R-value is resistance to conductive heat flow; air leakage is bulk air through gaps, carrying both heat and moistureIn: Building insulation and air sealing
The aluminum box or backing behind channel letters that holds the wiring and supplies and consolidates penetrationsIn: Electric sign installation
Rack and pinion
The toothed rack on the mast and the powered pinion gear that meshes with it to drive the platform up and downIn: Mast-climbing work platform safety
Rack BBU
Battery backup unit: a lithium battery in the rack or a sidecar shelf that carries the load through a blip and bridges to the generatorIn: Rack BBU and ride-through
Rack cooling index (RCI)
How well rack intake temperatures stay inside the ASHRAE recommended range, at the hot and cold endsIn: Delta-T and return temperature
Rack unit, 1.75 in of vertical mounting space, the unit assets and rack space are measured inIn: DCIM and asset management
Rack-and-stack
The physical install of servers, storage, and network gear into racks after the facility is ready, plus the migrations that load workloads inIn: Rack-and-stack deployment field guide
Random traffic moves any direction and uses FF/FL; defined traffic follows a fixed wheel path and uses F-min or wavinessIn: Floor flatness and levelness
Running Average Power Limit, the CPU power-cap feature, and the baseboard management controller that enforces caps out of bandIn: Power capping and oversubscription
Refrigerant concentration limit per ASHRAE 15, the most refrigerant allowed in the smallest occupied room if it leaksIn: VRF commissioning field guide
RCM
Reliability-centered maintenance; sets each asset's strategy by criticality, function, and failure modesIn: Data center PM program
RCR
Room cavity ratio, a shape factor for the room that selects the CU from the tableIn: Commercial lighting design
RCSC
The Research Council on Structural Connections specification for high-strength bolted joints, including the snug-tight and pretensioned methodsIn: Data center structural QA
RDHx
Rear-door heat exchanger, a water coil replacing a rack's rear door that captures the server exhaust heat at the rackIn: Rear-door heat exchanger commissioning
RDMA / NVMe-oF
Remote direct memory access and NVMe over Fabrics, the network protocols that let remote flash feed GPUs at near-local speedIn: AI storage tier architecture
A firestop pathway sleeve or box with a self-sealing intumescent insert that holds the rating as cable is added or removedIn: Cabling pathways and firestop
Reinforcing bar 1/2 in across, the minimum rebar size for a concrete-encased electrodeIn: Ufer ground field guide
Rebar cage
The cylinder of longitudinal bars and spiral reinforcement that gives the shaft tension, shear, and bending capacityIn: Drilled piers and caissons
Rebar cage / bond beam
The cage is the tied grid of reinforcing bar that gives the shell its tensile strength; the bond beam is the thickened, extra-reinforced top perimeter that carries the tile, coping, and deck edge and ties the walls together.In: Commercial pool construction
Rebound
Aggregate-rich material that bounces off and must not be reincorporated; rises overhead and on dry-mixIn: Shotcrete and gunite field guide
REC / PPA
Renewable energy certificate (an attribute) and power purchase agreement (a contract for specific output)In: Data center sustainability reporting
Recap
The summary that rolls labor, material, DJE, and quotes into job cost, then adds overhead and profitIn: Electrical estimating and bidding
Receiver
The air storage tank that buffers demand and smooths pressure; an ASME pressure vesselIn: Compressed air piping design
The designer's official set, produced from the as-builts by screening and incorporating the changes; carries the design professional's reviewIn: As-built record drawings
Recordable
A work-related injury or illness meeting the 300 log criteria, such as treatment beyond first aid or days awayIn: Incident investigation
Recordable vs reportable
Recordable goes on the 300 log; reportable means the call to OSHA for a fatality or severe injuryIn: Construction safety program
A dense board over the insulation or old roof that gives the membrane a hard baseIn: Reroof: recover vs tear-off
Recover vs restoration
A recover lays a new membrane over the old as a roof layer; a restoration coats the existing roof and usually does not count as a new layerIn: Roof coating restoration system
Recover vs tear-off
