On a re-roof, am I really choosing between these two?

Frequently not, and it is worth checking before treating it as a decision. If a sound deck is already up there, the honest question is whether to add battens or purlins ON TOP of it — creating a fastening plane and a vented cavity without a tear-off — rather than whether to have a deck at all. That is a both-and, not an either-or, and it is one of the more attractive things you can do to an old roof. The genuine either-or only appears when the deck is absent, or spongy enough that it is coming off regardless.

Which one is cheaper?

Structurally different answers, which is why this site refuses a figure. Purlins are members at intervals against a deck that is a full-area buy of sheathing, fasteners and underlayment — but the larger part of the purlin saving is usually upstream, in the primary frame it permits you to space wide, and that saving does not exist on a roof already framed at close centres. The other half of the ledger is the cost of changing your mind: adding battens over a deck later is trivial, adding a deck under existing panels is a strip-off. Run the row count and the sheet count against your own dimensions and your own quotes.

Is a metal roof loud on purlins?

Yes, and that is where the reputation comes from — a thin panel on open framing is a large, lightly damped membrane free to radiate on both faces, which is a drum. A deck fixes most of it by adding mass, by damping the panel against a solid surface, and by putting an underlayment between the two. What a deck does not fix is the other metal-roof noise: the tick and crack of thermal movement as panels grow and slip against their fixings. That belongs to panel length and the fastening system, not the substrate, and it is why clip systems and slip sheets exist.

Does a solid deck stop the roof sweating?

It moves the problem to a better place rather than solving it. Metal is impermeable and radiates heat to the night sky, so its underside routinely falls below the dew point of the air touching it — that happens over a deck too. What the deck changes is where the water goes: onto an underlayment that drains to the eave and into sheathing that can buffer it, instead of dripping onto whatever is below. The catch is that a metal roof lets a deck dry downward only, which is exactly why vented batten cavities and high-permeability underlayments are specified under metal. Over open purlins the same job is done by an anti-condensation backed panel, a taped membrane draped over the purlins, or insulation in direct contact with the metal — pick one deliberately, because doing nothing is also a decision.

Can standing seam go straight over purlins?

It depends entirely on the profile, and the span table settles it. Structural standing seam is designed to span open framing and is what steel buildings use. Architectural and low-rib profiles are non-structural by definition — they are deck-required, and fitting one over purlins is not a shortcut but a misuse that shows up as oil-canning, deflection and a warranty that no longer applies. If a particular panel is the reason you are doing this roof, read its span table before deciding the substrate, because the panel can settle this comparison on its own.

Can I add the deck later if I regret it?

Only by taking the roof off. Retro-decking means stripping the panels, adding a full-area material, dealing with purlins that now either come off or push the roof higher than the eave, verge and abutment details were built for, and re-fastening everything. The reverse direction costs almost nothing, which is the asymmetry to weigh at the outset: if there is a realistic chance the building will later be lined, heated, converted or re-covered with something that is not a spanning panel, the deck is cheapest bought once, at the beginning, when it is only material.

Honest comparison

Metal Roof on Purlins vs Solid Sheathing

Over purlins the panel is the structure and carries itself between rows; over a deck it is only a skin. The deck buys a supported second water barrier, an airtight plane, a walkable roof and a substrate for whatever comes next — but its cost is close to pure area, and the frame you already have usually settles the argument before any of that is weighed.

9Factors compared
6Questions
None, deliberatelyPrices

Banded stack of sawn lumber on grass at a house building site

How the two differ in kind

The difference is not a layer of material, it is which component is doing the structural work. A profiled metal panel is a spanning member: the depth of its ribs and the thickness of its coil give it a section, and the manufacturer's span table says how far that section may reach between supports under snow, under wind uplift, and under the concentrated load of somebody standing on it. Laid over purlins, the panel earns its keep — it spans eave-to-ridge from one purlin row to the next and the purlin spacing is simply the largest number that table permits for your load case. Laid over a solid deck, the panel spans nothing at all. It is a weather skin on a continuous surface, and the sheathing and the rafters beneath it carry every load the roof will ever see. That is why the same box of panels can behave like a structure on one roof and like a cladding on another.

Which means the honest first question is not about the panel. It is about the frame that is already there, because the two answers want different frames and one of them is often not available. A portal frame, a steel building or a barn on widely spaced trusses puts its primary members metres apart — far beyond anything a sheathing panel is rated to bridge — so on that roof a solid deck is not an alternative to purlins, it is an addition to them: you would fix purlins to span the frame, then sheathe over the purlins, then panel over the sheathing. Turn it around and the same logic reverses. A house or a garage framed at ordinary rafter centres already has supports at the spacing sheathing was designed for, so the deck is the direct, uncomplicated option and purlins become an extra layer bolted onto a frame that never needed one. On most real roofs the structure has already voted, and the interesting cases are the minority where both are genuinely buildable.

