DC rowhouse with no cellar: where the sprinkler riser goes and how the pipe gets upstairs

Most of what we write about rowhouse sprinklers assumes a cellar. A lot of the District is not built that way. The postwar brick rows in Deanwood, Fort Davis, and Congress Heights sit on a slab. Plenty of older houses in Brookland and Trinidad have a crawl space you can barely get a shoulder into. Carriage houses and alley dwellings have nothing under them at all. In those houses the first design question is where the riser lives, and the second is how the pipe reaches the second floor without a cellar to hide in.

What the riser needs

A residential sprinkler riser on a 13D system is short. The water service comes up, a control valve shuts the system, a gauge reads the pressure, a drain lets the system empty, and in a standalone system a check valve or a backflow assembly separates the sprinkler pipe from the house water. Our riser gauge article walks through the parts. The whole assembly needs a space about the size of a water heater closet, heat enough to keep it above freezing, access to the valve without moving furniture, and a place for the drain to go. A cellar gives all four without trying. A slab house gives none of them until somebody picks a spot.

The three usual spots

• The front closet. In a DC rowhouse the water service comes from the street under the front yard, so the shortest run ends in the closet beside the front door or under the stair. DC Water meters sit in a pit at the sidewalk, so what comes through the slab is just the service line. The closet works if it is inside the conditioned house and the valve can be reached. It fails when the closet is on the cold front wall with the door shut all winter, which is the freeze case below.
• The mechanical closet. Postwar rows put the furnace and the water heater in a rear closet or a kitchen corner. The service is already there for the water heater, the space is already warm, and the floor already has a drain or a standpipe nearby. The downside is the run from the front of the house to the back under or through the slab, which is a trench in the floor or a route through the first floor ceiling.
• Under the stair. Central, warm, and usually dead space. The access panel has to be large enough to work through, and the drain has to be routed to a fixture, because nothing under a stair drains anywhere on its own.

Getting the pipe upstairs

In a cellar house the feed main runs the length of the house in the joist bay and drops heads up through the first floor ceiling, then a single riser goes up to feed the second floor from its ceiling. Without a cellar there is no bay below the first floor, so the layout flips. The main runs in the first floor ceiling, which is also the second floor framing, and feeds heads down into the first floor rooms and up into the second floor through sidewall heads or through the top floor ceiling. In a two story slab row that means one horizontal run at the first floor ceiling line and a second at the top floor ceiling or in the attic.

The attic run is where slab houses get into trouble. A flat roofed DC row often has a shallow attic that is vented and unheated, and CPVC in that space freezes. The fix is the one in our freeze protection article, pipe laid on the ceiling with the insulation over it, never under it, or a route inside the conditioned envelope through a soffit. Where the first floor ceiling is plaster and the owner wants it kept, the pipe runs in a boxed soffit along one wall, and our low ceiling article shows how small those can be made with the right head.

A multipurpose system changes the picture in a slab house more than anywhere else. The domestic water already runs up the walls to the bathrooms and the kitchen, and a multipurpose layout lets the sprinkler heads tee off those same lines. When the house is getting new plumbing anyway, the walls are open, and the sprinkler pipe rides along for a fraction of the patching.

The freeze problem

A cellar riser sits in ground temperature air and rarely freezes. A slab riser sits in whatever the room around it does. A front closet on an uninsulated brick wall can drop below freezing on the wall face during a cold week even while the house is warm, and the service stub coming through the slab is the coldest point of all. The design answers are small. Keep the riser off the exterior wall by a few inches. Insulate the stub from the slab to the valve. Leave the closet door louvered or leave it off. In a house that will sit empty, our vacant house article covers the thermostat setting that matters more than any insulation.

Crawl space houses are the hard case. The crawl is unheated, the service runs through it, and the easy route for the first floor pipe is along the crawl space joists where a cellar run would go. That route freezes. We either keep every foot of sprinkler pipe above the floor inside the house or treat the crawl as the unheated space it is and insulate and heat trace the service through it. The second is cheaper to build and more expensive to live with.

The drain problem

A 13D system has to be drainable, and a slab house has no floor drain next to the riser. The drain line runs to the nearest fixture that can take it, a laundry standpipe, a utility sink, or a hose bibb through the rear wall into the yard. Running it through the front wall onto the public sidewalk is the answer we do not give in the District, because the sidewalk is public space and nobody wants to explain the ice. For routine service the flow on a 13D drain is small enough that a hose to a bucket does it.

Quick answers