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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Balcony Waterproofing:
Slope, Sills and
Systems That Last

Where balconies leak, which membranes work over concrete and wood framing, how California balcony inspections work, and a quick check for slope, door clearance and vent area.
Balcony quotes

Leaking balcony or planning a new one? Get it detailed right.

Tell us whether it is concrete or wood framed, what sits underneath it and roughly how big it is. We will connect you with a certified ArmorThane applicator who quotes the slope work, membrane, mils and flashing in writing.

  • Concrete or wood-framed balcony
  • Area and number of balconies
  • Living space, parking or open air below
  • Tile, coating or pavers on top
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1/4 in
Minimum fall per foot of run, about 2%
IBC low-slope drainage minimum
1/150
Net free vent area for enclosed balcony framing
IBC / CBC 2304.12.2.6
6 ft
Height above ground that puts a California balcony under SB 721
Inspected every 6 years
15%
Minimum share of each balcony type an SB 721 inspector must check
Apartment buildings, 3+ units

Balcony waterproofing fails in a predictable order: the door sill first, then the joint where the deck meets the wall, then the railing posts. Water that slips past the membrane rarely drips on anyone right away. It soaks into the framing or the slab, and on a wood-framed balcony it can rot joists for years before a stain shows up below.

Quick answer

Balcony waterproofing means a continuous membrane between the walking surface and the structure, sloped at least 1/4 inch per foot away from the building, turned up the wall, lapped under the door sill and finished with a drip edge at the outside. Walkable urethane or polyurea deck coatings work over concrete and plywood. Tile and pavers need a membrane underneath them.

What's in this guide
  1. 1Where balconies leak
  2. 2Systems compared
  3. 3Slope, sill and vent check
  4. 4Details that decide the job
  5. 5Inspection laws
  6. 6Questions
Failure points

Where water gets into a balcony

Most leaking balconies have a perfectly sound membrane across the middle. The trouble is at the edges and the holes, where the membrane has to turn a corner, stop against something or let a bolt through. Before anyone quotes a recoat, these six spots deserve a hard look, and the underside deserves a flashlight.

Most common

Door threshold

The sill under a slider is the lowest, busiest joint on the balcony. If the membrane stops at the face of the door instead of running under a sloped sill pan, wind-driven rain gets pushed straight into the subfloor.

Very common

Wall-to-deck joint

The membrane has to turn up the wall and tuck behind the siding, stucco screed or building wrap. A bead of caulk where the coating meets the wall lasts a season or two, then lets water run down behind it.

Often missed

Railing posts

Every post bolted down through the deck is a hole in the membrane. Side-mounted rails, or posts on raised blocks with the coating run up and over the base, hold up far better than a bolt and a dab of sealant.

Shows from below

Outer edge

Without a drip edge, water curls back under the edge and runs along the rim board or the slab soffit. Stains, peeling paint and spalled concrete on the underside usually point here.

Curbed balconies

Drains and scuppers

A balcony with a solid parapet or curb needs a drain or scupper and an overflow. One clogged drain turns the whole surface into a pond that sits against the door.

Tiled balconies

Cracked tile and grout

Tile and grout are not waterproof. With no membrane under them, each hairline crack is a leak, and a real fix means taking the tile up.

Options

Balcony waterproofing systems compared

The right system depends on three things: whether the balcony is concrete or wood framed, what people want to walk on, and whether anything lives underneath it. A balcony over a bedroom is a roof with furniture on it and should be treated like one.

