The 6 inch vs 9 inch blocks question decides how strong your walls are, how much floor space you keep, and how much of your budget survives to roofing. The answer is not a compromise: use 9 inch for every external and load bearing wall, and 6 inch for internal partitions. Here is exactly why, with the numbers, so you can defend the decision to your mason and to anyone selling you a shortcut.
The only real difference is the width
Both blocks share the same face: 450 x 225mm. Only the width changes.
| 6 inch hollow | 9 inch hollow | |
|---|---|---|
| Face size | 450 x 225mm | 450 x 225mm |
| Width | 150mm | 225mm |
| Blocks per square metre of wall | 10 | 10 |
| Typical weight per block | 18 to 22 kg | 25 to 30 kg |
| Weight per square metre of wall | 180 to 220 kg | 250 to 300 kg |
| Finished thickness with render both sides | about 175mm | about 250mm |
| Load bearing | No, partitions only | Yes |
| Indicative price per block | ₦750 to ₦1,050 | ₦950 to ₦1,350 |
The line most people miss is the third one. Because the face is identical, you need exactly the same number of blocks either way. Ten per square metre of wall, whichever you choose. So switching a wall from 9 inch to 6 inch never reduces your block count. It only reduces the price per block, and the strength.
Strength: what each wall can actually carry
A 225mm 9 inch wall has 50 percent more material through its thickness than a 150mm 6 inch wall, and it is far more resistant to buckling because it is stockier relative to its height. That matters most on a 3m storey height, where a slender wall is working hardest.
In practice, on a Nigerian residential build:
- 9 inch carries load. Roof loads, ring beams, first floor slabs on a duplex, and anything sitting above an opening.
- 9 inch resists lateral force. Wind on a long external wall, and the impact a boundary wall takes over its life.
- 6 inch carries itself and nothing else. It holds a door frame, a socket back box and a coat of plaster. That is its job, and it does it well.
Put a roof load on a 6 inch external wall and you have not saved money, you have moved a structural problem into the future. Cracks appear first at the corners of window openings, then along the bed joints, and no amount of filler holds them because the wall is still moving.
Cost: what you actually save on 6 inch vs 9 inch blocks
Take a typical 3 bedroom bungalow needing about 3,500 blocks in total, with roughly 1,400 of those in internal partitions.
| Approach | Internal blocks | Saving per block | Total saving |
|---|---|---|---|
| All 9 inch throughout | 1,400 at 9 inch | ₦0 | ₦0 |
| 6 inch internal partitions | 1,400 at 6 inch | about ₦250 | about ₦350,000 |
You also save on mortar, because a 150mm wall needs less bedding material than a 225mm wall, and you save a little on render because the wall is straighter and lighter to handle. Call the realistic total saving on a bungalow somewhere between ₦350,000 and ₦450,000.
That is a genuine saving, and it is why 6 inch exists. The mistake is trying to extend the same logic to external walls, where the saving is roughly the same size but the cost of being wrong is your entire building.
Floor space: the strongest argument for 6 inch inside
This one is underrated. A 6 inch partition finishes about 75mm thinner than a 9 inch one. On a bungalow with about 45 linear metres of internal wall, that is 45 x 0.075, which is roughly 3.4 square metres of floor area handed back to you inside the same external footprint.
3.4 square metres is a decent store, a proper wardrobe run across two bedrooms, or the difference between a kitchen you can work in and one you shuffle sideways through. You get it for free, simply by not putting 9 inch blocks where they do no structural work.
Heat, noise and Lagos rain
Heat. A 225mm wall has more thermal mass, so it absorbs the afternoon sun and releases it slowly. Rooms behind 9 inch external walls run noticeably cooler in the afternoon than rooms behind 6 inch, which means less air conditioning and less generator time. Over a Lagos year that is real money.
Rain. Lagos rain does not fall, it is driven sideways. A 150mm external wall gives water far less thickness to travel through before it reaches your paint. Damp patches on internal faces of external walls are one of the most common defects in Nigerian homes, and thin external blockwork is a leading cause.
Noise. Heavier walls block more sound. On an external wall facing a street, a generator or a neighbouring compound, 9 inch is a real improvement over 6 inch.
The Lagos building reality
Walk any developing estate in Ikorodu, Ikeja, Ibeju Lekki or Mowe and you will see 6 inch external walls going up. It happens for one reason: someone is short of money at blockwork stage and the saving looks like a rescue.
What follows is predictable. Hairline cracks at opening corners within the first two rainy seasons. Damp on internal wall faces. A wall that cannot take the load if the owner later decides to add a floor, which is the most common upgrade a Nigerian homeowner makes. And a house that is harder to sell, because any surveyor or serious buyer checks wall thickness in the first ten minutes.
There is also a planning reality. Most competent structural designs assume 225mm external walls, so building thinner than the drawing is a variation nobody approved.
Our recommendation, stated plainly
9 inch for every external wall, every load bearing internal wall, every boundary and fence wall, and all blockwork below damp proof course. Use solid 9 inch blocks from footing to DPC where ground conditions are wet, which covers a lot of Lagos.
6 inch for internal partitions that carry nothing but themselves. Bedroom dividers, bathroom walls, kitchen partitions, corridor walls. On a duplex, 6 inch upper floor partitions also cut dead load on the slab, which can genuinely reduce the reinforcement your engineer specifies.
4 inch only for light screen walls and non structural divisions, and never where a heavy door or wall hung fittings will be attached.
Do not use 6 inch externally to save money. If the budget is that tight, build a smaller footprint in 9 inch rather than the same footprint in 6 inch. A smaller strong house is an asset. A larger weak one is a liability with a roof.
Where 6 inch genuinely wins
To be fair to the 6 inch block, it is the correct choice in several places:
- All internal non load bearing partitions in a bungalow or duplex
- Upper floor partitions where reducing dead load on the slab matters
- Infill panels in a framed structure, where the reinforced concrete columns and beams carry everything and the block is only cladding the frame
- Interior fit outs and subdivisions in existing buildings
In a properly designed frame with adequate columns and beams, 6 inch infill panels are engineering, not corner cutting. The distinction is whether a designed frame is carrying the load. If there is no frame, the wall is the structure, and the wall needs to be 9 inch.
What this means for your project
Your block count does not change between 6 inch and 9 inch, so the only decision is where each width belongs. Put 9 inch everywhere load, weather or ground moisture is involved, put 6 inch in the partitions, and you get a stronger, cooler, drier building, roughly ₦350,000 to ₦450,000 back in your pocket, and about 3.4 square metres of extra usable floor area. That is the version of this trade off that pays you twice.
Order the right blocks from Naydo Keystone
Naydo Keystone Ventures manufactures 4, 6 and 9 inch hollow blocks, solid blocks, cement bricks and face bricks in Satellite Town, Lagos, and every batch is tested before it leaves the factory. Send us your plan or your wall measurements and we will split your order correctly between 6 inch and 9 inch blocks, quote both, and schedule delivery around your build programme. Delivery is free within our Lagos service area. Request your free quote today.
Related
See sizes, strengths and uses side by side on our concrete blocks in Lagos page, check current costs in the 9 inch block price guide, and work out quantities with how many blocks a 3-bedroom house needs.
