Why Upstands and Penetrations Are Critical on Commercial Flat Roofs

On a large commercial flat roof, the main waterproofed areas can appear relatively straightforward. In practice, some of the most important parts of the roof are the smaller details around its edges, junctions and penetrations.

Rooflights, pipework, ducts, plant bases, outlets and other services all interrupt the waterproofing system. Each creates an area that needs to be carefully incorporated into the roof construction.

When commercial roofs are refurbished or replaced, these details can significantly influence the waterproofing system, specification and amount of work involved.

For building owners, managing agents and property managers, understanding the importance of upstands and penetrations can help explain why two apparently similar commercial flat roofing projects can require very different solutions.

What Is a Roof Upstand?

An upstand is a vertical or raised section where the waterproofing is taken above the main roof level.

Upstands are commonly found around:

• Parapet walls

• Rooflights

• Plant bases

• Access hatches

• Doorways

• Service penetrations

• Changes in roof level

• Other structures projecting through the roof

Rather than terminating the waterproofing at roof level, the roofing system is continued vertically and appropriately detailed at the junction.

These areas form an important part of the overall waterproofing system.

What Is a Roof Penetration?

A penetration is anything that passes through or interrupts the roof construction.

Commercial buildings can contain a considerable number of them, particularly where the roof accommodates mechanical and electrical services.

Common examples include:

• Ventilation ducts

• Pipework

• Cable routes

• Soil vent pipes

• Flues

• Air-conditioning equipment

• Structural supports

• Roof drainage

• Access systems

Each penetration creates a junction between the waterproofing and another component.

The greater the number and complexity of these junctions, the more detailed the roofing works can become.

Why Are These Areas So Important?

Large open areas of waterproofing can often be installed relatively efficiently.

The situation changes around a penetration.

The roofing system needs to transition from the horizontal roof surface and form a watertight detail around an object that may have a completely different shape and material.

Corners, changes in direction and restricted spaces can make these areas considerably more complicated than the main roof.

If detailing isn't appropriate, these junctions can become vulnerable points within the roof construction.

Why Do Commercial Roofs Have So Many Penetrations?

Modern commercial buildings frequently use their roofs to accommodate building services.

Warehouses, schools, hospitals, offices and other large buildings may contain ventilation systems, air-conditioning equipment, pipework, communication equipment and other installations at roof level.

Additional services may also be installed throughout the life of the building.

This can mean an older commercial roof contains considerably more penetrations than were present when it was originally constructed.

We've previously looked at why rooftop plant equipment creates roofing challenges, particularly when equipment and services need to be incorporated into refurbishment works.

Existing Penetrations Need to Be Considered During Refurbishment

When a commercial roof is refurbished, the new waterproofing system needs to integrate with everything already passing through the roof.

That doesn't necessarily mean every penetration needs to be removed.

However, its condition, position and existing detail need to be considered as part of the refurbishment.

Questions may include:

• Is the penetration still required?

• Is the existing detail suitable?

• Is there sufficient upstand height?

• Can the new waterproofing be installed around it correctly?

• Is equipment restricting access?

• Will insulation change the finished roof level?

• Does the penetration need alteration before roofing works begin?

Addressing these questions during planning can help avoid trying to solve complicated details once the main roofing works are already underway.

Insulation Can Change Existing Upstand Heights

Upstands become particularly important when insulation is being upgraded.

Increasing the thickness of a roof build-up raises the finished waterproofing level.

An upstand that was adequate for the existing roof may therefore become significantly lower in relation to the new roof surface.

This can affect rooflights, plant bases, parapets, access doors and numerous other details.

The relationship between insulation and existing roof details needs to be considered when the new roof build-up is developed.

Otherwise, increasing thermal performance in one part of the construction could create problems elsewhere.

Plant Bases Can Be Particularly Complicated

Large items of rooftop equipment are often supported on kerbs, frames or structural supports.

Waterproofing around these areas can become complicated where access is restricted or numerous supports penetrate the roof.

In some situations, equipment may need to be temporarily raised, altered or otherwise accommodated to allow the new roofing system to be installed correctly.

This can require coordination between the roofing contractor and other trades.

It is another reason commercial roof refurbishment can involve considerably more planning than simply measuring the overall roof area.

Pipework and Small Penetrations Still Matter

A small pipe passing through a roof may appear insignificant compared with a large area of waterproofing.

But every penetration still needs an appropriate watertight detail.

Where several pipes or services are grouped closely together, there may be limited space available to install the waterproofing properly between them.

Older roofs can also contain redundant penetrations left behind following previous alterations to the building.

Where appropriate, removing redundant services and making good the roof construction can sometimes simplify the refurbishment and reduce unnecessary detailing.

