Commercial Roof Drainage: Why It Should Be Considered During Refurbishment

When planning a commercial roof refurbishment, much of the attention naturally focuses on the waterproofing system.

However, even a newly refurbished roof needs to manage rainwater effectively.

Poor falls, inadequate outlets, deteriorated gutters or changes to the existing roof build-up can all affect how water drains from the roof. If these issues aren't considered as part of the refurbishment, problems with standing water and drainage can remain even after new waterproofing has been installed.

For this reason, drainage should be considered alongside waterproofing, insulation and detailing when planning a commercial flat roofing refurbishment.

Why Is Roof Drainage So Important?

Commercial flat roofs are designed to direct rainwater towards outlets, gutters or other drainage points.

Unlike a pitched roof, water doesn't leave the roof particularly quickly simply because of its shape. The falls within the roof construction help direct water towards the appropriate drainage points.

Problems can develop when:

• Outlets become blocked or deteriorated

• Gutters are damaged or undersized

• Roof falls are inadequate

• Previous roofing works alter drainage routes

• New plant or equipment obstructs water flow

• Roof levels change

• Existing outlets are positioned poorly

• Debris accumulates around drainage points

On a large commercial roof, even relatively small drainage problems can affect significant areas.

What Causes Standing Water on Commercial Flat Roofs?

Some temporary surface water following rainfall can occur on flat roofs, but persistent areas of ponding can indicate that water isn't reaching the drainage system effectively.

Possible causes include inadequate falls, settlement within the roof construction, blocked outlets or localised low points.

Previous refurbishment works can also influence how water moves across the roof.

For example, localised overlays or repairs carried out over many years can gradually create changes in level that weren't part of the original roof design.

The result can be areas where water regularly collects rather than flowing towards an outlet.

Does Standing Water Mean the Roof Is Leaking?

Not necessarily.

A properly specified waterproofing system should be capable of protecting the building from normal exposure to rainwater.

However, persistent standing water can highlight underlying drainage issues and may place additional demands on certain areas of the roof.

It can also allow dirt and debris to accumulate and make routine maintenance more difficult.

Most importantly, if a roof is being substantially refurbished, it makes sense to understand why water is collecting rather than simply installing new waterproofing over the same roof profile.

Why Drainage Should Be Reviewed During Roof Refurbishment

A major refurbishment provides an opportunity to consider the roof as a complete system.

That means looking beyond the existing waterproofing and considering how water actually leaves the roof.

Before or during the works, this may include considering:

• Existing roof falls

• Number and position of outlets

• Condition of outlets

• Gutters

• Downpipes

• Overflow arrangements

• Localised low points

• Roof levels

• New insulation

• Rooflights

• Plant and penetrations

Where existing drainage arrangements are suitable, they may simply need to be incorporated correctly into the new roofing system.

Where problems already exist, refurbishment may provide an opportunity to address them.

Roof Outlets Are Critical Details

Roof outlets are relatively small components compared with the overall roof area, but they perform an essential role.

They also create a junction within the waterproofing system, making correct detailing particularly important.

Problems around outlets can include deterioration of the existing waterproofing, blocked drainage, poor previous detailing or changes in level that prevent water reaching the outlet effectively.

When a roof is refurbished, the new waterproofing needs to be properly integrated with the drainage system.

The exact detail will depend on the selected roofing system and outlet arrangement.

Large Commercial Roofs Can Collect Significant Amounts of Water

Drainage becomes particularly important on warehouses, industrial units, schools and other buildings with very large roof areas.

Thousands of square metres of roof can collect a substantial volume of rainwater during heavy rainfall.

That water needs to be directed efficiently towards the drainage system.

A drainage arrangement that is problematic on a small roof can become much more significant when applied across a large commercial building.

What About Gutters and Downpipes?

Not every commercial flat roof drains through internal outlets.

Many buildings direct water towards perimeter or valley gutters before it enters downpipes.

The condition and capacity of these components therefore need to be considered alongside the main roof.

Problems may include:

• Corrosion

• Failed joints

• Damaged linings

• Blockages

• Poor falls

• Deteriorated outlets

• Inadequate capacity

• Previous repairs

Installing a new waterproofing system while leaving a failing gutter arrangement untouched can mean water-management problems remain after the refurbishment.

Insulation Upgrades Can Affect Drainage

Commercial roof refurbishment can involve upgrading the existing insulation.

This is particularly common where a new warm-roof construction is being installed.

Adding insulation changes the roof build-up and can potentially affect finished roof levels.

This means outlets, upstands, thresholds, rooflights and other details need to be considered as part of the new construction.

In some situations, insulation can also be used to help create or improve falls within the roof build-up.

The proposed insulation and drainage strategy therefore need to work together rather than being designed independently.

Rooflights, Plant and Penetrations Can Affect Water Flow

Commercial roofs are rarely completely clear.

Rooflights, air-conditioning equipment, pipework, ducts and other rooftop installations can all influence how water moves across the roof.

Poorly positioned equipment or alterations made after the original roof was constructed can create areas where water becomes trapped.

We've also previously explained why rooftop plant equipment creates roofing challenges, including the additional detailing required around equipment and penetrations.

Can New Waterproofing Fix Poor Roof Drainage?

New waterproofing can address deterioration within the existing waterproofing system, but it doesn't automatically correct the shape or drainage characteristics of the roof beneath it.

If water is collecting because of inadequate falls, poor outlet positioning or other construction issues, simply installing another waterproofing layer may leave the same drainage pattern in place.

This is particularly important when considering an overlay.

An existing roof may be suitable to receive a new waterproofing system, but its drainage still needs to be considered as part of that decision.

Our guide to commercial flat roof repair, overlay or replacement explains some of the wider factors that determine whether an existing roof is suitable for refurbishment without complete removal.

What Happens if Roof Drainage Is Ignored?

Drainage issues don't necessarily mean a newly refurbished roof will immediately fail.

However, ignoring known problems can leave the building with avoidable maintenance issues.

Persistent ponding can make maintenance more difficult, encourage debris accumulation and place greater demands on outlets and localised roof details.

More significantly, unresolved drainage defects can mean the refurbishment hasn't addressed one of the underlying problems affecting the existing roof.

How Does the Roofing System Affect Drainage Details?

Different waterproofing systems require different detailing around outlets, gutters and other drainage components.

Built-up felt, single-ply and cold-applied liquid systems can all be used successfully on commercial roofs when appropriately specified.

The important point is that the drainage details form part of the overall system.

PRC works with established roofing manufacturers including Bauder, Garland, Langley, Alumasc, Soprema, Triflex, WestWood, Proteus, Polyroof and Nimbus.

Manufacturer requirements can influence how outlets and other critical details are incorporated into the finished roof.

Drainage Should Be Part of the Overall Roof Specification

Commercial roof refurbishment works best when individual elements aren't considered in isolation.

The waterproofing, insulation, drainage, rooflights, plant, penetrations and existing roof construction all interact.

The specification should therefore consider how these components work together.

This can be particularly important on older buildings where previous repairs and alterations have changed the roof over many years.

Addressing these issues as part of a planned refurbishment can provide a more complete solution than simply replacing the visible waterproofing.

Commercial Roof Refurbishment Across London, Essex and Kent

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

Our work includes warehouses, schools, public buildings, housing developments and other commercial properties.

We install built-up felt, single-ply and cold-applied liquid waterproofing systems from established manufacturers, with specifications developed around the requirements of the individual roof.