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5 common problems you should check in your loft during snagging

Soil stack pipe venting to loft leaves disgusting mark on loft wall

If your new home is covered by an NHBC warranty then you will be familiar with the two-year snagging period which begins when you move in. We’re all pretty good at reporting the issues we see in our living areas and gardens – but how many of us are looking in the loft with the same critical eye?

Here’s a handy list of the things you really need to look out for. It doesn’t take much effort but could save you from serious problems in years to come. Once your two years is up then most of these issues are no longer covered. For the complete legal documentation of what is covered by your NHBC Warranty and what is not, see your relevant NHBC Buildmark policy booklet here.

So borrow or buy a step ladder, grab a torch and take 5 minutes to spot these usual suspects:

1. Soil Vent Pipe Venting Into Loft

Yep, the soil pipe is the one you’re thinking of. The ventilation for the waste from your toilet. Yuck. This large, grey pipe is supposed to reach your roof or an external wall and be vented to the outside air. Not, as we so often see it, venting to the loft space. Disgustingly, we can often tell when a loft has this issue as soon as we open the loft hatch just from the smell.

Soil stack pipe venting to loft leaves disgusting mark on loft wall
10 years of fumes from the soil stack. Shown with the black cap (Air Admittance Valve) fitted by us.


According to the Scottish Building Regulations Technical Handbook Section 3.7.8:

A Wastewater drainage system serving a building should […] minimise the possibility of foul air entering the building.

The NHBC Standards 2019 (Chapter 8.1.6) goes into more detail, stating that soil and waste systems should be:

  • arranged to ensure foul air from the drainage system cannot enter homes
  • fixed neatly and securely to provide the correct falls
  • fitted to prevent the entry of vermin

Clearly, any soil stack which is open-ended in the loft as per the image above does not meet these requirements.

There are a huge number of requirements surrounding the correct installation of soil stack vents like this in new build homes, so it is important they are fitted by a professional plumber. Requirements specifying their geometry, their connection to the roof, the fitment of AAVs (Air Admittance Valves) and more are all tightly controlled and must be adhered to so that problems can be avoided down the line.

More detailed requirements can be found in BS EN 12056 Gravity drainage systems inside buildings, and BS EN 12380 Air admittance valves for drainage systems.

2. Flexible Ducting Used for Soil Vent Pipe

In loft soil vent pipe poorly connected to flexi pipe with yellow tape and venting fumes into the loft

For exactly the reason demonstrated in the image above, it is explicitly stated that flexible ducting is not suitable for connecting the soil vent pipe to the roof vent. NHBC Technical Guidance document 8.1/07 is very clear on the issue:

The soil & vent pipe should be terminated close enough to the vent tile to use the pipe supplied with the ventilation tile without extension. Flexible ducting (of the type used for extract ventilation) may not be sufficiently durable for this use. It may be difficult to achieve an air-tight joint at the ends of the flexible duct.

Flexible ducting (of the type used for extract ventilation) is not suitable for this purpose. Where necessary, the soil & vent pipe should be extended using rigid plastic fittings.

There is no excuse whatsoever for fitting flexible ducting like this. Choosing and fitting the correct rigid pipe parts takes longer but instead it seems all too common to see cheap, unsuitable flexible pipe used like this.

It should be noted that for ducting from extractor fans however, flexible duct can be used in some cases – but must be done in accordance with the regulations. See the next item…

3. Extractor Fan Ducting Poorly Designed or Fitted

Flexible ducting is generally quite flimsy and easily squashed. Often, we see it collapsed under insulation or held too tightly by brackets. This can prevent good extraction of the moisture from your bathroom and allow damp or mould to occur.

Squashed flexible ducting in loft from extractor fan

The relevant guidelines for compliance with building regulations are provided in the Scottish Government’s Technical Handbook 2017: Domestic Buildings Section 3.14.9 (Mechanical Ventilation). These guidelines are supported by the Scottish Government’s Building Standards Supporting Guidance – Domestic Ventilation 2nd Edition, which provides the following recommendations for flexible ducting when used for mechanical ventilation.

