Roofing
Roof crickets and tapered insulation: sloping a flat roof to drain
How crickets and tapered insulation move water off a low-slope roof, the slopes that actually work, and the layout that keeps water away from the laps.
Direct answer
A roof cricket, also called a saddle, is a raised sloped diverter that splits water around a curb, wall, or penetration so it drains instead of ponding. On a dead-flat structural deck, tapered insulation builds the slope. Both target the industry minimum of 1/4 in per ft, but the membrane manufacturer's warranty and the adopted code control the numbers.
Key takeaways
- The industry and code minimum slope for a low-slope roof to drain is 1/4 in per ft, about 2 percent, toward the drains.
- Roof crickets are sloped steeper than the field, commonly double, so the diagonal valleys stay above the drainage minimum.
- Ponding is standing water remaining more than 48 hours after rain; most membrane warranties require positive drainage and exclude ponding.
- Every drain on a tapered roof gets a sump, a recessed tapered area, so water reaches the drain bowl instead of ringing it.
- Recent IECC editions require above-deck tapered insulation at least 1 in thick at its lowest point, at the drain or scupper.
Crickets, tapered insulation, and the job they do
A roof cricket, also called a saddle, is a raised sloped diverter built behind or around an obstruction so water splits and runs past it instead of pooling against it. Tapered insulation is the wider tool. It is insulation board manufactured at an angle so that stacking and laying it out builds slope into a structural deck that was poured or framed dead flat.
The two solve the same problem at different scales. The deck has no slope, or not enough, and water has nowhere to go. Tapered insulation gives the whole field a fall toward the drains. The cricket handles the local trouble spots, the places where an obstruction sits in the path of that flow and would dam the water if you let it. A rooftop unit, a parapet corner, a wide wall, the flat dead zone between two drains. Each is a spot where water wants to sit, and each gets a cricket to push it where it needs to go.
Get this right at design and the roof drains itself for thirty years. Get it wrong and you own a ponding callback that no amount of patching fixes, because the water is doing exactly what the slope you built tells it to do. The fix for bad slope is more slope, and that means opening the roof back up. So this is a design-and-takeoff job first and an install job second.
Why ponding is the enemy
Ponding is standing water that does not drain, and it is the single failure that ends a low-slope roof early. The NRCA commonly defines ponding water as water that remains on the roof more than 48 hours after a rain during conditions that should let it dry. Confirm the exact wording in the current NRCA Roofing Manual, because it is the definition a warranty claim will be measured against.
Standing water attacks the roof four ways at once. It finds every lap and seam and works in where the membrane is weakest. It accelerates membrane aging, since constant wetting plus UV breaks down most single-ply and bituminous surfaces faster than the same roof kept dry. It carries dirt and debris that build a biological mat and hold even more water. And it adds dead load the deck may not have been designed to carry, because a few inches of water over a wide area is a lot of weight sitting in one low spot.
Here is the part that costs money. Most membrane manufacturers exclude ponding water from the warranty, or condition the warranty on positive drainage and no ponding past a stated window. Read the warranty language on the system you are installing. If it says positive drainage is required and the roof ponds, the manufacturer can walk away from the claim, and the leak becomes the contractor's problem. The 48-hour rule is not a guideline on those jobs. It is the line the warranty is written around.
What is the minimum slope for a low-slope roof to drain?
The industry and code minimum for a low-slope roof to drain is 1/4 in per ft, which works out to about 2 percent. That figure shows up in the International Building Code for roof drainage and in NRCA recommendations, and it is the number you design the field of the roof to hold toward the drains or scuppers. Confirm the requirement against the adopted code edition and the project specification, since both can be stricter.
A dead-flat structural deck does not give you that slope, and that is the whole reason tapered insulation exists. A deck poured level, or framed level, drains nowhere. Even a deck built with a little structural slope often loses it to deflection and tolerance, so a roof that looked like it sloped on the drawing ponds in the field. You add tapered insulation to get from whatever the deck gives you up to the 1/4 in per ft the membrane needs.
