Where there is no ceiling pipework and running some would be expensive, disruptive or impossible.

Corridors are the classic case: one pipe along a wall serves the whole length with no ceiling penetrations.

Hotel bedrooms, small rooms and refurbishments of occupied buildings use them for the same reason.

Historic and ornate ceilings that must not be penetrated are another common application.

Where a ceiling void and pipework already exist, pendents are usually simpler and cover more evenly.

The decision is normally driven by construction constraints rather than by fire performance.

It is asymmetric - the head throws forward across the room and does not project backwards over its own wall.

That gives a defined throw distance and a defined width rather than a circle.

Room proportions therefore matter: a room wider than the throw needs heads from both sides.

Horizontal and vertical sidewall patterns exist for different mounting conditions.

Extended-coverage sidewall heads increase the throw and are common in residential applications.

The listed coverage for the specific head is what the design uses, not a general figure.

Within specified distances both below the ceiling and from the wall face - the pattern is developed relative to both.

Too low and the head throws into the room below the hot gas layer that should operate it.

Too far from the wall and the coverage close to the wall behind it is lost.

The permitted ranges are narrower than for pendents because the geometry is asymmetric.

Sloped ceilings and soffits change the requirements again.

These are design dimensions checked at commissioning rather than installation preferences.

Because everything in the room lies in the direction of throw rather than beneath the head.

A pendent's obstructions are the things directly under it; a sidewall's are everything in front of it.

Deep beams, ductwork, ceiling fans and light fittings across the path all cast shadows.

In a corridor a single duct run can shield a long section from a head at one end.

Tall furniture, partitions and stacked goods have the same effect at lower level.

Obstruction rules for sidewalls are correspondingly more restrictive in most standards.

Only within the head's listed throw, and beyond that heads are needed from more than one wall.

The throw distance is a listed property of the specific head rather than a general figure.

Extended-coverage sidewall heads reach further and are widely used in residential.

Two opposing walls of heads is a valid arrangement where the design supports it.

Beyond a certain room size, ceiling-mounted heads become both cheaper and more reliable.

The hydraulic demand of extended-coverage heads is higher, which can affect the water supply.

Very - a sidewall in a bedroom or hallway avoids ceiling pipework entirely.

Residential systems are frequently retrofitted or installed in buildings where ceiling access is limited.

Concealed sidewall heads with flush cover plates are used where appearance matters.

Residential sprinklers are a distinct listing with a life-safety objective, and sidewall versions are listed the same way.

Coverage areas for residential heads are specific and larger than commercial equivalents.

Whether residential sprinklers are required at all differs enormously between markets.

Anything that changes the surface the pattern develops against - and coving in particular.

The spray is designed to develop along the ceiling before falling, so ceiling profile matters.

Coving, bulkheads and dropped soffits near the head interfere with that.

Textured or heavily profiled ceilings can disrupt the initial distribution.

Painting or covering the head itself is prohibited for the usual reasons.

Where a ceiling is altered during fit-out, the sidewall design should be reviewed rather than assumed still valid.