How the two differ in kind
A flat roof is not flat: it has to shed water to its outlets, because standing water is what shortens a roof covering's life, loads the structure, and finds every defect. There are two ways to create that fall and they operate at different scales.
TAPERED INSULATION builds the primary fall out of the insulation layer itself. Factory-cut boards of varying thickness are laid to a layout drawing so that the finished surface slopes toward the drains while the deck beneath stays level. It works on any roof shape, it can be designed around any drain arrangement, and it delivers the thermal layer and the fall in one operation.
A CRICKET is a local diverter: a small framed or built-up wedge that splits flow around an obstruction and stops water ponding behind it. The classic locations are upslope of a chimney, behind a rooftop plant curb, and in the valley between two drains where the primary fall leaves a flat line. Crickets are not an alternative to a primary fall; they are what a primary fall needs at the places it cannot solve on its own, which is why most roofs use both.
The constraint that catches people out on tapered systems is THICKNESS. The depth grows across the run, so a large roof can finish very much thicker at its high point than at its drain — and that dimension has to be accommodated everywhere it matters: at parapet heights, at upstands and door thresholds, at rooflight kerbs, at the transition to an adjoining roof, and in the additional dead load on the structure. On a big roof this is a design constraint set early rather than a detail resolved late, and it is frequently what pushes the design toward more drains rather than longer runs.
One more thing decides whether either works: DEFLECTION. A structural deck sags under load toward mid-span, and a drawn fall of the bare minimum gradient can be cancelled by that sag exactly where it matters — leaving a pond in the middle of a roof that slopes correctly on paper.
The factors that actually differ
Show
| Tapered insulation | Framed crickets | |
|---|---|---|
| Scale it works at | The whole roof — the primary fall to the drains. | Local — diverting water around a specific obstruction or along a specific line. |
| What it changes | The insulation layer. The deck stays level. | The structure or a built-up wedge above it, at one place. |
| Thickness consequence | Substantial and cumulative. The high point can be far thicker than the low point, which has to be allowed for everywhere. | Local and modest. |
| Structural load | The added insulation mass across the roof, plus the load path of the extra depth. | Small, though a framed cricket is structure and is designed as such. |
| Design effort | A layout drawing showing every board type and position — produced by the supplier and followed exactly on site. | A framing detail, sized by the area it drains and the fall it has to achieve. |
| Where it is indispensable | Any low-slope roof needing a designed fall to outlets on a level deck. | Behind any obstruction more than a modest width — a chimney, a curb, a rooflight — and in valleys between drains. |
| Effect of deflection | Serious. A minimal gradient can be cancelled by deck sag at mid-span, producing a pond on a roof that falls correctly on paper. | Less exposed, since it is local and usually near a support or an upstand. |
| Getting it wrong | Boards laid out of sequence, which produces a flat spot or a reverse fall that nobody can see once the covering is on. | Omitted entirely behind an obstruction — the commonest cause of a persistent pond and the failure directly upslope of it. |
| Cost | Per square metre, and it rises with the average thickness — so a long run to one drain costs more than a short run to several. | Per cricket, modest against the roof. |
| Typically used | As the primary system. | Alongside it, at the places the primary fall leaves flat. |
Which one, and when
Choose tapered insulation when…
- The deck is level and the roof needs a designed fall to its outlets.
- The insulation is being installed or replaced anyway, so the fall comes with it.
- The structure cannot be re-framed to create a fall, which on a refurbishment it usually cannot.
- There are several drains and the layout can be designed to keep runs short and thicknesses modest.
Choose framed crickets when…
- There is an obstruction the water has to get around — a chimney, a plant curb, a rooflight, a parapet return.
- A valley between two drains would otherwise be a flat line where water sits.
- The roof already has an adequate primary fall and the problem is local.
- The area is small enough that a framed wedge is simpler than re-laying tapered boards.
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 does the roof still pond when it was designed with a fall?
- Usually because of deflection, and occasionally because the fall was laid wrong. A structural deck sags under its own weight and under imposed load, with the sag greatest at mid-span — and a minimal design gradient can be entirely consumed by that sag, leaving water standing in the middle of a roof that slopes correctly on the drawing. That is why guidance generally recommends designing to a gradient meaningfully above the theoretical minimum, so there is fall left after deflection. The compounding risk is ponding itself: standing water is load, load causes more deflection, and more deflection collects more water — which is why persistent ponding is treated as a structural matter and not only a durability one. The other cause is simpler: tapered boards laid out of sequence, producing a flat area or a reverse fall invisible once the covering is on.