Watch the difference between design slope and built slope. A deck framed at 1/4 in per ft can sag at midspan under its own dead load and the roof load, and the low point that creates is exactly where water collects. The conservative move on a structural-slope roof that is close to the minimum is to add a tapered package anyway, or at least crickets at the dead spots, because the deck rarely drains as cleanly as the drawing claims.
What slope does a roof cricket need?
A cricket is sloped steeper than the roof field it sits in, and the common convention is double. A roof field at 1/4 in per ft gets crickets at 1/2 in per ft. A field at 1/2 in per ft gets crickets near 1 in per ft. The reason is geometry. A cricket has a diagonal valley running off each side of its ridge, and water on that diagonal travels at a flatter effective slope than the straight fall of the field, so you build the cricket steeper to keep the diagonal above the drainage minimum.
Run the cricket at the same slope as the field and the diagonal valleys end up below 1/4 in per ft, which means the cricket itself ponds along its own valleys. That is the quiet failure. The cricket looks like it is doing its job, the ridge sheds, and water sits in the two diagonal gutters it created. Doubling the slope keeps those valleys draining.
Treat double as the starting rule, not a law. The tapered manufacturer runs the actual layout and will set the cricket slope to keep every diagonal at or above the minimum for the cricket's size and the drain spacing. On a long cricket or a tight drain layout the slope may need to go higher than double. Confirm the cricket slope on the stamped tapered layout, not from habit.
Cricket geometry: the ridge, the valleys, and the two-way slope
A cricket is a little hip roof laid on the big flat one. It has a ridge line running away from the obstruction, and from that ridge the surface falls two ways, one slope to each side, so water splits at the ridge and runs off in opposite directions toward the drains. Those two falling planes meet the field of the roof along a pair of diagonal valleys, and those valleys are the actual drainage paths you have to keep above minimum.
Size the cricket to two things: the width of the obstruction it diverts around, and the distance from the obstruction to where the water is going. The cricket has to be at least as wide as the thing it protects, measured across the flow, so water cannot sneak around the back and pond there anyway. A common proportioning rule from tapered suppliers puts the width of a full diamond cricket somewhere between a third and a quarter of its length, which keeps the diagonal valleys at a workable slope. Confirm the proportion on the layout.
The ridge has to start high enough at the obstruction to give the cricket its full fall over its run. That starting height is where the cricket interacts with the rest of the tapered package. The cricket sits on top of the field taper, so its high point stacks on whatever the field thickness already is at that spot, and that combined height is what the structural and flashing details have to absorb. A cricket designed without checking the stack-up is the one that ends up burying a curb flange or running out of parapet flashing height.
Crickets behind wide curbs, walls, and penetrations
Any obstruction sitting in the flow of water needs a cricket on its uphill side, and the wider the obstruction, the more it matters. A rooftop unit, a large skylight curb, a wall, a row of equipment. Water hits the uphill face, has nowhere to go, and ponds against the base of the curb, which is exactly where the flashing is and exactly where the leak starts.
There is a code trigger worth carrying in your head. The International Residential Code requires a cricket or saddle on the upslope side of a chimney or penetration more than 30 in wide, measured perpendicular to the slope, and the masonry-chimney provisions carry a similar rule. That code lives in the steep-slope residential world, but the principle reads straight across to commercial low-slope. Once a curb gets wide, water cannot wrap around the ends fast enough, so it dams behind the face. Confirm the actual trigger against the adopted code, and treat anything wide and in the flow path as a cricket candidate regardless.
On a low-slope roof the practical rule beats the dimension. Any curb wide enough to create a dead zone of standing water behind it gets a cricket, and on the big rooftop units that is most of them. Run the cricket from the uphill face out to where its diagonals tie into the field slope that is already heading to a drain. The mistake is building the cricket too short, so it diverts water a few feet and then dumps it back into a flat spot that ponds anyway.