A new membrane over the old one (recover) versus stripping to the deck (tear-off); moisture and code layers decideIn: Roof maintenance program
Recoverable depreciation
Depreciation held back on the first check and released after the work is completed and documentedIn: Storm and hail damage restoration
Recovered binder
The aged asphalt binder still coating the RAP aggregate, recovered or accounted for in the mix designIn: Asphalt recycling and FDR
Light reflected by one event, and total reflected by the link; more negative is betterIn: Fiber OTDR certification
Reflected apparent temperature
The temperature of the surroundings that reflects off the target and into the camera, which corrupts the reading on low-emissivity metalIn: Infrared thermography field guide
The old crack reappearing through the overlay, and the stress-relief layer (SAMI, fabric, geogrid) that slows itIn: Asphalt mill and overlay
Refrigerant stacking
Refrigerant pooling in the condenser at high load and very low condenser-water temperature, costing capacityIn: Chiller plant sequencing optimization
Refrigeration rack
Parallel compressors on shared piping feeding a store, split into medium-temp and low-temp suction groups, with staged capacity and redundancyIn: Commercial refrigeration field guide
Refusal
When a pier stops advancing before design depth against rock, cobbles, or fill; resolved by relocating, advancing past, or load testing, not forcing.In: Helical piers and screw piles
Refusal and set
The point where the pile stops advancing under the hammer, and the penetration per blowIn: Driven pile foundations
Contaminated material handled under medical-waste rules: red-bagged or in sharps containers, manifested, and sent to licensed disposalIn: Biohazard and trauma cleanup guide
Regulated waste
Asbestos debris that must be wetted, double-bagged, labeled, manifested, and disposed of at a permitted facilityIn: Asbestos abatement procedures
Rehabilitation / reconstruction
Restoring structure and ride to failed-down pavement, versus rebuilding failed pavement from the subgrade up; neither is preservationIn: Pavement preservation treatments
Reheat
Adding heat to dehumidified air to raise its supply temperature, ideally from recovered heat rather than new energyIn: DOAS field guide
Restricted-entry interval, the label time after application before re-entry without the required PPEIn: Landscape IPM field guide
Reimbursable
A job cost billed back to the customer, usually with a contract markup, as opposed to overhead the company absorbsIn: Job expense and receipt tracking
Reinforced zone
The compacted, grid-bound block of soil behind the face that acts with the wall as a mechanically stabilized earth massIn: Segmental retaining wall build
RH is how full the air is as a percent; dew point is the temperature at which water condenses, and it sets which surfaces sweatIn: Natatorium HVAC field guide
Release agent
A bond breaker that keeps the stamp from sticking; pigmented powder versions also leave a secondary accent colorIn: Stamped concrete install guide
Release agent (form oil)
The film applied to the form face so it strips clean without bonding to or staining the concreteIn: Formwork types and systems
Release detection / ATG
Required monitoring that finds a UST leak early; an automatic tank gauge is one in-tank method, interstitial monitoring is the method for new secondarily contained systemsIn: Fuel storage tank field guide
The share of expiring agreements that renew, and its mirror, the share that leave; two views of whether the base holdsIn: Service agreements and recurring revenue
Renovate Right
EPA's lead-hazard pamphlet that the firm must deliver to owner and occupants before a covered renovationIn: Lead-safe renovation (RRP)
Rent commencement
The lease date the tenant starts paying rent, which the fit-out schedule is built to meetIn: Tenant improvement fit-out
Repair behind the bar
Removing the chloride-contaminated concrete behind a corroded bar, about 3/4 in clearance, so the full circumference is cleanedIn: Parking structure restoration
Repair, restore, replace
The three actions: fix a local defect, coat or recover a worn but sound roof, or tear off and rebuildIn: Roof asset management
Repeat rate
The share of work that comes from customers you have served before; a direct measure of whether follow-up is workingIn: Customer communication and reviews
Repellent
A scent or taste product that makes a plant unpleasant, reducing browse but not eliminating it, and subject to wash-off and habituationIn: Deer and wildlife protection