Where both are buildable, understand what the deck's money buys and how that money scales. Purlins are members at intervals — timber or light steel across the rafters, a few rows up the slope, plus the licence to space the primary frame wide, which is where the larger saving usually hides. A deck is a full-area buy: sheathing over every square of roof, fasteners over every square, and the underlayment that a continuous surface finally makes possible over every square again. So the deck's premium grows with the roof, while the things it buys — a walkable surface, a secondary water barrier, a plane to seal an air barrier to, a diaphragm that braces the building, a substrate that will accept any future covering — mostly do not grow at the same rate. They matter as much on a small heated garage as on a large barn, and cost far less there. The other asymmetry is time: battens or purlins can be added over an existing deck in a morning, whereas adding a deck under existing panels means stripping the roof to the frame. Four questions decide it in practice. What is holding the roof up, and at what spacing. Is anything underneath heated, lined or occupied. Is the panel profile you actually want rated to span open framing, or is it deck-required. And do you expect this covering to be the building's last.

The factors that actually differ

Show

	Purlins over rafters or trusses	Solid plywood / OSB deck
What carries the load	The panel does. Rib depth and gauge give it a section, and the manufacturer's span table for your snow, uplift and foot load sets the maximum purlin spacing — a number you read off, not one you choose.	The deck does. The panel spans nothing and is supported continuously, so its profile becomes an appearance and durability decision rather than a structural one.
What the primary frame must be	Free to be wide. Purlins exist so trusses, rafters or portal frames can sit metres apart, which is why every steel building and most agricultural roofs are built this way.	Close-spaced, at the centres sheathing is rated to span. Over a wide frame a deck does not replace purlins — it sits on top of them, and you buy both.
Shape of the material buy	Linear members at intervals, plus a real saving upstream in a primary frame you are allowed to space wide. The quantity is a row count up the slope.	A full-area buy: sheathing, fasteners and underlayment over every square of roof. The premium is close to pure area, so it scales with the building.
Where you can put your feet	On the purlin lines and nowhere else. Mid-span is a dent at best; on a wide spacing with a light gauge it is a fall through the roof.	Anywhere. That changes the install, and it changes every later visit — flashings, aerials, snow guards, solar, a chimney repair, clearing a valley.
Where the fasteners land	Every screw must hit a purlin. A miss is a hole in the roof with nothing behind it, and mis-hits multiply on wide spacings and long panels — which is why the row layout gets chalked before anything is lifted.	Substrate everywhere, so nothing misses. But the uplift path gains a link — panel to screw to sheathing to the sheathing's own nailing to the rafter — and that nailing then carries the whole roof, which is why edge and corner zones are fastened harder.
Secondary water barrier	Only an unsupported one. A membrane draped over the purlins is real and is normal practice, but it sags into troughs between rows, so it drains only if it was laid to fall and detailed into the gutter, and it cannot be walked on or repaired from above once the panels are down.	A supported one. Underlayment on a continuous deck is a drainage plane along its whole length, is walkable while the panels go on, and has something solid behind it when a lap fails or a screw backs out.
Condensation and the air barrier	The panel's cold underside is exposed to the space below and drips straight onto it, so control has to be chosen deliberately — a felt-backed anti-condensation panel, a draped and taped membrane over the purlins, or insulation in contact with the metal. There is no rigid plane to seal an air barrier against, so laps and penetrations have nothing solid behind them.	The underlayment and sheathing intercept the drip and buffer the moisture, and the deck gives an airtight plane you can actually tape. The trade is that metal is impermeable, so a wet deck can only dry downward — vented battens and a high-permeability underlayment exist for exactly that reason.
Bracing the roof plane	Purlins do not make a diaphragm. The roof plane needs its own bracing scheme, or a panel system with a tested shear value — which then constrains the fastener pattern and makes later panel removal a structural act.	Sheathing nailed to the framing is the diaphragm, and it comes free with the deck. On a building relying on the roof to transfer wind or seismic load to the walls, that is not a bonus, it is the requirement.
What can go on next	Only coverings that span or that bring their own battens — sheet metal, fibre cement, corrugated, or tile and slate where the purlin grid suits the batten spacing. Anything needing continuous support underneath is out: no shingles, no membrane, nothing fully adhered.	Anything, with no qualification. Shingles, membrane, tile on battens, different metal — the deck outlives the covering and is the reason a re-roof is a covering job rather than a rebuild.