Tapered insulation layout
A tapered system is a base layer of flat insulation plus tapered boards stacked on top to build the fall. The flat base carries most of the R-value and gives the tapered boards something level to start from. The tapered boards, sloped at a set rate, build up from the low points toward the high points so the finished surface falls everywhere toward a drain.
Tapered boards come in standard slopes, commonly 1/8, 1/4, and 1/2 in per ft, and you mix slopes and add filler layers to build the design fall and the crickets. The layout is a plan, drawn and stamped by the manufacturer, that shows every board by slope and thickness, the high points, the valleys, the ridge lines of the crickets, and the sumps at the drains. It is not a field improvisation. The crew lays board to the plan the way a mason lays to a course chart.
The shape you are building is a set of high points draining down to low points at the drains. On a roof that drains to interior drains, the high points run along the perimeter and the field slopes inward, with crickets between drains to keep the long flat runs moving. On a roof that drains to the edge, the high point runs down the middle or the back wall and the field falls out to gutters or scuppers. Either way, the layout walks water from every square foot to a drain without crossing a flat spot.
The sump is the detail that gets skipped. A sump is a recessed tapered area right at the drain, usually a set of steeper boards in the last few feet, that drops the membrane below the surrounding field so water actually reaches the drain bowl instead of sitting in a ring around it. Without the sump the field slope dies a couple of feet short of the drain and you get a donut of standing water around the very thing meant to drain it. Every drain on a tapered roof gets a sump in the layout.
Drainage design: primary drains, sumps, and overflow
Drainage design is where the slope you built actually pays off, and it has two jobs: get the normal rain off through the primary drains, and handle the storm that exceeds them or the drain that clogs. Primary drains or scuppers sit at the low points the tapered layout creates, and the whole field is sloped to feed them. Put a drain where the layout makes a high point and it drains nothing.
Secondary, or overflow, drainage is a code requirement, not an option, on most roofs that can trap water. The International Building Code and the plumbing code require secondary overflow drains or scuppers wherever the roof construction could pond water if the primary drains back up, for instance any roof ringed by a parapet. The common detail is overflow drains the same size as the primary drains set with their inlet about 2 in above the low point, or overflow scuppers sized larger than the primary drains with a minimum opening height and the inlet set above the low point. Confirm the size, count, and inlet height against the adopted code and the structural design, because the overflow inlet height is what caps how deep water can get on the roof.
That overflow inlet height ties straight back to the structure. The roof is designed to carry water only up to a certain depth, and the overflow sets that depth. Set the overflow too high and you let water stack deeper than the deck was designed for. The two have to be coordinated, and on a tapered roof the sump depth, the primary drain, and the overflow inlet all stack at the same low point, so the detail there gets busy and deserves a careful section drawing.
Scuppers deserve their own note. A scupper is an opening through the parapet that lets water out the edge, used as a primary drain on some roofs and as the overflow on many. When scuppers are the overflow, the code commonly sizes them larger than the primary drains and sets a minimum opening dimension. The trap with scuppers is the sump in front of them: water has to be sloped to the scupper opening, and a scupper set at the parapet with a flat run in front of it ponds just like a drain with no sump.
The takeoff and the estimate
Tapered insulation is a design-and-takeoff exercise before it is a material order, and it is where roofing estimates go wrong. You cannot price a tapered roof off the square footage alone, because the system is a stack of boards of varying slope and thickness, and the count depends on the drain layout, the slope, and the crickets. The manufacturer or distributor runs the takeoff from the roof plan and the drain locations and produces a board-by-board layout with quantities.
Get the layout before you bid, not after you win. The cost of a tapered package runs well above flat board of the same average thickness, because the slope means more material toward the high points and more cutting and labor to lay it out. Bidding a tapered roof at flat-board prices is a classic way to bury margin, and the gap shows up the day the real layout comes back thicker than the guess.
