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My contractor says we can go for 6" walls for the complete house and there is no need for 9" walls on the outer size. Will there be a problem with 6" outer wall? PLease suggest.

Ashok,

If you plan to do column and beam construction then 6" outer walls and 6"/4" inner walls should be good enough. The only problem with outer 6" walls might be that if your column width is more than 6", then you would have 2-3" step in the walls.

When you go for burnt bricks, there is no choice. The walls should be 9" or 4.5" due to brick size.

Cement blocks give you the flexibility as they come in 8"/6"/4" sizes.

By saying Cement blocks come in 8"/6"/4" sizes would it mean that after plastering the total thickness of wall would be (8"/6"/4" ) + 1" ? in addition to that i have seen Cement blocks come in solid and hollow blocks and anyone has an idea whats the reason for this and which is the best choice ? the reason for asking is i dont understand the hollow concept as after construction if we hammer a nail it would crate a big hole in the wall.

6 inch outer wall should be fine for a framed structure. The plastering depends on quality of masonry(aka line) could be 1" or 2" or 3" and can be done to eliminate offsets in wall created by different size of column and brick size. However going for very slender columns like 6 inches is not advisable. Your structural engineer can take care of it.

Hollow Blocks density is 1400Kg/m3 or less . Clay bricks and Solid Cement bricks are over 2200kg/m3. AAC bricks are around 600 kg/m3. These have different moisture retention properties and of different Compressive strength . The strength of Hollow block equivalent of Solid Cement block can be purchased. Hollow does not mean its strength is reduced ,Hollow bricks are not completely hollow , they are cellular. There is no creation of big hole in wall when you hammer a nail. It can be nailed drilled just like any other brick.
The disadvantage of Clay bricks is high water retention and very heavy and low strength . ( You can check the weight of bricks before and after 3 days of forced immersion in water) . For rest of the mechanical properties of Concrete bricks , check the company data sheet and test reports. I suggest Buy from reputed companies,not the local cement block manufacturers, local ones are very very poor in quality compared to standard companies ( APCO, Basant Baton).

Advantages of Hollow Blocks , I have used 8 inch cellular\hollow blocks in my construction and did not face any problem.
Costs less than Solid blocks
better Acoustic insulation and Thermal Insulation due to cellular structure.
Given the column and beam construction, why to increase dead load. Given the density , Steel requirement can be considerably reduced.

For a local contractor , Hollow block is too difficult to digest. How can brick be hollow ? He will tend towards traditional and conventional and will ridicule anything that he has not used it. Go by Scientific test reports while choosing bricks. Every aspect of a brick can be measured by any local manufacturer or individual for a nominal cost,demand test data. Your structural engineer can interpret it better.

It is better to go for 8 inch columns. Walls perpendicular to 8 inch measurement can be 6 inches in width whereas walls along the 8 inch dimension can be 8 inches to get proper alignment.