Elevated house design in the Philippines: raising the floor above floods

An elevated house lifts its main floor off the ground, on posts or on a tall base, so that water, damp and the heat of the ground stay below the rooms people live in. The bahay kubo was built this way for centuries, and the town houses of the colonial period kept the family floor upstairs too, as the bahay na bato guide explains. In low-lying parts of Pampanga, Bulacan and the other provinces where rivers and rain regularly put streets under water, it is still the most direct answer to a flood.

Why raise the floor

• Floods. Water that passes under the house does not reach the furniture, the appliances or the wiring.
• Damp. A floor off the ground stays drier, and the walls and the timber in them last longer.
• Air. Breeze moves under the floor as well as through the rooms, and a raised house catches more of it.
• Pests. A clear space under the house is easier to inspect for termites and rats.

How high

High enough that the worst flood the lot is likely to see stays below the floor, with a margin. That level comes from the lot, so start with what is known about it:

• The neighbours and the barangay. People who have lived there for years remember how high the water came, and marks on old walls often show it.
• Hazard maps. The government’s HazardHunterPH service, at hazardhunter.georisk.gov.ph, shows the flood susceptibility of a location, and the city or municipal disaster risk reduction office may have local flood maps.
• The road. If the road outside floods, so does the lot unless it is higher, and a street that is raised in a future project can leave an older house below it.

Raising the whole lot with fill is the other way to get height. It works, but the water it displaces goes somewhere, often onto the neighbours, and soft fill can settle under a house for years. Raising the floor on a structure leaves the ground and the water’s path as they were.

What holds it up

There are two main ways to lift a house.

On columns. Reinforced concrete columns on footings carry the floor, and the ground level stays open. Water flows through instead of pushing against walls, which is what a flood does to an enclosed ground floor. The columns need bracing, by beams or walls placed to resist the wind and earthquakes, because a house on tall posts with nothing between them sways.

On a raised base. The floor sits on a tall perimeter wall filled or built up inside. It looks like a conventional house on a high plinth. It keeps water out up to its height, and above that it holds water in, so its height has to be right.

Either way, the structural engineer sizes the columns, footings and bracing for this house on this ground. A raised floor also catches the wind from below in a typhoon, so the roof and the floor both need to be tied down. The house roof design guide covers fixing the roof.

Using the space below

The traditional name for the space under a house is the silong. On columns, it makes a good carport, a shaded work area or a place for a dirty kitchen and laundry, as long as nothing kept there minds getting wet. Enclosing it later as rooms brings the flood back into the house, so decide at the start whether the ground level is open space or a lower floor built to be washed out.

Getting up to the door

A raised floor means a stair at every entrance, and the stair is part of the design from the start. Covered stairs stay usable in the rain. The stair dimensions checker works out the steps for the height. For a parent who cannot climb, a ramp needs a great deal of length at a gentle slope, so plan the space for it early; the BP 344 accessibility guide gives the published ramp gradients.

Water, power and waste

• Wiring and the meter belong above the flood level, with outlets placed high.
• Water tanks and pumps placed under the house need to sit above the water or be built to survive it.
• The septic tank sits in the ground, and in a flood it can overflow back up through the drains. Its placement and sealing matter more on a lot that floods. The septic tank guide covers the distances and the design.

Permits

A raised house is a building like any other, and it needs a building permit with structural drawings and computations signed by the engineer. The building permit guide covers the folder.

Where this practice fits

We design houses for flood-prone lots across Central Luzon, starting from how high the water has come and what the lot will allow. Our custom house design service covers the house, and the sustainable design guide covers keeping it cool once it is up.