CHB Wall Rebar Spacing: What NSCP Actually Requires

The vertical bars go up first and the blocks come down around them, one course at a time. Illustrative photo.

Short answer: NSCP 2015 doesn't have one CHB rebar spacing. It has different rules for different walls. A non-structural partition or infill panel inside a reinforced concrete frame needs, at minimum, one 12 mm bar every 1.2 m in one direction, or joint reinforcement (Section 709.4.1.3). That lighter rule is written for panels detailed to be isolated from the frame (709.4.1.2); infill built tight to the columns is the frame designer's call. A wall that carries load or resists earthquake needs steel both ways, bars no more than 600 mm apart unless the wall is solid grouted, and total steel of at least 0.2% of the wall's cross-section (Sections 706.1.12.3 and 709.5.2). The note you see on many house plans, 10 mm at 600 mm vertical and a horizontal bar every 2nd or 3rd course, sits between those two: more than a partition needs, and sometimes less than a structural 6-inch wall needs. For a 10 m long, 3.0 m high 4-inch infill wall at 10 mm @ 600 mm and every 2nd course, that's 18 vertical bars, 7 horizontal runs, 25 lengths of 6 m bar (about 93 kg) and roughly ₱4,375 of steel at our 2026 rebar prices. At 3.0 m that wall is also too tall for a 4-inch block to be treated as a standard panel: its height-to-thickness ratio is 33, over the 30 at which the code wants it designed from a structural analysis.

The calculator below does that takeoff for your own wall and flags when a wall has stopped being something you copy off a standard note and needs an engineer's design. After it: why the numbers online disagree, what Chapter 7 actually says, how the three kinds of wall differ, the details that matter more than spacing (dowels, grout, lintels), cracks, and whether a hollow block house can go up more than one floor.

1. Why Sources Say 400, 600 and 800 mm

Search this and you'll find every combination: vertical bars at 400, 600 or 800 mm, horizontal bars every 2nd course, every 3rd course, or "every 3 layers." Some sites say 10 mm, some 12 mm, a few say no bars at all for interior walls. They aren't all wrong. Most of them are describing a different wall and not saying so.

Start with the geometry. A Philippine CHB is 400 × 200 mm on its face (390 × 190 mm actual plus a 10 mm joint, per our CHB sizes guide). In running bond the hollow cells line up vertically every 200 mm along the wall. So vertical bars can only sit at multiples of 200 mm: 400 mm is one bar per block, 600 mm is every block and a half, 800 mm is every second block. Horizontal bars sit in bed joints, and a course is 200 mm high, so every 2nd course is 400 mm and every 3rd course is 600 mm.

Then map each number to where it comes from:

• 1.2 m with 12 mm bars is the NSCP minimum for masonry that isn't part of the structure: partitions, screen walls (Section 709.4.1.3). It's also the ceiling for any wall in Zone 4 where the wall is solid grouted with qualifying units (709.5.2).
• 600 mm is the maximum bar spacing NSCP allows in a structural masonry wall that isn't solid grouted (709.5.2). It's also what most house plans carry, as a practice default.
• 800 mm is a spacing some builders use to save steel. In a hollow, partly grouted structural wall it's over the 600 mm limit. In a partition it's inside the 1.2 m limit.
• 400 mm is the conservative end: one vertical bar in every block, or horizontal bars every 2nd course. Our own CHB quantity estimator defaults to 10 mm at 600 mm vertical and 400 mm horizontal, and that's the default here too.

So the honest answer to "what's the spacing?" is another question: what is the wall doing? A bedroom partition, a road-side fence and a wall holding up a second floor are three different problems, and one note for all three is how walls end up over-built or unsafe.

2. Is Rebar Really Necessary in Hollow Block Walls?

This comes up because in some countries plain block walls without bars are normal. In the Philippines they aren't an option under the code, for a simple reason: earthquakes.

• NSCP Section 208.4.4.1 puts Palawan (except Busuanga), Sulu and Tawi-Tawi in Seismic Zone 2 and the rest of the country in Zone 4. There is no Zone 1 or 3.
• Section 706.1.12 requires masonry structures (walls that are part of the structure) in those zones to follow the special provisions for each zone. For Zone 2 that already means vertical steel at wall corners, beside openings, at wall ends and at no more than 1.20 m, and horizontal steel at openings, floors, roof and wall top and bottom (706.1.12.2 items 2 and 3). Zone 4 adds more on top (706.1.12.3).
• Section 709.2.1 keeps the empirical method of Section 710, the one that allows unreinforced walls sized by height-to-thickness rules, for masonry in areas lower than Zone 2. The Philippines has no such area.
• Even a wall that carries only its own weight is covered: 709.4.1.3 says partition and screen walls "shall be reinforced in either the horizontal or vertical direction."

