How to Build a Porch That Allows Water to Drain

Excavate the area around the house where the concrete structure will be placed. Remove any organic material like grass and weeds, and dig into the ground 2 inches beyond the frost line. Tamp the ground with a tamper.

Construct a wooden frame to hold the concrete while it cures. You may use 2-by-8 pieces of lumber, or whatever size lumber is large enough to form a concrete slab to your specifications. The wooden frame should be constructed to form a U-shape against the side of the house. The wood does not need to be of high quality, but it should be straight. The interior of the wooden form should match the dimensions of the concrete porch. Use a table saw or circular saw to cut the wood down to size, then nail the boards together at the corners with 6d nails.

Drive a wooden stake into the ground at each corner of the wooden frame, and drive a wooden stake into the ground at both locations where the wooden frame meets the house. Tie a string between the two stakes to the right of the door to the house and another between the stakes to the left of the door. The strings should be positioned so they are even with the top of the wooden frame. Check the strings with a level to ensure they are exactly straight, then use this string as a guide as you hammer the front end of the wooden frame with a mallet so that the front of the structure is pressed into the ground and is slightly lower than the end of the frame that meets the house. This downward slope in the frame will ensure that your concrete structure will drain off rainwater. To determine the exact pitch that your concrete structure should have, measure the depth of the wooden form from the front to the back where it meets the house. For every foot of depth, the concrete form should slope downward 1/8 inch.

Fill the concrete structure with 2 inches of gravel.

Pour wet concrete into the wooden form and fill it halfway. Tap the sides of the wooden frame repeatedly with a mallet to level the concrete and bring trapped air bubbles to the surface.

Lay a grid of rebar into the wet concrete.

Check the frame with a level to ensure that it is still sloped. Fill the frame to the top with wet concrete.

Tap the sides of the wooden frame repeatedly with a mallet to level the concrete and bring trapped air bubbles to the surface. Smooth the top of the concrete with a concrete trowel or squeegee.

Remove the wooden frame after the concrete has cured for 24 hours. Fill in any gaps around the concrete slab with soil.