It states that flexible ducting:

  • should be pulled taut to ensure that the full internal diameter is obtained  and flow resistance minimised. This is considered to have been achieved if the duct is extended to 90% of its maximum length;
  • should be supported at suitable intervals to minimise sagging. Refer to manufacturer’s information but generally it should be supported at no greater than 600mm intervals;
  • bends in ducts should have a minimum inside radius equal to the diameter of the duct. If tighter bends are required, rigid bends should be used; and
  • perforated insulated flexi duct, used to minimise airborne sound transmission, should not be used between the fan unit and the external discharge terminal to prevent condensation occurring within the insulation material

The NHBC Standards 2019 Chapter 8.1.12 agrees with the building standards guidance, and states in relation to Extract ducts:

Rigid duct is preferable to flexible, but where flexible duct is used, it should be restricted in length to ensure that airflow resistance does not prevent the designed ventilation rate from being achieved.

Flexible duct should be installed straight

Bends should generally be formed with proprietary rigid components. Where flexible duct is used to form bends they should be restricted to a maximum of two for systems of up to 30l/s and one for extract rates higher than 30l/s

The Technical handbook also requires installation of mechanical ventilation be fitted in accordance with the requirements of CIBSE Guide B2:2001, Ventilation and Air Conditioning. The guide cautions:

[It] should be noted that ductwork lengths of as little as 1m can considerably impair performance due to pressure losses (pp20-21)


Long flexible insulated ducting in loft

Although not explicitly a requirement for housebuilders in Scotland, the Domestic Ventilation Compliance Guide (2010) for England provides further useful relevant information and recommends:

The maximum length for flexible ducting should be no longer than 1.5 metres for axial fans, 6 metres for centrifugal fans (up to 30 l/sec) and 3 metres (from 31 l/sec to 60 l/sec). Depending on the flow rate, the maximum number of bends the flexible ducting system can have is 2.

Given that we almost always only see axial fans in the houses we survey, the long flexible ducting we usually see is most likely rendering the bathroom extraction utterly useless – and may even be introducing more problems than it is fixing.

4. Extractor Fan Ducting Uninsulated

The Scottish Government’s Building Standards Supporting Guidance – Domestic Ventilation 2nd Edition requires that when a duct passes through an unheated space, the duct should be:

insulated with at least 25mm thick insulation having a thermal conductivity of 0.04W/m²K. This prevents the cooling of the duct, thus maintaining the stack effect and reducing the risk of condensation occurring within the duct.

The NHBC Standards 2019 agree and provide an identical requirement (Chapter 8.1.12 Control of Condensation). In addition, they provide two alternatives to using insulated ducting – either by using a condensate trap that discharges to the outside or by installing the ducting to slope continuously to the outside as shown in the following image:

Diagram, NHBC 8.1.12 Control of Condensation

If uninsulated flexible ducting is used, the condensation forming in the pipe can run back down through the extractor fan, as this tradesman discovered and posted in the IET Forum:

Screenshot of IET forum - extractor fan leaking water due to condensation from uninsulated flexible ducting in loft

This problem will be especially noticeable in winter, when there is the biggest difference in temperature between the warm air of the bathroom and the cold air of the loft.

5 Connected with Duct Tape (or similar)

Duct tape is rarely a suitable construction material and in some documents is explicitly forbidden. The NHBC Standards (8.1.12 Installation) specifies that ductwork:

should be installed in a neat and workmanlike manner, be securely fixed, and have:

  • adequate support throughout its length
  • sealed mechanically fixed joints and connections

I would argue that connections held by sticky tape of any kind could not be considered as sealed mechanically nor securely fixed.

In England and Wales Building Regulations provide high level requirements and are supported by more detailed Approved Documents. Approved Documents F, L1A and L1B which apply when installing fixed ventilation systems in new buildings make further references to specific documents which give more detailed advice on how to comply with all requirements. One of those is The Domestic Ventilation Compliance Guide 2010.

Connection of lengths of flexible duct must use a rigid connector and jubilee clips or similar to ensure a long term seal is achieved. Connections of lengths of flexible duct should not be taped-only.

Domestic Ventilation Compliance Guide, Table 5: System 3 installation guidelines

The Building (Scotland) Regulations 2004, Schedule 5, Section 3: Environment, Paragraph 3.14 states:

Scotland Building Standard 3.14 Ventilation

Arguably, failure to comply with the guidelines above and the resulting damp issues could pose a long-term threat to the building or occupants and breach this mandatory Regulation.

This is not an exhaustive list of the issues we experience in new-build lofts, so look out for our upcoming articles covering some of the other issues we often encounter in our customers’ lofts. I hope this has been useful to you.




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