And the cement? Two parts to that. Mortar in every bed and head joint is plain workmanship: for hollow units NSCP 704.3.4 wants the joints filled solid for a distance in from the face at least equal to the face shell. The cells that hold bars are a different matter. They're meant to be filled with grout, a flowable mix the code specifies at not less than 15 MPa (Section 703.4.1; a footnote to Table 703-2 in the same section gives 13.8 MPa at 28 days, so the two figures don't quite agree). Grout is what makes the bar and the block act together, and it's what keeps the bar from rusting in an open cell. A bar standing loose in an empty cell, or in a cell stuffed with mortar droppings, does much less than the drawing assumes.

3. What NSCP 2015 Chapter 7 Actually Says

These are the clauses that decide CHB reinforcement, read off the NSCP 2015 scan. The clause numbers are the code's own.

Two things stand out. First, the partition rule is the one most houses actually fall under, and it's lighter than the plan note most people copy: one 12 mm bar every 1.2 m in one direction. Second, a single 10 mm bar (78.5 mm²) or even a single 12 mm bar (113 mm²) doesn't reach the 130 mm² that 706.1.12.2 wants at the corners, openings and ends of a structural wall. Those spots need a 16 mm bar or two smaller ones. That's a detail a standard note won't tell you and a designer will.

On 709.4.1.2: read strictly, it wants non-structural masonry separated from the frame so the frame's sway doesn't load it. Many Philippine house plans do the opposite and build infill tight to the columns and beams with dowels. That's the usual local detail, but it's the reason infill cracks at column lines, and it's one reason we don't count the infill as part of the structure when we design the frame. Whichever way your plans go, it should be the frame designer's decision, not the mason's.

4. Does 10 mm @ 600 mm Pass? The Steel Ratio Check

For a structural wall the test isn't spacing alone, it's steel area as a fraction of the wall's cross-section. A 10 mm bar is 78.5 mm²; at 600 mm that's 131 mm² per metre of wall. On a 6-inch wall (150 mm nominal, so 150,000 mm² per metre) that's a ratio of 0.00087. Here are the common schedules worked out. The table counts the running bars only; the extra bars at corners and openings (706.1.12.2 items 2–3) may also count toward the 0.002 total (706.1.12.3 item 2.3).

Ratios are on nominal thickness, which is conservative; the code's calculations use the actual (specified) dimensions, 140 mm for a 6-inch block (Sections 706.1.1 and 706.2.3.1: a hollow single-wythe wall's effective thickness is its specified thickness). On 140 mm the ratios come out a little higher (10 mm @ 600 vertical is 0.00094, every 2nd course 0.00140, total 0.00234; with every 3rd course the total is 0.00187) and none of the verdicts change. The 4-inch column is there for partitions and infill; a 4-inch wall can't be a bearing wall in Zone 4 anyway (707.1.3.1).

What this means in plain terms:

• As a partition or infill panel, every schedule in the table gives more steel per metre than the 709.4.1.3 minimum (one 12 mm bar per 1.2 m is 94 mm² per metre). The catch is diameter: the clause names 12 mm bars, and most plans use 10 mm. On the numbers the 10 mm schedules carry more steel, but the plan should say so, and it's your designer's call, not ours from a blog.
• As a structural 6-inch wall, the popular "10 mm @ 600, every 3rd course" doesn't make the 0.2% total. Moving the horizontal bars to every 2nd course passes 709.5.2, but not the tighter 706.1.12.3 that applies when the wall is designed under Section 706. 12 mm at 600 mm, or 10 mm at 400 mm, clears both.
• Section 709.5.2 (0.0007 each way, 0.002 total) applies to every structural masonry wall in Zone 4. If the engineer designs the wall under Section 706 rather than 709.2.1, 706.1.12.3 also applies and raises the per-direction minimum to 0.001. Which one governs depends on how the wall is designed. Our perimeter wall assessment guide screens against the 709.5.2 figures.

5. Partition, Fence, Bearing Wall: Three Different Rule Sets

Partitions and infill in an RC frame

This is most of the CHB in a Philippine house. The frame (columns, beams, slab) carries the building. The CHB fills the gaps and carries its own weight down to the slab or beam under it. The steel keeps the panel in one piece when the building sways and stops it falling out as debris. Exterior infill takes wind and earthquake out of plane, so it should be dowelled into the columns and the beam above as the plans show, and it's worth using 6-inch on long or tall exterior panels. Watch the height: Section 707.2.14.2 requires walls with a height-to-thickness ratio over 30 to be designed from a structural analysis that accounts for axial load and deflection. The code measures thickness on the actual block (90 mm for a 4-inch CHB; under Sections 706.1.1 and 706.2.3.1 a hollow single-wythe wall's effective thickness is its specified thickness), so on a 4-inch wall that's anything over about 2.7 m between supports, which is exactly where a 3 m storey, a tall ceiling or an open stairwell wall ends up. The weight matters too: about 1.5–1.8 kPa for 4-inch CHB with the bar cells grouted at 600 mm, plus 0.24 kPa for each plastered face (NSCP Table 204-2; ungrouted 4-inch is 1.05–1.39 kPa depending on block density), and that load has to land on a beam designed for it.