Which one, and when

Choose purlins over rafters or trusses when…

The primary frame is already wide — portal frames, steel bents, barn trusses at spacings no sheathing panel can bridge. A deck there means adding purlins first anyway.
The building is unheated and unlined: a machine shed, a hay barn, an open carport, where the panel is honestly allowed to be the whole roof and drip is handled with a backed panel rather than an assembly.
The roof is large and the deck's premium is close to pure area — this is the one case where the substrate decision moves a serious share of the total.
It has to be weathertight fast, in an uncertain window. Purlins never need drying in, so nothing on the frame is spoiled by a wet fortnight.
Non-combustible construction, where adding a full-area timber layer changes what the building is rather than just what it cost.

Choose solid plywood / osb deck when…

The rafters are already at sheathing centres — a house re-roof, or a new stick or truss roof at ordinary spacing. The deck is the direct option and purlins would be a layer over a frame that already carries one.
Something underneath is heated, lined or lived in. That is when the airtight plane, the buffered condensing surface and the real underlayment stop being refinements.
You want a second water barrier you can rely on rather than a draped one, because the consequence of one panel-level failure is a ceiling, a floor or a store of something that must not get wet.
The roof plane has to brace the building, and a sheathed diaphragm is simpler to justify than a separate bracing scheme.
The panel you actually want is deck-required, or you expect this covering to be replaced within the building's life and want the next choice to be free.

Now run your own numbers

This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.

Purlin Row Spacing Calculator →Plywood and OSB Sheet Calculator (Subfloor, Wall and Roof) →

Frequently asked questions

On a re-roof, am I really choosing between these two?: Frequently not, and it is worth checking before treating it as a decision. If a sound deck is already up there, the honest question is whether to add battens or purlins ON TOP of it — creating a fastening plane and a vented cavity without a tear-off — rather than whether to have a deck at all. That is a both-and, not an either-or, and it is one of the more attractive things you can do to an old roof. The genuine either-or only appears when the deck is absent, or spongy enough that it is coming off regardless.
Which one is cheaper?: Structurally different answers, which is why this site refuses a figure. Purlins are members at intervals against a deck that is a full-area buy of sheathing, fasteners and underlayment — but the larger part of the purlin saving is usually upstream, in the primary frame it permits you to space wide, and that saving does not exist on a roof already framed at close centres. The other half of the ledger is the cost of changing your mind: adding battens over a deck later is trivial, adding a deck under existing panels is a strip-off. Run the row count and the sheet count against your own dimensions and your own quotes.
Is a metal roof loud on purlins?: Yes, and that is where the reputation comes from — a thin panel on open framing is a large, lightly damped membrane free to radiate on both faces, which is a drum. A deck fixes most of it by adding mass, by damping the panel against a solid surface, and by putting an underlayment between the two. What a deck does not fix is the other metal-roof noise: the tick and crack of thermal movement as panels grow and slip against their fixings. That belongs to panel length and the fastening system, not the substrate, and it is why clip systems and slip sheets exist.
Does a solid deck stop the roof sweating?: It moves the problem to a better place rather than solving it. Metal is impermeable and radiates heat to the night sky, so its underside routinely falls below the dew point of the air touching it — that happens over a deck too. What the deck changes is where the water goes: onto an underlayment that drains to the eave and into sheathing that can buffer it, instead of dripping onto whatever is below. The catch is that a metal roof lets a deck dry downward only, which is exactly why vented batten cavities and high-permeability underlayments are specified under metal. Over open purlins the same job is done by an anti-condensation backed panel, a taped membrane draped over the purlins, or insulation in direct contact with the metal — pick one deliberately, because doing nothing is also a decision.
Can standing seam go straight over purlins?: It depends entirely on the profile, and the span table settles it. Structural standing seam is designed to span open framing and is what steel buildings use. Architectural and low-rib profiles are non-structural by definition — they are deck-required, and fitting one over purlins is not a shortcut but a misuse that shows up as oil-canning, deflection and a warranty that no longer applies. If a particular panel is the reason you are doing this roof, read its span table before deciding the substrate, because the panel can settle this comparison on its own.
Can I add the deck later if I regret it?: Only by taking the roof off. Retro-decking means stripping the panels, adding a full-area material, dealing with purlins that now either come off or push the roof higher than the eave, verge and abutment details were built for, and re-fastening everything. The reverse direction costs almost nothing, which is the asymmetry to weigh at the outset: if there is a realistic chance the building will later be lined, heated, converted or re-covered with something that is not a spanning panel, the deck is cheapest bought once, at the beginning, when it is only material.