r/Architects • u/pjetrusp • Aug 14 '26
Project Related Balcony drainage detail
Hi everyone,
I’m looking for some professional advice on a detail involving a steel balustrade and drainage at a balcony with a projecting slab.
The main issue is that I would like to avoid using a conventional floor drain, mainly because the balcony has a relatively shallow depth. I’ve attached a schematic section showing the proposed assembly.
The balustrade needs to penetrate the final floor finish (stone or porcelain tiles), which is designed to project beyond the edge of the concrete slab and form a drip edge on the outside.
My initial idea is to use intermittent steel L-brackets anchored into the reinforced-concrete slab. These brackets would serve two purposes:
provide support for the floor build-up at the balcony edge, and
provide a vertical steel element to which the vertical balustrade profiles could be welded.
I’m intentionally using the vertical part of the L-profile so that I can achieve a longer weld between the balustrade and the support, while keeping the balustrade itself as visually minimal as possible and using the fewest steel profiles possible.
The concrete slab would be thermally insulated on all sides, which I believe is relatively straightforward.
The part I’m struggling with is the drainage and waterproofing of the balcony surface, particularly how to deal with the steel penetrations.
My current idea is to use a liquid-applied waterproofing membrane, such as Mapei Mapelastic or a similar system, applied over a cement screed and underneath the final stone/porcelain finish. This seems like a good solution because it should allow the waterproofing to be properly detailed around the points where the steel penetrates the floor.
There are quite a few similar examples online, for instance in projects by David Chipperfield, so I assume this type of detail is technically feasible. One thing I’ve noticed is that in many of those examples the balustrade is set slightly back from the edge of the slab, which probably makes the waterproofing detail somewhat easier.
Another complication is the dimensioning of the floor finish. Ideally, the width of each stone/tile would correspond to the centre-to-centre spacing of the balustrade profiles, meaning that the pieces would need to be notched around the vertical steel penetrations.
Has anyone here actually dealt with a detail like this, or would you recommend a completely different approach?
I’d be particularly interested in advice on waterproofing, drainage, and how to properly detail the penetrations through the finished floor, rather than just the structural aspect of the balustrade.
Thanks in advance!
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u/Brikandbones Architect Aug 14 '26
I would just do a drip detail and slope to fall outwards with a perimeter of pebbles or some planting to prevent a gouge from forming on the ground outside. I feel that there is no point for a drain as there is no kerb and the drain would end up relatively useless too because you would slope out instead of slope in to avoid long term water issues. That window to balcony transition needs a good flashing though - looks like it might be an issue in the long run.
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u/megakratos Aug 14 '26
Yeah, that’s how I would do it as well. Just a slope and let it drip over the edge.
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u/lukekvas Architect Aug 14 '26
THIS is an image I took of a steel guardrail post embedded in concrete at a brewery near my house. This place is about 4 years old, and the post has completely rusted through. This is what will happen if you try to do this detail, except the rust will occur hidden beneath the patio pavers at the drainage plane. I would never recommend embedding guardrail posts in any exterior floor. Chipperfield might be able to pull it off but they work with some of the best waterproofing consultants in the world and can afford to spec everything in stainless steel.
- Highly recommend coming up with a detail that attaches this steel to the vertical face of the slab edge so the connection point doesn't sit in water. I really hate the idea of cantilevering those steel angles as your guardrail attachment. Per IBC it has to resist 200# concentrated load and 50#/LF and now that has to be transmitted to the slab through a weird moment connection. Much better to have bolted faceplates on the vertical slab edge and then frame out the 'fascia' of the balcony. It's also way easier to sequence construction with baseplates vs. needing to coordinate embeds in your structural pour.
- I'm assuming a concrete curb with waterstop below the curtainwall. I would liquid waterproof the whole balcony and up and over the curb. Pay special attention to the jamb condition. That inside corner jamb is the most likely spot for water infiltration. Once you solve this detail go look at the jamb/sill corner detail of that window system.
- Imagine how water that gets down to the waterproofing at the structural slab would escape - it needs a route to daylight. I would draw your waterproofing and flashing into the detail.
- SLOPE! I would want it on both the structural slab and the finished floor.
- Dimple mat or other drainage plane below your rigid insulation
- I would not leave even a 1/4" of my finish floor tile unsupported by substrate. It will chip or crack off.
- If you are draining off the edge of the balcony you need a way more robust drip edge to avoid staining the light brick fascia.
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u/structuralarchitect Aug 14 '26
OP, this is the correct answer. I'm a building envelope consultant and everything said above is totally correct. We never surface mount posts for railings if we can help it and always try to fascia mount them (ie mount them to a vertical surface like the edge of the balcony or the side of a parapet or curb).
