Solid Deck vs Open Purlins for Metal Roofing
Panels can go on a continuous deck or straight onto strapping. The choice changes the panel span, the condensation risk, the sound, and how the roof handles wind.
Metal panels can be installed two fundamentally different ways: over a continuous solid deck, or spanning between purlins with nothing but air behind them. Both are legitimate assemblies. They are not interchangeable, they do not share the same failure modes, and choosing the wrong one for the building is one of the more expensive mistakes available in this trade.
What Each Assembly Actually Is
A solid deck assembly is continuous sheathing — plywood or OSB — over the rafters or trusses, covered with underlayment, with the panels fastened through to the deck. This is the standard residential assembly and what almost every architectural standing seam system is designed for. An open purlin assembly runs horizontal strapping across the rafters at a calculated spacing, with the panels screwed directly to the strapping and open air between them. It is the standard for agricultural buildings, shops, pole barns and pavilions.
The Panel Has to Carry the Span
This is the first and most concrete difference. Over a solid deck, the deck carries the snow load and foot traffic and the panel just sits on it. Over open purlins, the panel itself has to span between supports and carry that load. Its ability to do so is a function of panel gauge, rib depth and rib spacing — which is exactly what a manufacturer's span table tells you, cross-referenced to a design load.
Southern Ontario snow loads are not trivial, and purlin spacing has to be derived from a span table for the actual load, not copied from the last building someone put up. Stretching the spacing to save a few pieces of strapping is how you end up with panels that deflect between purlins, oil can badly, and eventually deform permanently under a heavy winter.
Condensation Is the Real Open-Purlin Problem
Every experienced installer in this climate has been called to the same job: a heated shop or a barn with a metal roof on open purlins that rains on the inside. Nothing is leaking. Warm, moist interior air rises, contacts the underside of a panel that is sitting at outdoor temperature, and condenses. It collects, runs down the underside of the rib, and drips onto whatever is below. In winter it can freeze on the panel underside and then release all at once during a thaw, which people invariably report as a roof leak.
- ✓A solid deck with continuous underlayment gives the moisture a much less favourable surface to condense on and a path to dry, which is why the problem is far less common on housed roofs.
- ✓On open purlins, condensation has to be designed for: a factory-applied anti-condensation backing on the panel, a purpose-made condensation-control blanket, insulation with a properly placed vapour retarder, or a genuinely ventilated cavity.
- ✓The one thing that never works is doing nothing and hoping the building is dry enough. Any heated, humid or animal-occupied space will produce moisture.
- ✓Retrofitting condensation control to an installed open-purlin roof usually means working from below with spray foam or a lined ceiling — considerably more awkward than getting it right the first time.
Wind Uplift
A solid deck spreads uplift load across a continuous surface and gives every fastener a consistent substrate. On purlins, uplift concentrates at the fastener lines, and the load path runs panel to screw to purlin to the fastening of the purlin to the rafter. That last connection — strapping to rafter — is frequently the weak point, and it is invisible once the roof is on. Perimeter and corner zones see the highest uplift on any roof, and on a purlin assembly those zones need tighter fastener spacing and better attention to how the strapping itself is secured.
Sound, Walkability and Feel
A metal roof over a solid deck with underlayment is quiet in the rain — the mass and the damping of the deck absorb the impact, and the common belief that metal roofs are loud comes almost entirely from experience with open-purlin barn roofs, where the panel is a drum head with nothing behind it. Walkability is the same story: on a deck you can walk the roof normally; on purlins you have to know where the strapping is, because stepping between purlins dents a panel and the dent is permanent.
Architectural standing seam and structural standing seam are not the same product. Architectural panels are designed to be supported continuously and are not rated to span open framing. Structural panels are engineered to span between supports. Putting an architectural panel on purlins because it looked similar in the brochure is a straightforward way to end up with a deflecting, oil-canned roof.
Strapping Over an Existing Roof
There is a third arrangement worth naming: strapping laid over an existing shingle roof, with panels fastened to the strapping and screwed through into the rafters. It creates a ventilated cavity behind the panels, which helps with heat, and it avoids a tear-off. The trade-offs are real. The deck is never inspected. Moisture can be trapped against the old shingles. Every eave, rake, penetration and wall detail now has to accommodate the extra thickness, which is where these jobs usually look wrong. And the fasteners have to reliably reach structure through the strapping, the shingles and the sheathing.
Choosing Between Them
For a house — anything heated, occupied, and expected to look sharp from the street — a solid deck is the assembly. It gives you a continuous secondary water barrier, quiet performance, a flat surface so the panels look right, and the fastener holding a clip system needs. For an unheated equipment shed, a pole barn or a pavilion, open purlins are efficient and appropriate, provided the span is derived from a real table and the condensation question is answered honestly. For a heated shop, treat it as a house.
Frequently Asked Questions
Why is my shop roof dripping when nothing is leaking?
Almost certainly condensation on the underside of the panels. Warm moist air inside meets cold steel and gives up its moisture. The fixes are all about controlling the moisture and the temperature of that surface — insulation with the vapour control on the warm side, a condensation-control layer, or ventilation — not about sealing the roof.
What purlin spacing should I use?
Whatever the panel manufacturer's span table gives for your panel profile, gauge and design snow load. Spacing is a structural answer, not a habit, and it changes with the panel.
Can standing seam go over open purlins?
Structural standing seam profiles are designed to, within their published span limits. Architectural standing seam is not — it requires continuous support. Check which one you have before the framing goes up.
Is a solid deck worth adding to a barn re-roof?
It depends on what the building is used for. For unheated storage, usually not. For anything heated, occupied by livestock, or used as a workshop, a deck or an equivalent condensation strategy is what keeps the building dry, and it is far easier to do during a re-roof than afterward.
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