How to Strap a Roof for Metal: Spacing, Fastening and When You Need a Solid Deck
Horizontal strapping over an existing roof or open framing, where it is allowed, how far apart it goes and why some panels cannot use it.
- Advanced
- 1 to 2 days
- Updated
The short answer
Strapping, also called purlins or battens, is horizontal lumber, commonly 1 by 4 or 2 by 4, fastened across the rafters or over old shingles to give exposed-fastener metal panels a fixing line. Typical spacing is 24 inches on centre, set by the panel manufacturer and loads, and screws must reach the rafters. Concealed-fastener panels such as snap-lock and standing seam usually need a solid deck instead.
Strapping is how a lot of exposed-fastener metal roofs are built. The panels need something to screw into every few feet, and strapping provides that without the cost of a full deck. It is also the standard way of putting a new metal roof over an existing shingle roof without tearing off, because the strapping creates a clean, flat, ventilated plane above the old surface.
The limits matter. Strapping works only for panels that the manufacturer approves over open or strapped substrates, and most concealed-fastener panels such as snap-lock and standing seam need a solid deck. Strapping laid out badly telegraphs through the roof as waves and oil canning. Screws that miss the rafters or go into thin strapping pull out in the first windstorm.
Read metal roof panel types to see which families can use strapping, and underlayment for metal roofs for what goes under it.
Tools
- Framing nailer or cordless drill
- Chalk line
- Tape measure
- Circular saw
- Stud finder or rafter layout marks
- Speed square
- Fall protection equipment
Materials
- Strapping lumber, typically 1 by 4 or 2 by 4, pressure treated or dimensional with a stated moisture content
- Structural screws or ring-shank nails long enough to reach rafters
- Self-adhered membrane or breathable underlayment where the design calls for it
- Vented closure and bug screen at eave and ridge
- Counter-batten strips if a ventilated airspace is being created
Safety first
- Loose strapping over old shingles creates trip hazards and a slippery surface. Tie off and keep lumber staged on the roof in small stacks, not on the slope.
- Fasteners that miss the rafter can pass through the ceiling. Know the rafter layout and mark it before you fasten.
- Use fall protection at 3 metres (about 10 feet) or any height where a fall could cause serious injury under WorkSafeBC rules.
Step by Step
Step 1: Confirm the panel and the building allow strapping
Exposed-fastener ribbed panels are commonly allowed over strapping or purlins at a stated maximum spacing. Concealed-fastener panels generally are not, because their clips and flanges need a continuous deck to transfer wind loads. The panel data sheet and the manual spell out the substrate, and that overrides any rule of thumb.
Also check the structure. One layer of shingles is usually fine to leave, but a second layer, sagging rafters or a soft deck is a reason to tear off instead. Weigh the roof: light steel panels are lighter than shingles, but strapping adds lumber and the building code may limit how many layers sit on the deck. Ask your building department when in doubt.
Watch out: Over two layers of shingles, or over a roof with any rot or sag, strap-over is not a good idea. Tear off and inspect the deck.
Step 2: Locate the rafters and mark them
Rafter spacing is usually 16 or 24 inches on centre. Use the nail lines in the old deck, the attic view, a stud finder or an exposed soffit to locate them. Snap vertical chalk lines up the slope at each rafter centre.
The fasteners that hold the strapping carry the wind uplift load from the metal, so they must bite into the framing with enough penetration, typically at least 1-1/4 to 1-1/2 inches of the fastener into the rafter, and more on exposed sites. A screw that only reaches the sheathing will pull out.
Step 3: Decide on spacing and lumber size
The right spacing comes from the panel's gauge and profile and the wind and snow load. Many 26 gauge ribbed panels are rated for 24 inches on centre over solid or open substrates, and thinner panels may need closer spacing, such as 16 or 12 inches. Use the spacing in the panel's installation manual or engineered load table.
A 1 by 4 is a common strap size over a solid deck because the deck carries the load. A 2 by 4 is typical on open rafters because it must span between rafters and hold screws. For panels over old shingles, use straps thick enough to hold screws and to build a vent gap, and consider a double layer where the roof is uneven.
Step 4: Install vertical counter-battens if the design uses them
A counter-batten layout places vertical strips, often 1 by 4, fastened to the rafters up the slope, and horizontal strapping across them. The vertical strips leave a channel for air to move from eave to ridge, carrying away moisture and heat, which reduces condensation under the metal. See underlayment and condensation control.
The channel must be open at the eave and at the ridge, with bug screen, and not blocked by the horizontal straps. Notch the straps or leave gaps at each counter-batten line so air still flows along the channel, or use a layout the manufacturer or the supplier has tested.
Step 5: Lay out and fasten the first strap at the eave
The bottom strap controls the eave detail. Position it so the screws land just above the closure and the panel overhangs the fascia or gutter by the amount in the manual, commonly 1 to 2 inches. Check that it is straight with a chalk line, because a crooked first strap shows in every panel above it.
Fasten with structural screws or ring-shank nails into each rafter, two fasteners at each rafter crossing for a 2 by 4 or one for a 1 by 4 depending on the load. Countersink slightly so nothing protrudes into the underside of the panel.
