Why main contractors should treat lift pits as engineered assemblies

Key Takeaways

• Correct lift pit formation is a programme-critical task, not a finishing detail, because it directly affects drainage, interfaces, and long-term reliability.

• Shop-approved drawings exist to translate design intent into buildable site reality, including void paths, penetrations, and drainage assumptions that are easy to accidentally close up.

• Waterproofing and tanking can sometimes trap water rather than solve it, especially if applied after key interfaces are already in place.

• Sesame Access supports main contractors by diagnosing root cause quickly and implementing practical corrective measures without turning the issue into a blame exercise.

• In this scenario the lift continued to operate correctly, and the solution focused on restoring predictable drainage and protecting the installation for the long term.

Introduction: Why main contractors should treat lift pits as engineered assemblies

A lift pit is often formed early, then surrounded by multiple trades across weeks or months. That makes it one of the most vulnerable parts of the build sequence for unintended changes.

Even small variations in levels, tanking build-up, screed interfaces, boxed-out voids, or sealed penetrations can change how water behaves around the installation. The result is not necessarily a lift fault. More commonly it is a drainage-path fault created by a change in the surrounding construction.

This is why Sesame Access issues shop-approved drawings and expects them to be followed and protected. They are designed to prevent rework, avoid schedule disruption, and keep the installed system serviceable for years.

For broader guidance on drainage fundamentals, see Lift pit drainage and installation guidance and Lift pit drainage best practices.

Common misconception: Why tanking can make pit water problems worse

Why waterproofing can trap water instead of preventing it

A common assumption on busy sites is that tanking a pit solves water. In practice, tanking is a barrier in both directions. If applied without protecting the intended void paths, it can seal natural drainage routes and create a hidden reservoir.

In the transcript discussion, the engineering concern was not simply water ingress. The concern was water becoming trapped behind interfaces and bleeding out through surrounding structure because there was no release path.

One key point expressed was:

“It’s not a case of water getting in being the problem. It’s water getting in and having nowhere to come out.”

The people-first takeaway is simple. Most teams are trying to do the right thing by improving waterproofing. The risk is that well-intended tanking and sealing works can unintentionally close the very voids that allow the system to drain.

Problem: When site changes unintentionally close the designed drainage paths

What typically changes after shop drawings are approved

Shop drawings lock down the engineered intent, including drainage assumptions. The most common causes of unintended drainage blockage during construction are:

• tanking applied after interfaces are installed, sealing voids that were intentionally open

• screed and slab build-ups changing the edge condition at the pit or gate box

• boxed-in channels or pockets filled or sealed during finishing works

• protective membranes or sealants applied across gaps to “make it neat”

• debris accumulation that blocks a low-level exit route

The practical reality is that a lift installation can be correct to drawing, while the surrounding build later alters the behaviour.

That point was made directly:

“We build the lift to the drawing. If something changes after that without us knowing, the system around it changes too.”

Solution: Sesame’s approach is collaborative, diagnostic, and schedule-aware

How Sesame diagnoses pit water issues without disruption

Sesame’s approach is to treat the situation like an engineering diagnosis, not a replacement exercise. The transcript includes several indicators used to narrow down root cause quickly:

• asymmetric staining and wetness can indicate an offset interface or a preferential flow route

• voids behind guide tracks or boxed sections can act as a “water column” if sealed at the base

• some services can be routed inside protected hollow sections, which changes what is genuinely at risk

• drainage restoration is prioritised before component replacement to avoid repeat failure after the next rainfall

The decision logic was also stated very clearly:

“The lift works perfectly. We just need to let the water out.”

This matters because it avoids unnecessary disruption. If the lift is operational and the condition is understood, the goal becomes restoring predictable drainage and preventing recurrence.

Decision Guide: When to intervene immediately vs when to monitor

Step 1: Confirm operational status and visible risk

If the lift is operating normally and there is no visible corrosion on critical components, treat the issue as a drainage-path problem first, not a lift failure.

If corrosion is visible on exposed metallic parts, standing water is persistent, or moisture is migrating into areas that should be dry, escalate to immediate intervention.

Step 2: Start with a minimal test intervention

A controlled test opening can confirm whether a sealed void is holding water. If water releases, the diagnosis is confirmed and the next step is sizing the drainage route appropriately.

This staged approach was described in the transcript as starting small, confirming conditions, then scaling:

“Start with a pilot hole… you’ll know straight away… and then once you hit water… punch a bigger hole… and let that water out.”