Replace vs restore
The adjuster's item-by-item call: restore when it costs less than replacement, replace when it does notIn: Contents pack-out and inventory
A titration that measures the buffer left in the fluid; falling against baseline means the inhibitor is depletingIn: Hydronic water and glycol field guide
Reserve area
Suitable soil set aside and protected for the future replacement field, required by many codesIn: Septic system field guide
Reservoir void ratio
The share of the open-graded stone layer that is open space, commonly near 40 percent, where the water storesIn: Permeable pavement install
Re-grinding and re-polishing an existing terrazzo floor back to a new finish, the reason terrazzo lasts for generationsIn: Terrazzo flooring install and polish
Restoration coating
A fluid-applied membrane coated over an existing sound roof to renew it and extend its life without a tear-offIn: Roof coating restoration system
The length of pipe each side of a fitting that must be restrained so soil friction holds the thrustIn: Thrust restraint field guide
Restraint
Anything that stops concrete moving freely as it shrinks or contracts, turning movement into tensionIn: Concrete crack types and causes
Retail vs wholesale colo
Retail sells by the cabinet or cage in shared halls; wholesale sells suites or data halls, usually 100 kW and up, with more tenant controlIn: Colocation fit-out field guide
Retainage held from you is an asset (receivable); retainage you hold from subs is a liability (payable); ask a CPA about classification and timing.In: Retainage and retention field guide
Retempering
A one-time, recorded jobsite water addition allowed under ASTM C94 within the design w/c limitIn: Concrete slump test
Keeping trained staff versus the rate at which they leave; churn costs the person, the knowledge, and the training investmentIn: Data center staffing and workforce
Retention / hold time
The minimum time the room must hold the agent at the protected height, commonly at least 10 minutes under NFPA 2001In: Clean agent room integrity
Retention bond
A surety bond posted in lieu of cash retainage so the held money is freed while the owner keeps equivalent security; availability varies by contract and state.In: Retainage and retention field guide
Tuning existing equipment and controls back to proper operation; an audit finds the waste, retro-commissioning corrects the operational part.In: Energy audit and management field guide
How steadily and how recently new reviews arrive; a steady weekly flow ranks better than a large pile of old reviewsIn: Customer communication and reviews
Revise and resubmit
A review stamp returning a submittal for correction, which restarts the review clockIn: RFI and submittal process
REX (request-to-exit)
A button or sensor that tells the controller a legitimate exit is happening so it does not alarm; it is not the means of egressIn: Access control system field guide
RF exposure zone
Controlled (occupational) and uncontrolled (general-public) areas with different FCC maximum permissible exposure limitsIn: Cell tower and antenna install
Location technologies ordered by precision and cost: RFID is cheap read-or-no-read, BLE is zone level, UWB is sub-meter, GPS is outdoorIn: Connected jobsite and RTLS
RGA / RMA
Return goods or materials authorization; the number that allows the defective part to be returnedIn: Warranty claim processing guide
Spreading stems, below ground (rhizome) or above (stolon), that let a grass fill in and self-repairIn: Turfgrass selection
RHO
Thermal resistivity of the soil or backfill in degrees C-cm per watt; lower RHO carries heat away and raises ampacityIn: Concrete-encased duct bank field guide
How the operating point slides along the pump curve when a valve throttles or the system resistance changesIn: Hydronic pump install field guide
Right plant, right place
Selecting a plant whose sun, soil, water, and mature-size needs match the conditions of its spot so it thrives with little inputIn: Landscape design and plant selection
The built-up tapered columns and rafters that form the main moment-resisting structure and carry the load to the anchor boltsIn: Metal building erection
Rigid pavement
Concrete pavement, a stiff slab that bridges and distributes load by beam actionIn: Asphalt vs concrete pavement
The matrix output, low, moderate, high, or extreme, that drives the monitor, mitigate, or remove decisionIn: Tree risk assessment
Risk register
