Should I remove my old concrete patio before building a deck, or build over it?
Short answer
Keep the slab if it is sound, drains away from the house, and sits low enough that a deck over it still meets the door threshold and the stucco weep screed. Remove it, or part of it, if it slopes toward the house, has heaved sections that trap water, or sits so high that a deck on it would land above the threshold. The decision is about water and height, not about saving the demolition. Often the answer is to cut out a strip, not the whole slab.
What a good slab gives you
A sound concrete patio is a gift to a low deck. It is a stable, compacted base that keeps the frame off the dirt, it already sheds water if it was poured with a slope, and it lets a freestanding low deck sit on pads or sleepers without digging footings for every post. Demolition is loud, dusty and produces a lot of heavy debris, so if the slab is doing its job, we would rather leave it and build on it.
The questions are whether the slab is actually doing its job, and whether the extra height it adds leaves room for the deck. Both get answered with a level, a string line and a garden hose before anything is drawn.
When the slab works against you
The first problem is slope toward the house. Many older patios have settled over the years so that water now runs to the wall instead of away from it. Build a deck over that and you have hidden a puddle against your foundation and under your framing, where it will feed rot and attract termites for years without being seen. A slab that slopes the wrong way has to be corrected, cut back, or removed.
The second problem is height. Stucco houses have a weep screed at the bottom of the wall that must stay clear of paving by a couple of inches so the wall can drain. A slab that already sits close to it, plus a frame and decking on top, can bury the screed and put the deck surface above the door threshold, which sends rain into the house. Heaved and cracked sections that create steps and traps are the third problem; sleepers can bridge small irregularities, but not a slab that moves.
The middle path: remove a strip, keep the rest
Most of the time the slab is fine in the middle and wrong at the edges. The practical fix is to saw-cut and remove a strip along the house, typically a foot or two wide, so the frame near the wall sits on gravel that drains freely, the weep screed is clear, and the ledger or a freestanding beam can be set at the right height. The rest of the slab stays as the base for the field of the deck.
Saw cuts also solve drainage within the slab. A cut channel filled with gravel across a low spot gives trapped water a way out. And where an attached or taller deck needs real footings, individual holes are cored or cut through the slab and the footings are dug below it, leaving the slab in place around them.
How we check a slab before deciding
- Run a hose on it for a few minutes and watch where the water goes; standing water near the house decides the question.
- Shoot the slab with a level against the door threshold and the weep screed to see how much height is available.
- Map cracks and heaves; hairline cracks are fine, offsets you can feel with your foot are not.
- Tap suspect areas; a hollow sound means a void beneath and a slab that may still be moving.
- Check how the slab meets the house: poured against the stucco, or with a gap and a sealant joint.
Permits and the finished height
Carlsbad follows the California Residential Code exemption for small, low, freestanding decks, and a deck over a slab is often low enough to qualify, but attaching it to the house or raising it near the 30-inch guard threshold changes that. The Building Division's page explains what needs a permit. Whatever the paperwork, the finished height should land a step below the door threshold or flush with it only when the threshold detail and flashing are designed for that, and the decking should stop short of the stucco with a gap so the wall can breathe.
Related questions
Can I build a deck over a cracked patio?
Over hairline cracks and small settled areas, yes, with sleepers shimmed level and pads under the frame. Over heaves that create steps, or cracks that are still widening, no; that section comes out or gets a proper base.
Does the concrete need to be sealed or covered before the deck goes on?
No. The slab needs to drain, not to be sealed. A layer of weed fabric and gravel in any removed strips, and clearance under the joists so air moves, does more for the frame than any coating on the concrete.
Is it cheaper to keep the slab?
Usually, because demolition and hauling are avoided, but a slab that forces a thicker frame, special sleepers or a drainage fix can erase the savings. We price both ways when it is close and let the water decide.
Sources and further reading
- City of Carlsbad Building Division (www.carlsbadca.gov)