Perimeter fences

A fence has no beam on top and no slab to lean on. Wind and ground shaking try to rotate it about its footing, so it's a cantilever, and it fails at the base first. It's also slender sooner than it looks: for a wall not supported at the top, NSCP 706.2.4 takes the effective height as twice the actual height, so a 1.8 m fence has a height-to-thickness ratio of about 25.7 in 6-inch CHB (140 mm actual) but 40 in 4-inch (90 mm), well past the 30 in 707.2.14.2. That's why the footing, the dowels out of it and the stiffener columns matter more than the spacing of the bars in between. Under the 2004 Revised IRR of PD 1096, Section 301.3 exempts "garden masonry walls other than party walls not exceeding 1.20 meters in height" from a building permit; fences up to 1.80 m are covered by an accessory (fencing) permit, and fences over 1.80 m are listed as a Group J Division 2 accessory structure. Individual Building Officials apply this differently, so ask yours. The calculator flags any fence over 1.2 m for an engineer's design, because that's where the permit exemption ends and where a 4-inch hollow wall stops being defensible. The costs are in our CHB fence cost guide.

Load-bearing CHB

A wall that carries a floor or roof is structure. In Zone 4 it has to be at least 150 mm nominal (707.1.3.1), built of load-bearing units (706.1.12.2 item 5 keeps non-load-bearing units out of the load-resisting system; under PNS ASTM C90:2019 a load-bearing unit has to reach 13.8 MPa on net area, as an average of three), laid in Type M or S mortar (709.5.4 and 706.1.12.2 item 5 rule out Type N, Type O and masonry cement), with steel both ways at the ratios in Section 4. None of that is in a standard note. If your wall is load-bearing, the calculator will always say "needs an engineer's design," and it means it.

6. Dowels, Grout, Lintels and Stiffener Columns

Arguing 600 against 800 mm matters less than whether the bars connect to anything. These are the details that decide how a CHB wall behaves in an earthquake.

• Dowels into the footing or tie beam. Every vertical bar needs something to lap with at the bottom: a dowel cast into the footing, grade beam or slab, sticking up at least 40 bar diameters (400 mm for 10 mm, 480 mm for 12 mm; 707.2.2.6). 706.1.12.2 item 3 also calls for horizontal steel at the top of foundations where it's dowelled into walls. Bars that just sit on the footing aren't anchored. Dowels have to be set before the pour, at the right spacing, so the cells land on them; drilling and epoxy-anchoring later is possible but is its own design question.
• Dowels into the columns and beam. For infill, horizontal bars hook into the RC columns (bars left projecting when the column was poured, or drilled in), and the top of the panel is tied to the beam or packed tight under it as the plans show. 709.4.2.1 sets a 1.2 m maximum between connectors for masonry shear walls, and 709.4.1.2 wants non-structural panels detailed for storey drift; your frame designer decides which applies.
• Grout the cells with bars. Fine grout per NSCP Table 703-2 is one part cement to 2¼–3 parts sand by volume (coarse grout adds 1–2 parts gravel), mixed fluid enough to flow to the bottom of the cell. Section 704.5 has the rules for pouring it, including lift heights and cleanouts for tall pours. Rodding it as you go matters more than the recipe.
• Lintels and bars around openings. Door and window openings concentrate stress at their corners, which is where the diagonal cracks start. For structural masonry, 706.1.12.2 wants horizontal steel at the top and bottom of openings extending at least 600 mm or 40 bar diameters past them, and vertical bars on each side. On most houses the head of the opening is an RC lintel beam that bears on the masonry at each side; the bearing length and bars are on the plans.
• Stiffener columns. Long walls and fences get small RC columns at intervals. For fences our cost guide assumes columns about every 3.0 m, which is common practice. For masonry shear walls in Zone 4 the code sets vertical steel at corners, near openings and ends, at no more than 3.0 m (709.4.2.3, which Zone 4 carries over), and bar spacing within the wall of no more than a third of its length or height, or 1.2 m (709.5.2.1). A stiffener column every 3 m doesn't replace the steel in the panels between.
• Bars at the wall top. 709.4.1.3.1 wants horizontal reinforcement within 400 mm of the top and bottom of a partition. On a fence the coping beam does that job.