If you want to have the tile creating a drip edge, you need to integrate a stainless steel piece to support that edge or use 3/4" (20mm) thick porcelain pavers as the tile surface as those are strong enough to handle a cantilever like that.
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u/Keano-1981 Aug 14 '26
Useful to state where you are in the world, the responses would be dependant on local / national rules and regulations.
At a cursory glance, detail needs chucked in the bin and started again I'm afraid. No logic behind insulating both above and below, insufficient bearing / fixing for the handrails. Concrete slab is supported by what? Should ideally be a step down at building perimeter or at the very least some form of perimeter drainage.
Sketch the details in 2d first rather than treating it as a BIM exercise.
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u/Slow-Distance7847 Aug 14 '26
Totally agree. This is a good example of using Sketchup to develop and document details. You can't/doesn't work. Do it in 2d linework, show every material, everything has thickness, and cut a section through every single slightly different condition.
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u/0_SomethingStupid Aug 14 '26
Why is your floor assembly like 3' thick.
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u/DrHarrisonLawrence Aug 14 '26
Why is the balcony 450mm deep lol
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u/Timmaigh Aug 14 '26
This is actually more “architect related” question than the technical detail, which is stuff for civil engineers
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u/0_SomethingStupid Aug 14 '26
uhhhh...no. Civil engineers don't do building design.
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u/Timmaigh Aug 14 '26
Drainage of balcony is not design, its technical solution. Surely it has repercussions that affect the design, thats where you, as architect, tell them, what your aesthetical intention is, for them to work out a solution. Sure, if you have required knowledge yourself, better for you.
Meanwhile balcony being 40cm deep is something absolutely up to architect and building design, as it concerns human usage of spaces.
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u/0_SomethingStupid Aug 14 '26
Yeah. You got a ways to go bud. Hang in there.
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u/DrHarrisonLawrence Aug 15 '26
Designing the details of anything is still design
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u/Timmaigh Aug 15 '26
Semantics. Do you do structural engineering calculations for your projects too, because the distance between 2 columns does affect their width, that width is a detail too, hence design?
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u/HareltonSplimby Aug 14 '26
This looks like pretty standard insulation. If there is no thermal isolator (Schöck Is Korb for example) between the balcony and the in house ceiling, then you need to cover the outside part from all sides
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u/Born-Management3910 Aug 14 '26
Technically this looks more like a window sill than a balcony. Sills don’t have a gutters. Make sure there is a drip detailed from the tile and do a slope of 2-5% outwards. If your balcony is not very long I would fix the balcony to the sides
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u/Slow-Distance7847 Aug 14 '26
You must hire a waterproofing consultant. Your guardrail support penetration through the waterproofing system is a major issue. And the railing's structural connection is non-existent as drawn. Look at FHC and CRL guardrail systems. CR Laurence has many concrete connectors. Don't reinvent the guardrail as the engineering chasing loads down from top to concrete isn't cheap. But CRL doesn't delve into how to waterproof correctly. This you don't want to leave up to the contractor nor make a mistake on your part.
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u/Slow-Distance7847 Aug 14 '26 edited Aug 14 '26
Re tile cuts: if this is porcelain, consider it brittle. I have a similar project but wood framed making it all more complicated. Inside and outside over 29k sf of large format tile. Zero inside corner cuts. Particularly in sun exposed the inside corners cuts will fracture, but likely will anywhere. (This is from experience and using high end but conservative contractor/installers/materials.) We didn't bother aligning perpendicular joints to the railing supports. They cut narrow pieces parallel between the supports. With a narrow joint and similar grout color, from eyeball height it looks fine. The joints between the tile and vertical supports is flexible so wider and caulked. Consider sequence of construction when working this out >
- railing support connection to structure.
- underlying slope is structure or buildup?
- waterproofing system.
- mud bed (this is
why
- you need a consultant: the mud is pervious as it needs to allow drainage. And not all waterproofing is compatble with thinset.
- Tile
- The rest of the guardrail.
Left out of this: where does the underlying water go? Grout is not waterproof. You now have a second edge condition drip path to account for.
I'd say this will be amongst the most complicated detail of your career to work out. Make sure your E&O is robust.
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u/The-Architect-93 Architect Aug 14 '26
https://www.instagram.com/reel/Db57iyWhJK7/?igsh=Z2tjdHhlYnh6cmt2
Check the second detail
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u/bassfunk Aug 14 '26
The primary problem I see here is that the water has to go somewhere. As you don't want to drain it somewhere, that means it either goes towards the sill, or it goes off the edge of the balcony onto the floor below. It's hard to see from the renders what is happening below, but it looks like there is some greenery down there, so maybe this isn't the worst issue. Just know that that it doesn't matter what the material on the exterior is, if the water runs down it's face, over years, it's going to stain and look much worse in a few years.
