Who this guide is for
- Owners adding or replacing guarding on a terrace or balcony
- People whose guarding base fixings are showing staining or movement
- Renovators making an existing flat roof accessible
- Anyone coordinating a roofing, surfacing and metalwork package
- Readers briefing an engineer about an edge over occupied space
Two requirements meeting at one hole
The anchorage is where the load path completes, and it is normally covered by a plate, a finish or a surface build-up as soon as it has been made. On an external edge it is simultaneously a hole in a layer that is shedding water, and the two requirements work against each other.
The published entry sets out the tension precisely: a rigid fixing concentrates movement at the closure around it, while a fixing allowed to move slightly protects that closure but softens the chain. Neither is resolved by sealant alone, which is why guarding over a water-resisting layer is designed in from the start rather than added later.
Sealing a moving structural fixing is a different problem
Where the base anchorage passes through a layer shedding water, the closure at that point has to survive a joint being loaded and moved. Closing a moving structural fixing is not the same problem as closing a static penetration, and treating it as one is named in the entries as a misunderstanding: sealing around a base fixing is thought of as waterproofing, when it is a joint loaded and moved every time the guard is used.
The corresponding condition to design out is standing water at the foot of a post. At an external base fixing, water reaches a structural connection and a penetration in the same place, and keeping it away does more than any product applied at the penetration itself.
What each surfacing permits
Loose units cannot support a guard, so posts pass through the paving layer to structure below, and unit cutting, pedestal positions and the closure at each pass all have to be coordinated with how the paving is set out. Every one of those passes is a penetration of the layers below that has to be closed and has to stay reachable.
On a seamless bonded surfacing, fixings breach the layer resisting water at a point that is also structurally loaded, so fixing positions belong in the surfacing design rather than in a later trade's method. On a timber deck the anchorage lands in joists, trimmers or posts forming part of the deck structure, which makes guarding a framing decision taken with the deck rather than after it.
- Loose-laid units: the post passes through to structure, and the pass stays reachable
- Bonded seamless surfacing: fixing positions are part of the surfacing design
- Bedded rigid paving: each penetration is a discontinuity in a bond that cannot drain
- Framed timber deck: the members have to be in the right places before the boards go on
The alternatives that avoid the penetration
The published entries describe two. On a roof terrace, the guard has to be supported by structure lying below insulation and a water-resisting layer, and reaching it means either penetrating those layers or using a support arrangement designed to avoid doing so, which is settled early.
On an externally supported balcony, guarding is often part of the same fabricated assembly as the platform frame, which removes fixings through the walking surface but concentrates load into the brackets. The two then form a single structural item and cannot be altered independently, which is a different obligation rather than a smaller one.
Where the load goes changes what can be reached
On a projecting balcony, a guard standing on the top surface puts those fixings inside the area the platform has to drain, while an edge-fixed guard moves them onto the face below the line water leaves at. The two arrangements ask different things of the drip and the falls.
They also differ afterwards. Fixing down onto the top surface delivers the load to the projection in bending, while fixing to the outer edge delivers it to the slab edge, and which member ends up receiving it settles whether the anchorage can be reached again once the surface build-up is complete.
The substrate question, which does not go away
Guarding fixed to a finish, a coping, an outer leaf or a surface build-up has been fixed to something that was never designed to take it. Where a parapet already exists the guard may sit on it, behind it or through it, and anchoring into a coping or into the top of a parapet puts the fixing into the most weather-exposed and often the least structural part of that construction.
The same applies at the top of a retaining structure. Guarding applies horizontal force at the very place a gravity wall is least able to accept it, and posts set into the top of a layered reinforced structure cut reinforcement locally while loading a face designed to carry very little.
Who owns the junction
This assembly meets a surface, a structure and often a water-shedding layer at the same point, and coordination between the trades responsible for each is where most of the difficulty lies. The guarding, the waterproofing and the surfacing are commonly drawn by three different people, and the junction belongs to all three and to none.
Post position and structure position have to agree, and the agreement has to be reached before the surface below is finished. Once a deck, terrace or slab is complete, the available positions are limited to whatever lies underneath, and the usual response of shifting a post or adding a plate changes both the loads on the remaining posts and the relationship of the fixings to the layers below.
Guarding penetration planning checklist
- 1Establish what structure lies below each proposed post position
- 2Confirm the post positions were agreed before the surface below was finished
- 3Establish whether the anchorage passes through a layer that is shedding water
- 4Ask what closes that pass, and how that closure survives being loaded and moved
- 5Design out standing water at the foot of every post
- 6Ask whether a support arrangement avoiding the penetration is available here
- 7Where guarding is part of a fabricated platform, note the two cannot be altered separately
- 8Decide whether the guard is fixed down onto the surface or to the edge, and what that means later
- 9Check the fixing is not landing in a coping, a finish or a surface build-up
- 10Coordinate unit cutting and support positions with the fixing route through the surfacing
- 11Ask whether the base fixings can be exposed, checked and tightened later
- 12Record the infill, the fixings and the substrate they went into
Common mistakes to avoid
- Adding guarding to a finished terrace and fixing it to whatever is there
- Treating sealant around a base fixing as the waterproofing
- Anchoring into a coping or the top of a parapet because it is convenient
- Fixing a post to loose-laid paving rather than through it to structure
- Substituting an infill panel and treating it as a like-for-like change
- Leaving the junction between three trades unowned by any of them
- Choosing guarding on appearance when the anchorage decides how it behaves
When to involve a professional
- Guarding is safety-critical and its loading requirements are set by others
- Existing guarding of unknown construction is assessed by a competent person
- Ask what each base fixing lands in, and how that was verified
- Ask how the water-shedding layer is closed where the anchorage passes through
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Can guarding be fixed to the paving on a terrace?
Loose units cannot support a guard. Posts pass through the paving layer to structure below, so the unit cutting, the support positions and the closure at each pass all have to be coordinated with how the paving is set out rather than resolved when the guard arrives.
Is sealing the fixing enough to keep water out?
The entries treat that as a misunderstanding. Sealing around a base fixing is thought of as waterproofing, when it is a joint loaded and moved every time the guard is used. Keeping standing water away from the foot of the post does more than any product applied at the penetration.
Why can't a post simply be moved to a better position?
Because the available positions are set by what lies underneath. Once the surface is complete, shifting a post or adding a plate changes both the loads on the remaining posts and the relationship of the fixings to the layers below, so it is rarely the small change it looks like.
Can I change the infill panels without touching anything else?
Substituting one infill for another is a change to the whole assembly. A different panel changes the weight the posts carry, the way force is delivered into them, and the gaps left between elements, and the entries treat that as a safety-critical alteration rather than a material preference.
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