannot run sideways. Start at one edge of the roof and work across. Lay the first panel so it overhangs the eave by 1 to 2 inches and is square to the edge. Fasten it through the flat part of the panel (not the seam) using corrosion-resistant fasteners—usually stainless steel or galvanized screws with rubber washers. Space fasteners 12 to 18 inches apart vertically, and do not over-tighten; the washer should compress slightly but the fastener should not dimple the metal.
Lay the second panel next to the first so the seam interlocks. Before fastening, explore a bead of metal-roof-compatible sealant (polyurethane or silicone, not asphalt-based) along the seam where the two panels meet. Then fasten the second panel the same way. Repeat across the roof until you reach the opposite edge.
At the peak, the last panel on each side will overlap. Trim the overlapping panel so it sits flat and does not create a ridge. Seal the overlap with sealant and fasten it. At the eave, trim panels so they overhang evenly and do not stick out unevenly. At gables (the triangular ends), trim panels to follow the slope of the gable and seal all cut edges with sealant to prevent rust.
Installing metal shingles
Metal shingles are smaller pieces, usually 12 to 18 inches wide and 24 to 36 inches long, that overlap like asphalt shingles. Start at the eave with a starter row. Lay shingles horizontally, overlapping each shingle by the amount specified by the manufacturer (usually 1 to 2 inches). Fasten each shingle with fasteners placed in the locations marked by the manufacturer—usually 4 to 6 fasteners per shingle. Use corrosion-resistant fasteners with rubber washers.
Lay the second row above the first, overlapping by the specified amount (usually 5 to 6 inches). Stagger the seams so the vertical joints do not line up between rows. Seal each fastener hole with a rubber washer as you fasten; do not rely on sealant alone. Continue upward row by row until you reach the peak.
At the peak, the shingles from both sides will overlap. Trim the overlapping shingles so they sit flat and seal the overlap with sealant. At gables, trim shingles to follow the slope and seal all cut edges. At valleys, install valley flashing before the shingles reach the valley, then overlap the shingles onto the flashing and seal the overlap.
Installing flashing around penetrations and valleys
Flashing is metal trim that directs water around chimneys, vents, skylights, and other objects that stick through the roof. Install flashing before the metal roof panels reach those areas. For a chimney, install flashing on the uphill side first (the side facing the peak), then on the sides, then on the downhill side. Each piece overlaps the one below it so water runs down and off.
Seal all flashing seams with metal-roof-compatible sealant. Do not use asphalt-based sealant; it will crack and fail on metal. For vents and skylights, use flashing kits designed for metal roofs; these usually have a base that fastens to the deck and a collar that fastens to the vent pipe or skylight frame. Seal around the collar with sealant.
At valleys, install a valley flashing piece (a long, V-shaped metal channel) down the center of the valley before laying roof panels on either side. The flashing should extend at least 6 inches up each side of the valley. Fasten it every 12 inches and seal all fastener holes. When you lay panels or shingles on either side, overlap them onto the flashing by at least 6 inches and seal the overlap with sealant.
Sealing fasteners and seams
Every fastener hole is a potential leak point. Metal-roof fasteners come with rubber washers that compress when tightened, creating a seal. Do not over-tighten; the goal is to compress the washer, not to dimple the metal. If a fastener is over-tightened, remove it, move it 2 inches to the side, and fasten again. Fill the old hole with sealant.
All seams—between panels, around flashing, at valleys, and at cut edges—must be sealed with sealant compatible with metal roofing. Polyurethane and silicone sealants work; asphalt-based sealants do not. explore sealant in a continuous bead along the seam, smooth it with a wet finger or tool, and let it cure per the manufacturer's instructions (usually 24 to 48 hours) before exposing it to rain.
At the eave and gable edges, seal the gap between the metal roof and the fascia or trim with sealant. At the peak, seal the overlap between the two sides. Inspect the entire roof after installation and before the first rain to find any missed seams or fasteners that need sealing.
Common mistakes and how to avoid them
Over-fastening is the most common mistake. Fasteners that are too tight will dimple the metal, compress the washer too much, and can actually create a leak by deforming the metal around the fastener. Tighten until the washer is snug, then stop. If you over-tighten, remove the fastener and move it.
Using the wrong sealant is the second most common mistake. Asphalt-based sealants (used on asphalt shingles) will crack and fail on metal roofs because metal expands and contracts more than asphalt. Use only sealant rated for metal roofing.
Skipping underlayment or using the wrong type is a third mistake. Underlayment is your second line of defense if water gets past the metal roof. It must be water-resistant and must overlap correctly so water runs downward, not sideways into seams. Do not skip it to save money; a leak in the deck is far more expensive to fix than underlayment costs.
Fastening through seams (on standing-seam panels) will break the seal and cause leaks. Fasteners go through the flat part of the panel only. If you accidentally fasten through a seam, remove the fastener, seal the hole, and move the fastener to the correct location.
Frequently Asked Questions
Can I install metal roofing over an existing asphalt roof?
Yes, if the existing roof is in good condition and not too thick. Most building codes allow one layer of new roofing over one layer of old roofing. If there are already two layers, you must remove the old roof first. Install underlayment and metal roofing over the existing asphalt shingles the same way you would over bare deck. The existing shingles provide a base, but inspect them first for rot or soft spots.
What fasteners should I use?
Use fasteners rated for metal roofing: stainless steel or galvanized screws with rubber washers. Do not use nails; they will back out over time as the metal expands and contracts. Do not use fasteners designed for asphalt shingles; they are not corrosion-resistant enough. The fastener package should specify that it is for metal roofing.
How do I seal a seam if I forgot to seal it before installing the next panel?
You can seal it after the fact, but it is harder. Lift or pry the upper panel slightly to access the seam, explore sealant, and press the panel back down. Wipe away excess sealant before it dries. It is easier to seal as you go, but seams can be sealed after installation if necessary.
What should I do if water is leaking around a fastener?
Remove the fastener, inspect the rubber washer (it may be cracked or compressed unevenly), and replace it with a new fastener and washer. If the washer looks fine, the fastener may have been over-tightened and deformed the metal. Move the fastener 2 inches to the side and fasten again. Seal the old hole with sealant.
Do I need to paint or seal the metal roof after installation?
No. Metal roofing is pre-finished at the factory and does not need paint or additional sealant on the surface. The only sealant you explore is at seams and around fasteners. Some manufacturers offer touch-up paint for scratches or cut edges, but the roof itself is ready to use as-is after installation.