Building a masonry blocks wall

Masonry walls must rest on a foundation, footers or concrete slab. The foundation must be clean so the mortar will adhere to it. It should also be relatively level.

Taking measurements from the foundation or floor plan, and transferring those to the foundation, footing or floor slab is the first step in laying out the wall. The openings for doors and windows must be placed exactly. It is important that the corners be set on the foundation exactly as given on the detail drawing on the foundation plan.

Once two points of measurement are established, either corner to corner, or corner to door opening, a chalk line will be marked on the surface of the foundation. It will establish the line to which the face of the block or brick will be laid. Since chalk can wash away, a grease crayon or a nail can mark the surface for key points along the chalk line, so that the chalk line can be re-snapped along these markings as needed.

Starting with the corners of the wall, lay the first level of block without mortar, so a visual check can be made between the dimensions on the floor plan and how the first course actually fits the plan. This dry run will tell you which block will need to be cut to fit openings, etc. During the dry run, block units will be strung along the entire width and length of the foundation, floor slab and even across openings. This will show how bond will be maintained above the openings. Since all mortar joints should be 3/8", the dry run of block will be spaced accordingly. This can be simplified by using a 3/8" piece of wood placed between block as they are laid during this dry run.

Exact corners is the most important construction of a masonry wall as corners guide the building of the rest of the wall. A truly vertical corner pole will make the job easier. Two such poles are set up, one on each corner, with the mason's line stretched between them. Corner poles for block walls should be marked every 4 to 8 inches, depending on how high the material is with which you are building the wall. Such marks must be absolutely level when the mason's line is stretched between them.

The first course is now ready to be laid in mortar. About 1" thick mortar is spread, full bed, on the foundation or slab. The only exception to spreading a full bed of mortar at the first course is where reinforcing rods or steel rebar are projecting from the foundation footing or slab. Since the cells of the block where these rods will be located will be grouted, no reason for a full bed of mortar. In such cases, leave a space around the rods so that the block will be seated in mortar, but the mortar will not cover that part of the foundation.

Picking up a small amount of mortar with the trowel, the trowel point is placed on the foundation. Rotate the trowel 180 degrees, moving it in a backwards motion in the direction you want to lay the mortar. The mortar should slide off the trowel with ease. It is the rotating and backward motion simultaneously that will release the mortar from the trowel.

After spreading a full bed of mortar on the marked foundation, the first block of the corner is carefully positioned with the marked lines in both directions. Press the block down until a 3/8" joint is made. If necessary, tap the top of the block lightly, usually with the handle of the trowel, to get the block into proper alignment. It is essential that the first level or course of block is plumb (straight up and down) and level.

Once the corner blocks placed, the lead blocks are set. This will entail about three or four blocks going from each side of the corner. When laying block against one already in place, you will trowel a vertical bead of mortar along the side of the block, creating a head joint (mortar placed along the block vertically). Press or shove the block gently against the one already in place. This shove will help make a tighter fit of the head joint. But the shove must not be so strong as to move the block already in place.

Corners and lead blocks are generally built 4-6 rows high, with each course being stepped back one block from the course below, creating a pyramid effect. The wall alignment, plumb and level should be checked on these corners before completing the wall. If everything checks out fine, it is now time to fill in each course between the corners. Make sure the mason's line is stretched taut and attached to the line holder at each corner. The line should be no further away than 1/8" from the face of the wall, and when the block is set into the mortar to form a 3/8" mortar joint, the top of the block should be even with top of the mason's line. Allowing the block to touch the mason's line is called "crowding the line" and could cause a bulge in the wall.

The first course of block is laid from corner to corner, allowing for openings or doorways. It is important that mortar for these blocks is spread on all vertical edges of the block before the block is carefully put in place. When laying the second row of block (and all rows above), mortar must also be placed on block directly beneath the block to be laid.

It is also important that, for each course, a block should not be stacked directly on top of the block below, rather, it should be staggered so the block being laid is placed half over one block below, and half over the other block below. Otherwise, a weak spot will be created within the wall.

As each block is put into place, the mortar which is squeezed out should be cut off with the edge of the trowel and care should be taken that the mortar doesn't fall off the trowel onto the wall or smear the block as the excess mortar is being removed. When building block walls, it is important to press firmly so that a good bond forms between the mortar and the block. Once dry, the mortar can be flicked off with the trowel. If there should be some stain on the face of the block, it can be rubbed off with a piece of broken block or brick, or brushed off with a stiff bristle brush. All squeezed mortar which is cut from the mortar joints can either be thrown back onto the mortar board or used to butter the next block. Mortar which has fallen onto the ground should not be salvaged.

The work of building the corners, stringing the line, laying the course between corners, will continue until you reach the next to last row. At this point, you will know whether a masonry wall cap or coping will be placed on top of the last row or if the top row will be filled with mortar and another type of coping placed on the top of the wall. If mortar is to go into the cells of the top course of block, a wire mesh has to be placed across the cells of the block below so that the mortar in top course will be held in place.

At this point, you will check that the wall is plumb and aligned. Look at it to make sure there is no holes or gaps in the joints. If there is any smear or splatter, let it dry before rubbing it off.

Once the mortar is hard (pressing with your finger doesn’t leave any marks), the vertical joints are tooled with an s-shaped jointer. The horizontal joints are also finished and any burrs which develop between the points where the horizontal and vertical joints come together, are flicked off with the blade of the trowel.

A final inspection should be made to be sure of a tight bond between mortar and block and that there are no ragged edges. The wall is now ready for cleaning.