The list of identified risks behind the contingency, each scored by likelihood and impact, used to size and then draw down the contingency.In: Contingency and risk pricing
Reverse osmosis; the purified water through the membrane is permeate, the concentrated leftover is reject or concentrateIn: Pure water RO/DI field guide
Robotic total station
A one-person instrument that measures angle and distance to a tracked prism, accurate to a few millimeters, for building layoutIn: Construction layout
RoCE
RDMA over Converged Ethernet, running RDMA on Ethernet made lossless with PFC and ECN, versus InfiniBand's credit-based lossless fabricIn: Network observability for the AI fabric
The fixed pass count, overlap, speed, and gap each roller runs, set on a test strip and held to keep density uniformIn: Asphalt compaction and rolling
Rolling sphere
A design sphere, commonly 150 ft radius, rolled over the structure to find exposed points needing terminalsIn: Lightning protection (NFPA 780)
Roof asset management
Treating each roof as a tracked capital asset: inventory, condition rating, remaining-life estimate, and a multi-year capital planIn: Roof asset management
Roof condition index
A score, commonly 0 to 100, that puts every roof on one consistent scale for comparison and rankingIn: Roof asset management
Roof condition rating
A consistent score of a roof's overall condition used to estimate remaining life and plan capitalIn: Roof maintenance program
The watertight roof detail over a structural expansion joint, built on two raised curbs with a flexible cover spanning the gap to take the building's movementIn: Roof expansion joint installation
The sudden release of a whole snowpack off a slick or steep roof, the hazard retention is built to preventIn: Snow guards and retention
Room integrity / door fan test
A pressurization test that measures enclosure leakage to confirm a clean agent room holds concentration for the required timeIn: Data center fire and life safety
Room pressure monitor
A permanently installed instrument that continuously shows the pressure direction at the door and alarms when it is lostIn: Healthcare HVAC ventilation field guide
Root ball
The mass of soil and roots dug with the tree, sized by trunk caliper per ANSI Z60.1In: Tree transplanting
Root barrier
A layer, or a certified root-resistant membrane, that keeps roots from penetrating the waterproofingIn: Green roof installation
The actual underlying reason a callback happened, usually a crew, a detail, or a product, as opposed to the symptom you fixed on the visit. Fixing it removes future callbacks instead of just covering them.In: Warranty reserve and cost management guideIncident investigation
Root cause analysis (RCA)
The structured search after restore for the real cause and contributing factors, not the first symptom, often using the 5-whysIn: Incident management and outage response
A continuous linear element that senses fluid anywhere along its length and reports the location, for distributed coverageIn: Liquid-cooling leak detection
Rotation / changeout
A scheduled replacement of the color display, pulling the spent planting and installing the next season's flowersIn: Seasonal color and annual rotation
The manufacturer's dimensioned drawing for a fixture, giving drain, supply, and clearance dimensions from the finished surfacesIn: Fixture rough-in and setting guide
Rough-in test
The test of the bare piping before fixtures and before close-in, proving the joints holdIn: Plumbing pressure test guide
Route and seal
Widening a non-structural crack into a groove and filling it with a flexible sealantIn: Concrete crack injection
EPA's Renovation, Repair, and Painting Rule (40 CFR 745, Subpart E) for work that disturbs lead paint in pre-1978 homes and child-occupied facilitiesIn: Lead-safe renovation (RRP)
RRP vs abatement
RRP is renovation that controls lead dust along the way; abatement is work whose purpose is to permanently remove lead, under separate stricter rulesIn: Lead-safe renovation (RRP)
RRU / antenna
Remote radio unit, the transceiver mounted at the antenna, and the antenna panel that radiates each sectorIn: Cell tower and antenna install
Acclimatization ramp: about 20 percent of normal workload day one, adding roughly 20 percent each dayIn: Heat illness prevention
Rule-based vs data-driven
Rules encode physics or sequence logic and are transparent; data-driven methods learn normal patterns and flag anomaliesIn: FDD and building analytics field guide