Rooflights Create Large Upstand Details

Rooflights are another common interruption within commercial roofs.

Depending on the building, there may be dozens of them across the roof area.

The waterproofing needs to integrate correctly with the rooflight kerbs and surrounding construction.

Their condition, height and expected remaining lifespan may therefore influence the refurbishment specification.

Rather than linking to our newly created rooflight article, I'd keep this section self-contained for now because you've asked that we only link to previously confirmed live blogs.

Why Liquid Waterproofing Can Be Useful Around Complex Details

Cold-applied liquid waterproofing systems can be particularly useful where a commercial roof contains complicated shapes, penetrations or restricted details.

The liquid material can be applied around irregular junctions to form a seamless waterproofing membrane.

This can make liquid systems useful around certain:

• Plant bases

• Pipes

• Rooflights

• Outlets

• Upstands

• Complex junctions

PRC installs liquid waterproofing systems from established manufacturers including Triflex, WestWood, Proteus and Polyroof.

However, the presence of penetrations doesn't automatically mean a liquid system is the correct specification for the entire roof.

The existing construction and wider project requirements still need to determine the most appropriate solution.

Built-Up Felt and Single-Ply Systems Also Require Careful Detailing

Complex detailing isn't exclusive to liquid waterproofing.

Built-up felt and single-ply systems also incorporate specific methods for waterproofing around upstands, penetrations and roof interfaces.

The appropriate detail depends on the roofing system, construction and manufacturer specification.

PRC works with a range of established roofing manufacturers, allowing different systems to be considered according to the requirements of the project.

Manufacturer requirements can be particularly important around critical details where different components of the roof meet.

Poor Detailing Can Be Mistaken for a Failure of the Main Roof

When water enters a building, the entire roof covering can sometimes be blamed.

In reality, the defect may be confined to a relatively small detail.

Water ingress may originate around a penetration, upstand, outlet or another junction before travelling within the roof construction and appearing internally somewhere else.

This is one reason identifying the source of persistent water ingress can be difficult.

Our published guide on why roof leaks keep coming back after repairs explains why treating the apparent location of a leak doesn't always resolve the underlying problem.

Future Building Services Should Be Considered Too

Commercial buildings change over time.

New air-conditioning systems, solar installations, ventilation equipment and other services may eventually be added at roof level.

Where future changes are already planned, it can be sensible to consider them during a major roof refurbishment.

Installing a new roofing system and then creating numerous new penetrations through it shortly afterwards can introduce additional interfaces into the finished roof.

Coordination between the building owner, roofing contractor and other relevant trades can therefore be important.

Access Around Penetrations Can Affect the Work

The number of penetrations isn't the only consideration.

Their position matters too.

A roof containing widely spaced equipment may be relatively straightforward to work around, while another roof may contain closely grouped plant with very limited working space.

Restricted access can affect preparation, waterproofing installation and future maintenance.

This supports something we've discussed previously: roof access can have a bigger impact on cost than roof size.

The total square metre area alone doesn't necessarily indicate how complicated a commercial roofing project will be.

Manufacturer Details Matter

Commercial roofing manufacturers develop standard details and installation requirements for their systems.

These can cover common interfaces such as upstands, outlets, penetrations and roof edges.

Following the appropriate system details helps ensure that the individual parts of the roof work together as a complete waterproofing system.

Where a manufacturer-backed warranty forms part of the project, installation in accordance with the relevant specification can be particularly important.

Upstands and Penetrations Should Be Considered Early

Many detailing problems become more difficult to resolve once roofing works have already started.

Identifying complicated areas during project planning allows them to be incorporated into the proposed specification and scope of works.

This is particularly important where other trades may be required to move equipment, alter pipework or remove redundant services.

Good planning can also make quotations more comparable because contractors have a clearer understanding of the detailing involved.

As we've previously discussed, roofing tenders often receive widely different prices, and differences in the complexity and scope of proposed detailing can be one reason.

The Main Roof Area Doesn't Tell the Whole Story

Two commercial roofs could each measure 1,000m² and still require very different amounts of work.

One might contain large uninterrupted areas of waterproofing.

The other could contain dozens of rooflights, plant bases, pipes, ducts, outlets and changes in level.

Although their overall roof areas are identical, the amount of detailing involved may be completely different.

This is why commercial roofing projects should be assessed on the construction and complexity of the roof rather than square metre area alone.

Commercial Flat Roofing Across London, Essex and Kent

Premier Roofing & Construction undertakes commercial flat roof refurbishment and replacement projects across London, Essex, Kent and the wider South East.

We work on commercial, public-sector and multi-residential buildings and install built-up felt, single-ply and cold-applied liquid waterproofing systems from established roofing manufacturers.

The proposed roofing system and detailing are developed around the requirements and construction of the individual building.