The takeoff also surfaces the problems early. The layout will show where the high point lands against a door threshold, where a curb needs a cricket, where the package gets so thick at the high side that it buries a flashing or runs past the parapet. Catching that on paper is cheap. Catching it when the crew is laying board, with the wrong quantities on the truck, is the expensive version. Treat the tapered layout as a design deliverable you review, not a number you accept.
How is R-value figured on a tapered roof?
On a tapered roof the insulation thickness changes everywhere, so R-value is figured two ways: the average across the taper, and the minimum at the thinnest point. The energy code cares about both, and they answer different questions. The average sets whether the assembly meets the prescriptive R-value for the climate zone. The minimum makes sure the thin edge does not fall below a floor.
Recent energy-code editions handle the average explicitly. IECC 2021 lets you demonstrate compliance for tapered above-deck insulation by multiplying the material's R-value per inch by the average thickness, where the average thickness is the total insulation volume divided by the roof area. Confirm the method and the required R against the adopted code edition. ASHRAE 90.1 takes a stricter prescriptive line in places, so the controlling document matters.
The thinnest point is the catch. The taper runs down to its minimum at the drains, the sumps, and the gutter edges, and the code sets a floor there. Recent IECC editions call for the above-deck insulation to be no less than 1 in thick at its lowest point, at the drain or scupper. That floor plus the average requirement is what drives the base layer thickness, because you build the base flat layer thick enough that even the thinnest tapered spot, plus the base, clears both the minimum and the average. Designing only to the average and letting the drain edge go to nothing fails the code and creates a cold, condensation-prone thin spot right where water concentrates.
Crickets between drains on a wide roof
On a wide roof with drains spaced out across the field, the dead zone is the flat line halfway between two drains. The field slopes to each drain, but along the ridge between them the slope flattens out, and that ridge is where water sits. The fix is a cricket, or a continuous tapered ridge, built along that line so the high point sheds to both drains instead of holding water in the middle.
This is the cricket most people forget, because there is no curb or wall to make it obvious. It is pure geometry. Two drains, a flat span between, and a tapered ridge built to split the span and feed both. On a long roof you get a whole series of these ridges and valleys, a sawtooth of tapered crickets between every pair of drains, and the layout reads like a watershed map once it is drawn.
The same logic runs at the perimeter and the corners. A parapet corner with the nearest drain twenty feet away is a dead pocket unless a cricket pushes the water out of the corner toward the drain. Corners pond because water has two walls trapping it and the field slope alone rarely reaches all the way in. Any corner, any long span between drains, any spot the field slope cannot reach, gets a cricket. The drains set the low points and the crickets fill in the high points between them.
Membrane type and ponding sensitivity
Every low-slope membrane wants drainage, but they do not all tolerate standing water the same way, and the warranty language tracks the difference. The slope you design and the membrane you specify are one decision, not two.
EPDM and TPO single-ply hold up to incidental wetting reasonably well, but the seams are the weak point under standing water. A TPO heat-weld or an EPDM seam tape sitting underwater for days is being tested in a way the field of the sheet is not, and manufacturers commonly condition the warranty on positive drainage for exactly that reason. PVC is generally the most water-tolerant of the common single-plies, which is why it shows up on roofs that genuinely cannot avoid some standing water, but tolerant is not the same as warranted, so read the language.
Built-up roofing and modified bitumen are asphalt-based, and standing water is hard on asphalt over time. It softens the bitumen, works into any surfacing defect, and accelerates the aging the UV already started. Ponding on a BUR or mod-bit roof shows up as blistering, surface erosion, and eventually seam and lap failure. Whatever the membrane, the slope is what protects it, and the manufacturer who sells the warranty is the one who decides whether your drainage satisfies the condition. The membrane choice can buy a little tolerance. It does not buy you out of designing the slope.