SystemTypical thickness (mils)Surface you walk onBack in useBest fit
Penetrating sealer (silane or siloxane)None, soaks inBare concreteSame dayOpen-air concrete balconies; repels water but bridges no cracks
Urethane deck coating, base plus topcoat30–50Textured coating with quartz or chips1–3 daysPlywood or concrete balconies, including over living space
Spray polyurea with aliphatic topcoat40–80Textured coating, chips or quartzOften same or next dayMulti-unit and commercial work where downtime costs money
Bonded sheet or liquid membrane under tile40–60 membraneTile or stoneSeveral days with tileOwners who want tile; needs slope built into the substrate
Heat-welded vinyl sheetAbout 60–80Printed vinylNext dayWood-framed residential balconies with simple shapes
Typical industry ranges for pedestrian systems. The manufacturer's data sheet and the project spec set the real numbers. Tap a column heading to sort.
A sealer is not a membraneA clear silane or siloxane sealer keeps rain from soaking into concrete, and it is a fine choice for an open-air slab. It will not bridge a moving crack or seal a sill. Over occupied space, or on any wood-framed balcony, use a real membrane.
Wood framed

Plywood over joists

Slope has to be built into the framing or with tapered sleepers. The membrane must be continuous over the sheathing, and the building code wants either preservative-treated or naturally durable framing, or an impervious moisture barrier that drains positively. If the ceiling below is closed in, it needs cross ventilation.

Concrete

Cantilevered or post-tensioned slab

The enemy is water reaching the rebar, which rusts, swells and spalls the slab edge. A flat slab can be sloped with a bonded mortar topping before coating. Test moisture and prepare the surface to the coating maker's profile, because most coating failures on concrete are bond failures.

ArmorThane's two-step ArmorDeck system pairs a moisture-cured aromatic urethane polyurea basecoat with an aliphatic topcoat for plywood and concrete decks. For bigger podium and plaza jobs, see plaza deck waterproofing.

Field check

Will it drain, and will the door still close?

Two numbers settle most balcony arguments before a drop of coating is bought: how much the surface falls from the door to the edge, and how far the door sill sits above it. Lay a 4 foot level at the door, measure the gap at the low end, and scale it to the full depth. Then measure from the deck to the top of the sill.

Slope and sill math

Balcony slope, sill and vent check

Slope = fall ÷ depth, compared with the 1/4 inch per foot (2%) drainage minimum. Fall to add = depth × 0.25 in, minus the fall you have. Building that slope up at the door comes out of the sill clearance along with the new system's thickness. Vent area = deck area × 144 ÷ 150 sq in, from IBC / CBC 2304.12.2.6 for enclosed framing. Build-ups are typical values.

—
Slope you have, inches per foot
—
Fall to add for 1/4 in per ft
—
Sill clearance left after the work
—
Net free vent area needed

Larger walking decks, stairs and patios follow the same logic. Our deck waterproofing guide covers wood decks with gapped boards and under-deck drainage, and the patio coating guide covers slabs on grade.

Details

Five details that decide the job

We would rather see an average coating with perfect details than a premium coating with lazy ones. The membrane in the middle of a balcony almost never fails first.

  1. Fix the slope before you coat

    Coatings follow the surface they go on. If the deck is flat or pitched back toward the door, correct it in the framing or with a sloped, bonded mortar topping. No coating moves water uphill.

  2. Pan the door sill

    On new work, a sloped sill pan with end dams and a back leg goes in before the door, and the membrane laps under it. On a retrofit where the slider stays, run the membrane tight to the sill, seal the joint with a compatible sealant and put that joint on a yearly check list.

  3. Turn up the wall, shingle fashion

    Carry the membrane up the wall far enough that the wall's own drainage plane (building wrap, weep screed, flashing) laps over it. Water running down the wall should land on top of the membrane, never behind it.

  4. Give the edge a drip

    A metal drip edge, or a coated edge with a formed drip, throws water clear of the rim board or slab soffit. Coat over the flange so the edge metal is part of the membrane, not a separate piece.

  5. Treat every hole as a leak

    Rail posts, light boxes, hose bibs and drains each get reinforcing fabric and extra coating at the base. Where you have a choice, mount rails to the face of the rim instead of through the walking surface.

Concrete balconies need sound, clean, dry concrete before any of this. Patch spalls, rout and fill cracks with a proper crack repair, then profile the slab as described in our surface preparation guide.