7. How Do I Prevent Cracks in Concrete Block Walls?

Some hairline cracking in plaster over CHB is normal. The cracks worth worrying about have causes you can do something about while the wall is going up:

• Soaking the blocks. Many masons wet CHB before laying so the mortar doesn't dry too fast. NSCP 704.2 item 4 says concrete masonry units "shall not be wetted unless otherwise approved." The reason given in masonry practice is that a soaked block shrinks as it dries inside the wall, and that shrinkage has to go somewhere. Dampen the joint surfaces if the weather is hot, but don't soak the blocks.
• Old mortar. 704.2 item 7: mortar can't be used more than two and a half hours, or grout one and a half hours, after water is added. Re-tempering is allowed within that window, but reviving yesterday's leftover mortar is a common shortcut and a weak joint.
• The wall-to-frame line. A vertical crack right where CHB meets a column is the frame and the wall moving differently. Dowels and horizontal bars hooked into the column reduce it; a proper joint (per 709.4.1.2) controls it. Plaster across the joint will crack either way unless there's mesh or a groove there.
• Openings. Diagonal cracks from window corners mean the corner steel or lintel bearing is short. Bars past the opening, per the previous section.
• Settlement. Stair-step diagonal cracks that follow the mortar joints and widen toward one end usually mean the footing is moving. That's a foundation question, not a plaster one.
• Chasing. Cutting long horizontal grooves for conduits through the face shells weakens the wall along exactly the line it bends about. Run conduit vertically inside the cells as the wall goes up.
• Rust. Bars in open or poorly grouted cells rust, swell and crack the face. Grout the cells that hold bars and keep the cover NSCP asks for (707.2.2.2: 20 mm minimum, 40 mm exposed to weather, 50 mm against soil).
• Long walls without joints. Long CHB walls and fences crack at intervals if there's nowhere for shrinkage and temperature movement to go. Control joints are layout practice; we haven't found an NSCP clause that sets their spacing, so ask your designer to show them.

If a wall has already cracked, the question is whether it's still moving. Our wall crack repair guide covers how to tell, and what each kind of repair costs. For plastering over new CHB, see the CHB plastering cost guide.

8. Can Concrete Block Houses Be Built Multi-Storey?

The code says yes. NSCP Table 208-11C lists masonry systems for earthquake resistance, including a bearing wall system of masonry shear walls (R = 4.5) and a building frame system with masonry shear walls (R = 5.5), each with a 50 m height limit in Zone 4. Reinforced masonry buildings of several storeys are built that way in other countries.

But the masonry Table 208-11C means is engineered reinforced masonry: at least 150 mm walls (707.1.3.1), load-bearing units, Type M or S mortar, grouted cells, steel both ways at the Section 4 ratios, and, under the allowable stress method, shear walls checked for shear at 1.5 times the seismic force (707.1.7). Without the special inspection Section 701 describes, 707.1.2 cuts the allowable masonry stresses in half. That's a designed, inspected structure. It isn't the partly grouted 4-inch CHB with 10 mm bars that most Filipino houses are built with.

That's why most two- and three-storey houses in the Philippines are a reinforced concrete frame with CHB infill. The frame carries the floors and resists the earthquake; the CHB just fills the walls. When we design these frames we include the CHB's weight (it's real load) but don't count on it for strength, and the infill details are chosen to keep it from creating short, stiff column segments. Adding a second floor over load-bearing CHB walls that were never designed for it is risky: the wall steel is usually unknown and 4-inch walls can't qualify. For what a second storey costs done the frame way, see our bungalow vs two-storey guide, and if you're weighing lighter blocks for the upper floor, the CHB vs AAC guide.

9. What to Check Before the Plasterer Covers It

Once the plaster is on, nobody can see the steel. A ten-minute walk along a freshly laid wall catches most of the problems:

• Bar size and spacing match the plans. Count the vertical bars along a measured length and check a cut end for size. 10 mm delivered where 12 mm was specified is common, and so is 800 mm spacing where the plan says 600. Our rebar price guide has the weights per metre if you want to check a delivery by weight.
• Dowels are there and lapped. At the base of the wall, each vertical bar should overlap a dowel by at least 40 diameters. A wall bar that stops at the footing with nothing beside it isn't anchored.
• Horizontal bars are in the joints and hooked into the columns. Look at the column faces: the bars should go in, not stop short.
• Cells with bars are grouted. Tap the face at bar locations; a hollow sound all the way up means an empty cell.
• Openings have their lintels and side bars before the frames go in.
• Blocks are sound. Broken corners, soft faces and blocks that crumble in the hand are rejects. Our CHB price guide covers what separates a load-bearing PNS ASTM C90 block from a cheap one.
• Mortar joints are full and 6–16 mm thick (704.3.1), not furrowed deep in the middle.

For how many blocks, bags of cement and bars a wall takes, the CHB quantity estimator does the full material takeoff. This page is about whether the steel in it is right.

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