How the two differ in kind
Wind does not press evenly on a roof. Air flowing over a building accelerates as it passes the edge and separates at a corner, forming vortices that produce SUCTION far greater than anything acting in the middle of the roof. The pattern is consistent enough to be codified: the corners see the highest uplift, the perimeter strip next, and the field the lowest — often by a factor of several between corner and field.
That is why fastening schedules are ZONED. The design establishes the uplift pressure in each zone from the building's height, its exposure, the local wind speed and the roof's geometry, then specifies a fastening pattern for each: closer spacing and often a different fastener at the corners, intermediate at the perimeter, and the base pattern in the field.
The alternative — one uniform pattern across the whole roof — is always wrong somewhere, and the two ways it is wrong are not symmetrical. Set the pattern for the FIELD, which is the natural instinct because the field is most of the roof, and the corners are under-fastened at the place where the load is highest. Set it for the CORNERS and every square metre of the field carries fasteners it does not need, which on a large commercial roof is a substantial and pointless cost.
The asymmetry matters because roof failures in wind are PROGRESSIVE. A roof does not lose a corner and stop: once the covering lifts at a corner, wind gets underneath it, and the pressure that was acting on the top surface now acts on both — so the covering peels back along the roof, taking the better-fastened field with it. That is why the corner detail governs the survival of the whole roof, and why a deficiency confined to a small area is not a small problem.
The zone WIDTHS are geometry rather than convention: they derive from the building's plan dimensions and its height, so a tall narrow building has different zones from a low wide one and a small roof can be almost entirely corner and perimeter.
The factors that actually differ
Show
| Zoned to the uplift pressure | One uniform pattern | |
|---|---|---|
| Uplift distribution | Recognised as non-uniform — highest at corners, then perimeter, lowest in the field. | Treated as uniform, which it is not. |
| What goes wrong | Nothing, if the zones are correctly derived and the patterns are actually installed where they belong. | Under-fastened corners if set for the field; a large waste if set for the corners. |
| Failure mode | Not applicable. | Progressive peeling from a corner, which takes the whole roof rather than the corner. |
| Cost on a large roof | Lower overall, because the field gets the pattern it needs and no more. | Either unsafe or expensive, with no middle setting that is right everywhere. |
| Zone widths | Derived from the building's plan dimensions and height — calculated, not assumed. | Not applicable. |
| Small buildings | Handles them correctly — on a small roof the zones can cover almost the whole area. | Particularly poor here, since 'the field' may barely exist. |
| Installation risk | Real: the zones have to be marked out and the right pattern used in each, which needs supervision. | Simple to install consistently, which is its only genuine advantage. |
| Applies to | Sheathing fastening, covering attachment, insulation fastening, edge metal and every layer in the assembly. | The same layers, wrongly. |
| The edge detail | Sized for its zone, and edge metal is frequently the first thing to fail in a wind event. | Often the same as the field, which is where the sequence starts. |
| Verification | Inspection against the zone drawing during installation, since the pattern is invisible afterwards. | Nothing to verify beyond the count. |
Which one, and when
Choose zoned to the uplift pressure when…
- Any roof being designed or re-covered — this is the standard approach and codes expect it.
- A large roof, where getting the field pattern right is a real saving.
- A small or tall building, where the corner and perimeter zones cover most of the roof.
- A high-wind location, where the corner pressures are highest and the consequences greatest.
Choose one uniform pattern when…
- Only where the uniform pattern adopted is the CORNER pattern, applied everywhere — safe, and wasteful.
- A very small roof where the zones cover essentially all of it anyway, so one pattern is the zoned answer.
- As a fallback where no design information exists — in which case the conservative pattern is the one to use.
- Never as a field pattern applied to the corners, which is the combination that fails.
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.
Frequently asked questions
- Why is uplift so much higher at corners?
- Because of how air behaves when it meets an edge. Wind flowing toward a building accelerates over the roof edge and separates from the surface, and at a corner the flow separates in two directions at once, forming conical vortices that trail back across the roof. Those vortices have very low pressure at their cores, and low pressure above a roof surface with normal pressure beneath it is suction — pulling the covering upward. The effect is strongest at the corners, substantial along the perimeter, and much weaker in the field where the flow has reattached. The ratio between corner and field pressure is large enough that a fastening pattern adequate in one is plainly inadequate in the other, which is the entire basis for zoning. It is also why parapets change the picture, since they alter where the flow separates.