Inspections

California balcony inspection laws

In 2015 a balcony in Berkeley collapsed during a birthday party, killing six people and injuring seven. Investigators traced it to rotted joists inside a closed-in balcony that had been taking on water. California answered with two laws that turned balcony waterproofing from a maintenance item into a legal one.

Apartments

SB 721

Applies to buildings with three or more rental units. Balconies, decks, stairs and walkways more than 6 feet above ground that rely mostly on wood for support must be inspected every 6 years, with at least 15% of each type checked. The first deadline was pushed to January 1, 2026. Owners then have 120 days to make the repairs found.

Condos

SB 326

Applies to condominium associations. Inspections run every 9 years, use a statistically significant sample (95% confidence), and must be done by a licensed architect or structural engineer. SB 721 also allows qualified A, B or C-5 contractors and certified building inspectors.

Inspectors look at the load-bearing framing, the connections, the railings and the waterproofing that protects them, and they often cut small openings to see inside. A balcony with a failing membrane at the sill or the wall line will usually show it here first.

Outside CaliforniaSome cities have their own facade and balcony inspection rules, so check locally. Either way, a yearly look at the sill joint, the wall line, the rail bases and the underside costs nothing and catches most problems while they are still a coating repair.
Waterproof it once

A balcony membrane detailed to the door sill.

A certified ArmorThane applicator can quote the system, the mil thickness and the sill, wall and edge details in writing, which is where most balcony leaks start.

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FAQ

Balcony waterproofing questions

How do you waterproof a balcony?

Get the surface sloped at least 1/4 inch per foot away from the building, prepare it, then install a continuous membrane that turns up the wall, laps under the door sill and ends at a drip edge. Flash every rail post and drain, and finish with a walkable topcoat, tile or pavers.

What is the best balcony waterproofing membrane?

For a coating you walk on, urethane or polyurea deck systems with a UV-stable aliphatic topcoat are the usual choice over plywood and concrete. If the owner wants tile, use a bonded sheet or liquid membrane rated for tile underneath. The details at the sill and wall matter more than the brand.

Can you waterproof a balcony without removing the tiles?

Clear sealers over tile and grout can slow light seepage for a while, but they do not bridge cracks or fix a failed sill or wall joint. If water is already reaching the framing or the room below, the lasting fix is to remove the tile, repair and slope the substrate, and install a membrane.

How much slope does a balcony need?

Plan on at least 1/4 inch per foot, about 2%, falling away from the door. On a 6 foot deep balcony that is 1.5 inches from the wall to the edge. Less slope leaves puddles against the threshold, which is the joint most likely to leak.

Why is my balcony leaking into the room below?

Usually water is getting in at the door sill, the wall-to-deck joint or a railing post, then traveling along the framing or slab before it shows up. The stain below is rarely right under the entry point, so test and inspect those details first.

Does SB 721 require new waterproofing?

SB 721 requires inspection, not a particular product. The inspector checks load-bearing parts and the waterproofing that protects them. If the report finds damage or a failed membrane, the owner has 120 days to repair it, which often means new framing, flashing and a new membrane.

AT
Written by the ArmorThane technical team. We have manufactured polyurea, polyurethane and polyaspartic coatings and plural component spray equipment in Springfield, Missouri since 1989. Code and law summaries here are general; your local building code, the engineer of record and the product data sheet govern any balcony repair.

Sources

  1. Bureau Veritas, The California Balcony Inspection Bill: SB-721 and SB-326. bvna.com/magazine/california-balconyinspection-billsb-721-sb-326
  2. Rimkus, California Balcony Inspection Law: SB 721 and SB 326 Guide. rimkus.com/article/california-balcony-inspection
  3. Nevada County, CA, Balcony and Deck Code Requirements (California Building Standards Bulletin 17-01). nevadacountyca.gov/DocumentCenter/View/15481
  4. Architectural Graphic Standards, Balcony Waterproofing. graphicstandards.com/2016/09/08/balcony-waterproofing

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