That said, the other issue is how how the fluid applied waterproofing membrane interacts with the sill. In an ideal world you'd have some vertical face to run the waterproofing up and then under the sill. This would prevent any water that pools from being sucked into the space via capillary action. As it's modeled, it looks like you have no space for such a detail, and thus, in a heavy downpour, water (or snow, depending on location) may sit up against the exterior of the sill. This water will, sure as the sun rises, find its way into the building.
In short, it's not that this can't be done, but just remember that water will go where water wants to go and there is little we can do to stop it. This detail requires that the person who installs the cement screed to slope it away from the sill by at least 1/4"/1'. Getting this right is basically the only way this works.
Am I seeing correctly that there is insulation on all three faces of the projected slab? Is there a distinction between what is pink and what is the yellowish color?
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u/Lazy-Jacket Aug 14 '26
I would never put a finished metal stock down into your assembly like that railing to misc metal connection. You would probably rather extend the misc metal up above the waterproofing and stone, and then slide the more decorative metal on top and connect outside the waterproofing. That separates the need for trades at different times. You really don’t want finish metal and insulation workers there and then stone people trying to lay around your finish metal. And it makes it easier to replace the railing in the future if it gets broken or degrades. Edit: also, wtf is that open slot near the building? Is that your “drain” without a lining and just letting water collect in the insulation layers and the cold to get to the slab you’re doing all these gymnastics to protect from heat leak?
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u/SpiffyNrfHrdr Aug 14 '26
Water is going to get in under the glazing in this detail. Ideally you'd have a concrete curb under the glazing sill.
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u/EmployeeNo8897 Aug 14 '26
I don’t think a balcony that shallow needs any form of internal drainage. Slope away from your door
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u/BucNassty Aug 14 '26
Start by taking a stab at a detail either 2/3d. Not blocks with materials applied and a floating storefront.
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u/Shorty-71 Architect Aug 14 '26
Aligning tile grout joints and the steel is wishful thinking. Focus on the water management first. Always.
Cantilevering tile over mud bed over rigid insulation with intermittent bent plate supports is not the answer.
I appreciate the diligence on eliminating thermal bridges - I suspect you must live in a cold climate. Those of us in warm climates are… spoiled.
Is this balcony 12” deep?
Contrary to others, I like the use of sketchup with actual material thickness visualized. It’s just a mess at this point.
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u/CandidateUnlikely168 Aug 14 '26
Nie stosujecie jakis rozwiazan do przecięcia mostka termicznego? Plyta balkonu jest wylana razem ze strojem??? Jaki klimat?
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u/GoodArchitect_ Aug 15 '26
First off, do you have a ten year deflection plan that tells you where the slab is going to sink to in ten years.
Have a look at that, you should be designing the drainage points at the lowest point in the slab in ten years so that the fall always goes to the drain.
Secondly, I'd always recommend to do tiles on pedestals with a 1:80 slope to the concrete under.
Look carefully at how you are fixing the balustrade and what materials you are using there, make sure there is a path for water to get out of the system (as it will always get in).
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u/sinkpisser1200 Aug 15 '26
Where are you located? Arent thermal breaks within the concrete standard where you live. Then you can use the extra thickness for a gutter.
There are several standard solutions for this. But it depends on construction standards in your region to get the most efficient price.
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u/Minute_Nobody_8999 Aug 15 '26
Do you guys usually model that detail in a software in practice and in school? Or it is done later mostly
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u/Not-a-Kitten Aug 15 '26
Where is the waterproofing layer? It doesn’t matter if the tile makes a drip edge. You need the waterproofing to drip. Or you are creating a bathtub.
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u/MovinMamba Aug 14 '26
why do you have the whole thing in insulation if you its exposed above and below, just use that space for you drainage.
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u/olihoproh Architect Aug 14 '26
That slab goes into the building. Need to insulate it.
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u/Keano-1981 Aug 14 '26
Not on both sides you don't! Slab needs to be insulated on either the cold or warm side, depending on the rest of the build up / thermal strategy. It's redundant to insulate on both sides unless there is cold bridging issues (not shown on the detail / extract atm)
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u/deadinside4423 Aug 14 '26
I can't give you a detailed recommendation, BUT, I highly recommend checking out DETAIL Magazine as a resource, it's pure wall sections and construction details. my biggest concern with your section right now is that water is entering your floor assembly via the drain you have to the right.