Step 6: Run the remaining straps at constant spacing
Mark the spacing on a story pole and transfer it from the eave, because drift builds up when each strap is measured off the last one. Butt joints in a strap must fall on a rafter, never between them, and adjacent straps should not share joints. Leave a 1/4 inch gap between strap ends to allow for swelling.
Check that the straps are in the same plane. A crown or sag in a strap shows as a bump in the roof and can cause oil canning in the metal above. Use shims or plane high spots, and avoid wet or badly bowed lumber.
Tip: Choose straight, dry lumber and stack it flat before installing. Twisted strapping is the most common source of visible waves in a finished metal roof.
Step 7: Strap around penetrations, ridge and hips
Ridge and hip caps need a strap close to the ridge line to fasten the cap and closures, and valley flashings need backing on both sides. Around chimneys, skylights and large pipes, frame a short curb or add blocking so the panel edges and flashing have something to fasten to. See ridge and hip caps and valleys.
Leave a gap at the ridge so the vent opening is not blocked if the ridge is vented.
Step 8: Close eave and ridge vents with screen, then underlay
If the assembly uses a ventilated airspace, install a vented eave closure that keeps out birds and insects while letting air in, and a matching vented ridge closure. If the strapped layer is over a solid deck, the underlayment generally goes below the straps, with the straps fastened through it.
Seal the nail and screw penetrations through self-adhered membrane where the design places the membrane below strapping. Self-adhered membrane reseals around fasteners, which is a good reason to use it under strapping.
When a Solid Deck Is Required
Most snap-lock, nail-strip and standing seam panels require a continuous deck, commonly at least 1/2 inch plywood and sometimes 5/8 inch, because the panel is fastened through a flange or clip every 12 to 24 inches and the deck spreads wind uplift. Strapping leaves big spans between fasteners, and the thin panels deflect and oil can.
Other reasons for a solid deck: local building code requirements in snow zones, fire ratings, low slopes where a leak is less tolerable, and any roof where the attic space is used and sound or condensation control matters.
Strapping Over Old Shingles: The Trade-Offs
Strapping over one layer of shingles saves tear-off cost and keeps the old roof as a secondary layer. The trade-offs are that hidden deck damage stays hidden, the old shingles can be a fire and rot trap if the ventilation design is poor, and you must find the rafters to fasten to, which can be hard through a finished ceiling. Many installers prefer a tear-off so the deck can be inspected and proper underlayment installed directly.
Common Mistakes
- Fastening strapping to the sheathing only, not the rafters. The roof can lift in a windstorm.
- Using concealed-fastener panels over strapping when the manufacturer requires a solid deck.
- Uneven or bowed lumber, which telegraphs as waves and oil canning in the metal.
- Measuring each strap from the last one, letting the spacing drift until the ridge strap is in the wrong place.
- Blocking the airflow with horizontal straps across counter-battens, which defeats the ventilated cavity.
- Leaving out extra framing at penetrations, hips and ridge, so trim and flashing have nothing solid to fasten to.
When to Call a Professional
If the roof has more than one layer of shingles, a sagging deck, unknown rafter spacing or snow loads to consider, call a roofer. The spacing and fastener decisions affect wind uplift and safety, and mistakes are hidden once the panels are on.
A roofer can also advise whether strapping over the old roof or a full tear-off is the better choice for your house and the panel you have picked.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Metal Roofing service or the towns we cover.
How to Strap (Purlin) a Roof for Metal: Questions
What is the spacing for metal roof purlins or strapping?
Twenty-four inches on centre is the common maximum for 26 gauge exposed-fastener panels, but it depends on the panel, gauge and local wind and snow loads. Thinner panels may need 16 or even 12 inches. The panel manufacturer's installation manual or engineered load table sets the exact spacing, and it overrides any general rule.
Can you install metal roofing over shingles with strapping?
Often yes, over a single layer of shingles in good condition, using strapping fastened through the shingles into the rafters. It saves tear-off, but hidden deck damage stays hidden and the building code may limit layers. Many roofers prefer to tear off so the deck can be inspected and underlayment installed directly.
Do I need strapping if I have plywood decking?
Not always. Concealed-fastener panels such as snap-lock and standing seam are usually installed directly over solid plywood with underlayment, with no strapping. Exposed-fastener panels can be screwed into the deck or into strapping. Strapping over a solid deck is used to create a ventilated airspace or to change the fastening line.
Should metal roof strapping be 1x4 or 2x4?
A 1 by 4 is common over a solid deck because the deck carries the load. A 2 by 4 is common over open rafters because the strap has to span and hold screws. Follow the manufacturer's manual and the load requirements. Use straight, dry lumber, and fasten into the rafters rather than only into the sheathing.
Related Guides
All Metal Roofing guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
Want someone to look at your roof?
Kory & Johnny have roofed homes across the Fraser Valley since 2016. Free inspections, honest estimates, no pressure.
604-728-0146Text UsEmailServing Hope, Agassiz, Chilliwack, Rosedale, Abbotsford & the entire Fraser Valley