- How are the zone widths determined?
- By geometry, from the building's plan dimensions and its height, rather than by a fixed distance. The codes define the edge zone width as a function of the building's least horizontal dimension and its height, subject to minimums, which means the zones scale with the building rather than being a standard border. Two consequences follow. A small building can be almost entirely edge and corner zone, with very little true field — so treating it as a field with small borders is substantially wrong. And a tall building has wider zones than a low one of the same plan, because height enters the derivation. This is why the widths are calculated from the specific building rather than taken from a previous project, and why the zone layout is drawn on the roof plan rather than described in words.
- Why is a corner failure not a corner-sized problem?
- Because it is the start of a progressive failure rather than the end of a local one. While a roof covering is intact and fastened, wind acts on its upper surface and the fastenings resist. Once the covering lifts at a corner, air gets underneath it — and now the same wind is pressurising the underside while still sucking on the top, which roughly doubles the force trying to remove it and applies that force at a peeling edge rather than across a fastened field. The covering then unzips progressively along the roof, tearing out fastenings that would have been perfectly adequate had the sequence never begun. That is why the corner detail governs the survival of the whole roof, and why post-storm investigations so consistently trace a lost roof back to an edge or corner initiation.
- Does this apply to more than the sheathing?
- To every layer in the assembly, and the weakest one governs. Sheathing fastening is the layer people think of first, and the same zoning applies to the attachment of the roof covering itself, to the mechanical fastening of insulation boards on a commercial roof, to the seam spacing or clip spacing on a metal roof, and — critically — to the EDGE METAL. Perimeter edge metal, fascia and coping are frequently the first components to fail in a wind event, because they are directly in the highest-pressure zone and are often specified with less attention than the covering they terminate. Standards for edge metal securement exist precisely because of that record. A roof with a correctly zoned covering and an under-specified edge detail fails at the edge and then peels.
- How is the right pattern actually achieved on site?
- By marking the zones out and supervising, because the pattern is invisible once the next layer is down. The zone layout goes on the roof plan with the fastening schedule for each, the zones are marked on the deck or the substrate before work starts, and the installation is inspected while it is in progress rather than afterwards. The failure modes are mundane: a crew running one pattern across the whole roof out of habit, the zone boundaries not marked so the transition drifts, and fasteners driven at the right spacing but to the wrong depth — over-driven fasteners that crush the substrate or under-driven ones that stand proud both reduce the holding capacity the schedule assumed. Pull-out testing on a sample is used on significant projects for exactly that reason.
- What does the deck material change?
- The holding capacity of each fastener, which changes how many are needed for the same uplift. The schedule is a product of the load in the zone and the capacity of one fastener in that substrate — so a fastener in structural concrete, in steel deck, in plywood, in OSB or in a cementitious wood-fibre deck all behave differently, and the same nominal spacing does not deliver the same resistance. Deck thickness and condition matter too, which is why re-roofing an existing building includes verifying what the deck actually is and what condition it is in, and frequently includes pull-out testing on site. An old deck that has been re-roofed several times may have lost capacity through previous fastener holes, which is a common finding and a reason schedules on a re-cover are derived rather than copied.
- Do parapets change the zones?
- They can, and generally favourably, which is one of the less obvious arguments for them. A parapet of sufficient height moves the point at which the airflow separates away from the roof surface, which reduces the intensity of the corner vortices acting on the covering below — so codes recognise parapet height in the uplift calculation, with taller parapets giving greater reductions. A low parapet can be less helpful and in some geometries can make matters worse locally. The parapet itself is then in the highest-pressure zone and has to be designed for it, including its coping, which is a frequent failure component. As with the rest of this subject, the effect is captured in the code's method rather than something to estimate, and it is one more reason the zones are calculated for the specific building.
- Is it worth zoning a small domestic roof?
- The zoning concept applies, and on a small building it mostly resolves into a simpler instruction: fasten the edges and corners more, because there is very little true field. Since zone widths scale with the building's dimensions, a modest house roof can be largely edge and corner, so the difference between a zoned schedule and simply applying the corner pattern throughout is small — and applying the tighter pattern everywhere is both safe and easy to supervise. Where it matters most on domestic work is at the eaves, verges and ridge, and in the edge details: the fixing of the first courses, the verge and barge details, and the attachment of any edge trim. In high-wind regions, domestic roof fixing specifications reflect exactly this and should be followed rather than replaced with habit.