How to Make a Bigger Concrete Porch With an Existing Concrete Porch

Butt the tape measure's clip against the existing slab. Measure away from the slab and mark the perimeter of the addition on the ground with marking paint. Pound stakes into the ground approximately 3 inches outside of the addition's corners. Setting the stakes and twine outside of the corner marks allows you to leave the string in place while you excavate and build forms.

Tie builder's twine between the stakes and adjust the twine's horizontal position with a line level. Adjust the string's pitch so that the slab addition's surface slopes away from the existing slab or structure, roughly 1/4 inch per foot. Excavate the slab addition's installation area with a shovel or excavation equipment. Allow 3 to 4 inches for a gravel base layer. Compact the soil with a tamper.

Measure and mark form lengths on the form material with a tape measure and pencil. Cut the form material to size with a circular saw. Install stakes every 4 feet along the perimeter of the excavated area, using the string to check the stakes' positions. Fasten the cut form material to the inside faces of the stakes with a hammer and framing nails, using the string to adjust the lumber's horizontal level.

Hold the pistol-grip of a reciprocating saw firmly with one hand and the saw's body with the other hand. Rest the saw's blade on the form material's top edge and snug the saw's rocker shoe against the form's face. Press the blade against the side of a stake. While holding the saw in position, activate the saw and slice through the stake's top to make it flush with the top edge of the form. Reciprocating saws are dangerous tools; to prevent injury, you must have a firm grip on the tool and constant awareness of the blade's position.

Slice the tops from the remaining stakes. Stakes flush with forms allow you to draw a screed across the wet concrete's surface. Position expansion joint material against the edge of the existing slab that abuts the addition. Spread construction adhesive on the expansion joint's face and press the adhesive against the existing slab. Shovel the gravel base into the form's interior. Spread and smooth the gravel with a rake.

Compact the gravel with a tamper. Fill the form with ready-mix concrete. Select a straight, spare piece of form material to use as a screed. Set the screed across opposite sides of the forms and pull the screed across the wet concrete's surface. Smooth and distribute the concrete with the screed. Fill voids and low spots with concrete. Pass the screed over the slab a second time.