The structural check: weight, deck, and rain load
Tapered insulation adds weight, and the weight is not even. The package is thin at the drains and thick at the high points, so the dead load climbs toward the high side and the perimeter, and on a deep tapered package over a big roof that added dead load is worth a structural look, especially on a re-roof adding taper over an existing system. Confirm the deck and structure can carry the added dead load with the structural engineer, not by eye.
The bigger structural story is rain load and ponding instability. ASCE 7 covers rain loads and the ponding instability check, where water collects, deflects the deck, which lets more water collect, which deflects it further. A roof that is too flexible and too flat can run away like that. ASCE 7 treats a roof sloped at least 1/4 in per ft toward free drainage as not subject to the susceptible-bay ponding check, which is one more reason the 1/4 in per ft minimum matters beyond just keeping the membrane dry. Confirm the rain-load and ponding analysis with the structural engineer and the adopted code.
Drain location is a structural decision too, not just a plumbing one. Drains belong at the low points the structure deflects toward, so the deck helps the water reach them instead of fighting it. A drain placed at a structural high point, or far from where the deck sags, makes the tapered layout work harder and the ponding risk worse. FM Global data sheets carry their own drainage and rain-load guidance for insured roofs, so on an FM job confirm the requirement against the applicable FM data sheet, since it can be stricter than the base code.
Field execution: cutting, fastening, and the markout
The install lives or dies on following the layout, and the first field task is the markout. Before any board goes down, the crew chalks the high points, the ridge lines, the valleys, and the drain sumps from the tapered plan onto the deck, so everyone is laying to the same map. Skip the markout and the crew builds slope by feel, and feel ponds.
Tapered boards lay from the low point up, in the sequence the plan gives, with each slope and filler layer stacked in order. Boards get cut to fit at the ridges, the valleys, the crickets, and around penetrations, and the cuts at the cricket diagonals are where sloppy work shows, because a gap or a high cut at a valley dams the very flow the cricket is built to move. The fasteners or adhesive follow the membrane manufacturer's and the FM or code attachment requirement for the roof, the same as flat board, with the pattern and density set by the wind uplift design. Confirm the attachment against the project's uplift requirement.
QA against the plan is the step that separates a roof that drains from one that ponds. Walk it before the membrane goes on. Check that the high points are where the plan put them, that the crickets are full height at the obstruction and run all the way out to the field slope, that the sumps drop in at every drain, and that no flat dead spots got built by a missed board or a wrong slope. Then flood-test or watch it in the first real rain. A board laid one slope wrong in the field is invisible until the water shows you, so find it before the membrane covers it.
How do you fix a ponding roof?
You fix a ponding roof by adding slope, and there is no patch that substitutes for it. The water is doing what the existing slope tells it to do, so the fix is to change the slope, which means one of three things: add a cricket to divert the water, add tapered insulation to re-slope the area, or add a drain at the low point where the water already collects. Which one depends on why it ponds.
Diagnose before you cut. Mark the pond after a rain, while the water is still there, and find the low point and the path water should be taking. A pond against a curb wants a cricket behind that curb. A pond in the dead zone between two drains wants a tapered ridge between them. A pond in a corner wants a cricket pushing water out of the corner. A broad pond across a flat field wants a tapered overlay over the whole area, which is the big-ticket fix.
Adding a drain is the move when the low point is real and permanent and there is structure and plumbing to reach it. Drop a new drain into the existing low spot, build a sump to it, and let the water that already wants to be there leave through it. It is often cheaper than re-sloping a whole field, but it only works if the low point is where you can run a drain line and the structure allows it.
The honest part of the conversation is that some ponding gets accepted, and that is a decision, not an accident. An owner may choose to live with a shallow pond that dries within the window rather than pay to re-slope, and a water-tolerant membrane like PVC supports that call. But accepting ponding voids the kind of warranty that requires positive drainage, so it is a decision made with the warranty language in hand, not a shrug. Put the trade-off in writing so the owner owns the choice.
