Construction glossary · Concrete and masonry
What is control joint in construction?
A control joint is a planned weakened line, sawcut or tooled into concrete or formed into masonry, that controls where shrinkage cracking occurs by forcing the crack to follow a straight, predetermined path. In slabs on grade it is typically cut to about one quarter of the slab depth and spaced at roughly 24 to 36 times the slab thickness in inches. Unlike an expansion joint, it does not fully separate the two sides; the concrete remains continuous below the cut.
Updated June 2026 · Reviewed by the Ruh construction team
How control joints are spaced
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Book a walkthroughA control joint (also called a contraction joint) is a deliberately weakened plane that tells shrinkage cracking where to go. In slabs on grade it is a sawcut made within hours of finishing, typically one quarter of the slab thickness deep. In CMU walls it is a continuous vertical joint built with sash block, a preformed gasket, and sealant on both faces. You will find control joints on foundation and slab plans as a joint layout grid, in the typical details (cut depth and timing), and in the specs under cast-in-place concrete in Division 03 and unit masonry in Division 04, with the joint sealant scope often parked in Division 07. The concrete and masonry subs perform the work, but the GC estimator has to confirm somebody carried it. New estimators trip on three things: pricing control joints as if they were expansion joints, missing the separate joint filler or sealant line item, and assuming the drawings give them a joint plan when the note says joint layout by contractor, which means the quantity has to be generated from the spacing rule.
Sawcut control joints are measured in linear feet (lf). If the drawings include a joint layout plan, take the lf directly off the grid. If the layout is left to the contractor, derive it from the spacing rule: count the interior joint lines in each direction and multiply by their lengths, or approximate with 2 times the slab area divided by the joint spacing. Price the cut per lf, then check Division 03 and Division 07 for joint filler or sealant, a separate per-lf item that often costs more than the sawcut itself. CMU control joints are counted each or in vertical lf, with sash units, gasket, and sealant on both faces carried in the unit price. Note the cut depth as well, since a deeper cut on a thicker slab changes the production rate.
Worked example
Take a 100 ft by 150 ft warehouse slab, 5 in thick, with joint layout by contractor. Apply the spacing rule of thumb of 24 to 36 times slab thickness in inches: 24 x 5 in = 120 in (10 ft) and 36 x 5 in = 180 in (15 ft), so panels should land between 10 ft and 15 ft. A 12.5 ft grid divides the slab evenly: 100 ft / 12.5 ft = 8 panels one way and 150 ft / 12.5 ft = 12 panels the other. That gives 7 interior joint lines at 150 ft (7 x 150 ft = 1,050 lf) plus 11 interior lines at 100 ft (11 x 100 ft = 1,100 lf), so 2,150 lf of sawcut, cut 5 in / 4 = 1.25 in deep. At an illustrative $1.50 per lf, the sawcutting line is 2,150 lf x $1.50 = $3,225, before any joint filler.
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How Ruh handles control joint
Ruh reads the slab plans, joint layout, and typical details, measures the sawcut control joints in lf (and counts CMU control joints), and prices the cut, filler, and sealant against the contractor's own price book instead of generic unit costs. The estimator receives a line-item draft with the joint quantities and the spacing assumption visible, then reviews and adjusts, which matters most when the layout is left to the contractor and panel spacing is a judgment call. Ruh drafts the takeoff; the estimator owns the final assumptions and the sign-off.
See concrete estimating softwareControl joint: frequently asked questions
What is the difference between a control joint and an expansion joint?+
An expansion joint (or isolation joint) is a full-depth separation with compressible filler that lets two elements move independently under thermal change or differential settlement, typically at building lines or where a slab meets walls, columns, or footings. A control joint only manages drying shrinkage; it is partial depth and gives the slab nothing but a straight place to crack. The cost gap is large: a sawcut is typically a per-lf item in the low single dollars, while a formed expansion joint with filler, sealant, and dowel load transfer runs several times that, so never let one stand in for the other in a takeoff.
How far apart should control joints be in a CMU wall?+
Masonry control joints are continuous vertical joints, and the common US rule of thumb is spacing at no more than 1.5 times the wall height and no more than 25 ft, whichever governs, plus joints at openings, intersections, and changes in wall height or thickness. They are typically built with sash block and a preformed rubber gasket, then sealed with backer rod and sealant on both faces. Estimators count them each or in vertical lf, and the sealant often lives in Division 07 rather than the masonry spec, so coordinate the two scopes.
Do control joints in concrete need to be filled or sealed?+
It depends on traffic and exposure. Industrial floors with hard-wheeled forklift traffic typically get a semi-rigid epoxy or polyurea filler installed the full depth of the cut to protect joint edges from spalling, ideally as late in the schedule as possible so more shrinkage has already happened. Exterior and wet-area joints usually get an elastomeric sealant over backer rod to keep water out, while interior slabs going under finishes are often left unfilled. Read the spec either way, because filler is a separate per-lf item that can cost more than the sawcut itself and is easy to miss in a bid.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.