Screen blocks and cement jali

What a jali does on a Karachi facade

Karachi has more daylight than any interior needs and more heat than any interior wants, and the two arrive together. A jali separates them. The openings pass diffuse light and the sea breeze while the solid mass between them takes the direct sun, so a stair core or a west facing corridor can be lit and ventilated without a pane of glass and without a cooling load sitting behind it.

Privacy is the other half of the job, and it is geometric rather than optical. Each opening is a short tunnel through the depth of the block, so somebody passing at street level sees the lit inside face of the reveal and very little of the room, while somebody standing square to the wall sees straight through. That asymmetry is what lets a ground floor room open onto a lane.

Passes

Diffuse light, moving air

Holds back

Direct sun, street sight lines

Cast solid, not pressed hollow

The blocks are cast solid rather than pressed hollow, which is why an arc or a fretted corner holds a clean edge and why a wall of them can be handled on site without losing pieces. A hollow unit gives way at exactly the places a jali is made of, the thin sections between the openings, and those thin sections are the drawing.

The reveal is deep for the size of the opening, and the depth is the point. It gives the block its shading, its privacy angle and its stiffness, and it is why the mass between the openings still behaves like a wall.

Body

Solid cast

Reveal

Deep for the opening

Dry cast and wet cast, and what changes

The same block can be made two ways, and the method is a specification rather than a preference. A dry cast block is made from a stiff, low water mix compacted into the mould and released straight away, so the face comes out textured and matte and reads as a rugged, industrial surface. A wet cast block is poured as a fluid mix and left to cure in the mould, so it picks up finer detail and comes out smooth and close grained.

That difference decides what a design can be. The six dry cast blocks run to squares, diagonals and rectangles, the geometry a compacted mix will hold. The nine wet cast blocks go where a fluid mix can: the quatrefoil, the scrolled rosette, the fretwork lattice and the ellipses all need the mould to be filled rather than rammed.

It also decides how the wall ages and how it reads under light. A matte dry cast face scatters light and holds its tone as it weathers, which suits a boundary wall or a car porch. A smooth wet cast face takes a raking light more sharply and shows the arris, which is what you want on an entrance screen or an interior partition that people pass close to.

Dry cast

Textured matte, 6 designs

Wet cast

Smooth, 9 designs

Detail

Finer on wet cast

Dry stacked or mortared

Dry stacked, the blocks go up quickly and read as a screen: joints stay tight, nothing is pointed, and the panel depends on something else for stability, a frame around it, a reinforced core, or a restraint at the head. Mortared, the panel becomes masonry and can carry more of its own height, at the cost of a bed joint visible in every course and a slower lift.

Either way, a jali panel is not a load bearing wall and should not be drawn as one. Give it an opening in the structure with a bearing under it and a lintel over, let the slenderness of the panel be calculated rather than assumed, and detail the perimeter as a joint rather than as a butt against structure that is going to move.

Dry stacked

Needs a frame or head restraint

Mortared

Bed joint visible in every course

The depth of the opening, and the light through the day

The opening is as deep as the block, which is what makes a jali more than a perforated wall. Every hole is a short reveal, and a reveal has a cut off angle: with the sun high the head of the reveal shades the opening and the wall goes quiet, and as the sun drops the light drives further in and the pattern walks across the floor.

Orientation therefore decides how the wall behaves. A north facing jali is an even light source all day. A west facing one is a clock, and it will lay a moving grid of light on the wall opposite through the last hours of the afternoon, which is either exactly why it was specified or a reason to set it behind a deeper reveal.

So depth is a specification and not a detail, and it is not the same on every design. Pick the pattern for the elevation, then ask for the module and the depth of that block before the opening is set out, because a deeper block throws a longer shadow and closes the oblique view much further than a shallow one at the same opening.

North facing

Even, all day

West facing

Moving pattern, late afternoon

Ask for

Module and depth, per design

Where a jali belongs

A boundary wall is the easiest case: a solid base course to keep dust and wheels off, a jali panel above it to let air cross the plot and to stop the wall reading as a barricade. A stair core is the most useful, because a stair is the one room in a Karachi house that can be lit and ventilated for nothing.

Beyond those, the reliable places are a car porch screen, which has to hide a vehicle and still let the heat out, a courtyard divider, a service yard screen over a plant compound, and the corridor side of a lift lobby. What a jali is not is glazing. It passes dust and wind driven rain, so whatever sits behind it wants a floor that can be washed and somewhere for water to go.

Boundary wall

Solid base, jali above

Stair core

Light and air, no glazing

Car porch

Screen, heat release