- How thick does tapered insulation get?
- As thick as the run length times the gradient, plus the minimum thickness at the low point — and on a large roof that is a substantial dimension. The consequence is not the cost of the insulation but everything the thickness has to fit under: parapet height, so the roof does not finish above the coping; upstand heights at every abutment, which have a minimum above the finished surface; door and threshold levels onto terraces; rooflight and plant curb heights; and the transition where this roof meets another. It also adds dead load. Because all of those are set early, the tapered design is not something to leave until the roofing package — and the usual remedy where the thickness becomes unworkable is more outlets, which shortens every run and reduces the maximum depth.
- Where are crickets actually needed?
- Upslope of anything that blocks the flow, and in valleys the primary fall leaves flat. The classic case is a chimney or a plant curb sitting across the fall: water running down the roof arrives at the upslope face, has nowhere to go, and ponds against the flashing — which is the detail least able to tolerate standing water. A cricket splits the flow and takes it around both sides. Most codes and guidance require one behind any obstruction beyond a modest width. The other case is the line between two drains: a roof falling to two outlets has a ridge between them on one axis and frequently a flat valley on the other, and the valley needs its own fall to the outlets. That is designed into the tapered layout rather than added afterwards, but it is the same problem.
- What gradient should a flat roof have?
- More than the bare minimum, because the minimum is a finished-surface requirement rather than a design target, and the difference between them is what survives deflection and construction tolerance. Guidance and codes state a minimum fall for the finished roof; designing exactly to it leaves nothing for the deck's sag, for tolerance in the deck's own level, or for compression of the insulation under traffic. Standard practice is therefore to design at roughly double the minimum, which costs a little more insulation and removes most of the ponding risk. The other half of the answer is drain layout: gradient times run length is thickness, so a roof designed with more outlets and shorter runs achieves a steeper gradient at a lower maximum thickness than one draining a long distance to a single point.
- Can a fall be added to an existing flat roof?
- Yes, and tapered insulation is the usual way — it is one of the main reasons the product exists. A refurbishment where the deck is level and cannot be re-framed can be given a designed fall by laying a tapered scheme over it as part of a new warm roof build-up, which solves the ponding and upgrades the thermal performance at the same time. The constraints are the ones described above, and they bite harder on an existing building because the parapet height, the upstands and the thresholds are already fixed. That frequently means raising upstands and kerbs as part of the work, or accepting more outlets to keep the build-up shallow. The alternative of re-framing to create a structural fall is possible and is a much larger intervention.
- Does the roof covering change the answer?
- It changes how much the ponding costs rather than whether to create a fall. Some coverings tolerate occasional standing water better than others, and manufacturers state their position on it — which matters because a warranty can be void where water stands beyond a stated period after rain. But none of them are improved by it: standing water increases the exposure of seams and details, encourages growth, carries dirt that abrades, adds load, and in freezing climates works at every joint. Guidance is consistent in treating ponding as a defect to design out rather than a condition to specify around. So the covering influences the tolerance and the warranty conversation; the fall is designed for the roof rather than for the membrane.
- How is a tapered layout actually installed?
- To a drawing, board by board, and that is the part that goes wrong on site. A tapered scheme is not a bundle of wedges to be laid by eye: the supplier produces a layout showing each board type and its position, with the taper direction and the build-up at every point, and swapping two board types or rotating a board produces a flat spot or a reverse fall that is invisible once the covering is on and expensive once it is discovered. The practical protections are checking the delivered board types against the drawing before laying, setting out and marking the roof, and — most usefully — flooding or inspecting the surface for standing water before the covering goes down, which is the last moment the layout can be corrected.
- What about drains and overflows?
- The fall exists to get water to them, so their number, position and capacity are part of the same design rather than a separate one. Position drives the tapered scheme: outlets near the middle of a roof give shorter runs and shallower build-ups than outlets at a parapet edge, and more outlets do the same. Capacity has to suit the roof area and the design rainfall intensity, including allowance for a blocked outlet, which is why overflows are required — a scupper or an overflow outlet set above the normal water line so that a blocked drain discharges visibly rather than loading the roof. A roof that ponds because an outlet is blocked is a maintenance problem; a roof with no overflow when that happens is a structural one.