What to document
The record on a tapered roof is what defends the drainage when a pond shows up and the question becomes who pays. A warranty claim turns on whether the roof was built to drain, and the tapered layout, the slopes, and the drain detail are the evidence. The crew that documents is the crew that keeps the warranty intact.
Capture the roof area and the deck slope you started from, the tapered package and slopes used, the cricket locations and their slopes, the drains and overflows with their inlet heights, the membrane and its warranty drainage condition, and the post-installation ponding check. Note any spot where ponding was accepted and the owner sign-off that goes with it. Keep the stamped tapered layout with the closeout, because it is the design of record for the slope, and the next person to touch the roof needs it.
| Field to record | Why it matters |
|---|---|
| Roof area and existing deck slope | Sets how much taper had to be added |
| Tapered package and board slopes | The design of record for the field slope |
| Cricket locations and slopes | Proves the dead spots were diverted |
| Drains and overflows, inlet heights | Acceptance, and the structural water-depth limit |
| Membrane and warranty drainage clause | What the slope had to satisfy |
| Post-install ponding / flood-test result | Evidence the roof drains within the window |
| Any accepted ponding and owner sign-off | Ties an exception to a decision, not a defect |
Common mistakes
- Building no cricket behind a wide curb or wall, so water dams against the flashing and starts the leak there.
- Running the cricket at the same slope as the field, so its diagonal valleys fall below the drainage minimum and pond.
- Stopping the tapered slope a few feet short of the drain with no sump, leaving a ring of standing water around the drain.
- Building a cricket too short, diverting water a few feet and dumping it back into another flat spot.
- Accepting ponding on a roof whose membrane warranty requires positive drainage, then losing the claim.
- Omitting the code-required secondary or overflow drainage, so a clogged primary drain has nowhere to go.
- Pricing a tapered roof at flat-board cost without the manufacturer's takeoff, then eating the difference.
- Letting the thinnest taper point at the drain fall below the energy-code minimum thickness.
- Placing a drain at a structural high point instead of where the deck deflects, so the layout fights the deck.
- Laying board by feel without chalking the high points, ridges, valleys, and sumps from the plan first.
Field checklist
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Standards and references
The NRCA Roofing Manual is the practical reference for slope, drainage, ponding, and crickets on low-slope roofs, and it is where the 1/4 in per ft recommendation and the 48-hour ponding definition are laid out for the trade. Treat its figures as the commonly cited industry practice and confirm them against the current edition.
The code framework lives in the International Building Code and the plumbing code. The IBC addresses roof drainage and the 1/4 in per ft minimum slope in its roof-assembly and drainage provisions, and the secondary or overflow drainage requirement appears in the IBC and the International Plumbing Code, with the residential equivalent in the IRC roof-assembly chapter. The IRC also carries the cricket-behind-a-wide-penetration rule for chimneys and penetrations more than 30 in wide. Section numbers shift between code cycles, so confirm the article and the requirement against the edition the jurisdiction has actually adopted and any local amendments before citing them.
Structural and energy standards round it out. ASCE 7 governs rain loads and the ponding-instability check, and it treats a roof sloped at least 1/4 in per ft toward free drainage as outside the susceptible-bay ponding analysis. The energy code, IECC and ASHRAE 90.1, sets the R-value, with recent IECC editions specifying the average-thickness method for tapered insulation and a minimum thickness at the drain edge. On an insured roof, FM Global data sheets add their own drainage and uplift requirements. Above all of it, the membrane manufacturer's warranty governs ponding, since it is the document that decides whether your drainage satisfies the condition that keeps the warranty in force.
Units, terms, and conversions
Roof slope and drainage get described a few ways across a drawing set, a tapered layout, and a spec, so the same fall reads differently depending on where you look.