Step 3: Do not replace components until drainage is resolved

Replacing components before fixing the drainage route risks repeated flooding and repeat replacement. The sequence is drainage first, then refurbishment or replacement only if required.

Site reality vs design intent: Why shop-approved drawings protect programme

Why shop drawings should be treated as construction-critical

Shop-approved drawings from Sesame Access are not generic. They translate design intent into buildable site intent and include details that are easy to miss in general arrangement drawings, including:

• specific voids that must remain open for services and drainage behaviour

• interface levels that must not be altered by floor build-ups

• intended water escape routes when ingress is unavoidable

• installation tolerances that preserve serviceability

When site conditions need to change, the best outcome usually comes from flagging it early so the design intent can be adapted safely, rather than unknowingly closing off an engineered assumption.

For related pit and drainage planning, link the build team to Lift pit drainage and installation guidance and Lift pit drainage best practices.

Product Integration Summary

The coordination principles in this article apply most strongly to bespoke and low-profile lifts where pits, gates, and finishes are tight, and where water behaviour can be altered by small construction changes.

Relevant Sesame systems include:

• Bespoke Lift with building-specific design for complex architectural constraints and controlled interfaces

• Wellington Lift for compact, design-led installations where pit and drainage details must be formed accurately

• Cavendish Platform Lift for premium fit and finish where sequencing and tolerances protect long-term serviceability

• Windsor Lift for discreet access integration where interface control reduces rework risk

• British Library Platform Lift for public-facing environments where build coordination and drainage control protect high-value finishes

Comparison table: Which situations demand stricter pit control

Situation on site	Why pit control matters	Typical Sesame fit
tight pit depth or low-profile interfaces	small level changes can remove drainage paths	Wellington Lift for compact, design-led installations, Cavendish Platform Lift for premium fit and finish
high-value finishes around the entrance	drainage must be predictable to avoid staining and rework	Windsor Lift for discreet access integration, British Library Platform Lift for public-facing environments
bespoke geometry and multi-trade sequencing	changes without review can close engineered voids	Bespoke Lift with building-specific design

Problem–Solution Summary

Problem:

Late-stage site changes, particularly around tanking, sealing, and finishes, unintentionally closed the designed drainage paths, allowing water to collect in hidden voids.

Solution:

Sesame identified the most likely trapped-water route, applied a staged diagnostic approach, restored a controlled drainage path, and protected the installation without unnecessary replacements.

Outcome:

The lift continued to operate correctly, disruption was minimised, and the installation was future-proofed against repeat water build-up.

Frequently Asked Questions (FAQ)

Why is lift pit formation a main contractor responsibility?

Because the pit is part of the building fabric. If levels, openings, or drainage routes are not formed to the shop-approved detail, the lift may still install but long-term behaviour can change.

Does tanking always prevent water issues in a lift pit?

No. Tanking can sometimes trap water if it seals voids or closes drainage paths. The goal is controlled water behaviour, not just sealing everything.

What is a “shop-approved drawing” and why is it different from an architect drawing?

A shop-approved drawing is a construction-ready document based on the actual lift design, tolerances, and installation realities. It captures interfaces that general drawings often cannot.

If the lift still works, should we just leave it?

Not usually. If water is trapped, it can create future corrosion risk or staining. The right response is often to restore drainage first and only replace components if needed.

What causes water to leak from one side of a lift pit wall more than the other?

Asymmetric wetting often points to a preferential flow path, an offset interface, or a sealed void behind one side that is filling and bleeding out through structure.

How do you fix a flooded lift pit without replacing the lift?

Start by restoring drainage paths. Use staged diagnostics, confirm where water is trapped, then implement a controlled outlet. Replace components only after drainage is stable.

What should a contractor check before pouring screed around a lift pit?

Confirm that planned voids, drainage paths, and interface levels remain open and unchanged from the shop-approved drawings. Screed build-up is a common cause of blocked routes.

Who should be consulted before changing pit tanking details on site?

The lift designer or lift manufacturer should be consulted because tanking changes can affect engineered assumptions around voids and drainage.

Next Steps

If you are managing a programme with a bespoke or low-profile lift, an early pit-formation and drainage review can prevent delay later.

Book a Teams meeting with one of our Project Managers here: https://www.sesameaccess.com/book-a-meeting