Slope is given in inches of rise per foot of run on most US roofing work, so 1/4 in per ft is about 2 percent and 1/2 in per ft is about 4 percent. The same slope shows up as a ratio on some structural drawings. Insulation is rated by R-value, the thermal resistance, and tapered boards by their slope in in per ft and their thickness in inches. Ponding is standing water that does not drain within the defined window. A sump is the recessed tapered area at a drain, and a cricket or saddle is the raised diverter that splits water around an obstruction.
- Cricket / saddle
- A raised, sloped diverter built behind or around an obstruction to split water past it instead of ponding
- Tapered insulation
- Insulation board manufactured at an angle to build drainage slope into a flat structural deck
- Slope (in per ft)
- Roof fall in inches of rise per foot of run; 1/4 in per ft is about 2 percent, the common drainage minimum
- Ponding
- Standing water that remains on the roof past the defined window, commonly cited as 48 hours after rain
- Sump
- A recessed tapered area at a drain or scupper that drops the membrane so water reaches the bowl
- Overflow / secondary drain
- Code-required backup drainage set above the low point to cap water depth if the primary drains clog
- R-value
- Thermal resistance of the insulation; figured on a taper as both an average and a thinnest-point minimum
FAQ
What is a roof cricket?
A roof cricket, also called a saddle, is a raised sloped diverter built behind or around an obstruction such as a curb, wall, or large penetration. It splits water so it runs past the obstruction to a drain instead of ponding against the uphill face, which is where flashing leaks usually start.
What slope does a roof cricket need?
A cricket is sloped steeper than the roof field, commonly double. A field at 1/4 in per ft gets crickets near 1/2 in per ft. The extra slope keeps the cricket's diagonal valleys above the drainage minimum, since water on the diagonal travels at a flatter effective slope than the straight fall of the field.
What is the minimum slope for a low-slope roof to drain?
The common code and industry minimum is 1/4 in per ft, about 2 percent, toward the drains or scuppers. It appears in the IBC roof-drainage provisions and NRCA recommendations. A dead-flat deck needs tapered insulation to reach it, and the adopted code edition and project specification can require more, so confirm both.
How do you fix a ponding roof?
You add slope, since no patch substitutes for drainage. Add a cricket to divert water around an obstruction, add tapered insulation to re-slope a flat area, or add a drain at the low point where water collects. Diagnose by marking the pond after a rain, then match the fix to why that spot holds water.
What is tapered insulation and why is it used?
Tapered insulation is insulation board manufactured at an angle so laying it out builds slope into a structural deck that was poured or framed flat. It is used because a flat deck drains nowhere, so the taper gives the field a fall toward the drains while also carrying R-value, getting the roof from no slope to the drainage minimum.
Does ponding water void a roof warranty?
It often does. Most membrane manufacturers exclude ponding water or condition the warranty on positive drainage and no ponding past a stated window, commonly tied to the 48-hour rule. Read the specific warranty language, because if it requires positive drainage and the roof ponds, the manufacturer can deny the claim and the leak becomes the contractor's problem.
Do I need overflow drains on a low-slope roof?
Usually yes. The IBC and plumbing code require secondary or overflow drains or scuppers wherever the roof could trap water if the primary drains clog, such as any roof ringed by a parapet. The overflow inlet sits above the low point to cap water depth at what the structure was designed to carry.
When does a curb or penetration need a cricket?
Any obstruction wide enough to dam water in its flow path needs a cricket on the uphill side. The IRC requires one behind a chimney or penetration more than 30 in wide, measured perpendicular to the slope. On a low-slope roof, treat any wide curb that creates a dead zone behind it as a cricket candidate.
How is R-value calculated on a tapered roof?
Two ways. The average R-value, the material R per inch times the average thickness, shows whether the assembly meets the prescriptive code value. The minimum at the thinnest point, at the drains and edges, must clear a code floor, with recent IECC editions requiring at least 1 in of above-deck insulation at the lowest point.
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Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.