Covered by an NHBC warranty? Then you’re familiar with the two-year snagging period. We’re all pretty good at reporting the issues in our living areas and gardens. But the loft is easily overlooked. So here’s a handy loft snagging checklist for you – based on our years of experience in surveying lofts in Scottish homes.
To get started, borrow or buy a step ladder, grab a torch and take 5 minutes to spot these usual suspects. And don’t dawdle: It’s a quick thing to do and it can save you from serious and costly issues in the future.
(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.)
1. Soil Vent Pipe venting into Loft
First of all, look for the soil vent pipe. This large grey pipe is supposed to reach your roof or an external wall to be vented to the outside air. Not, as we so often see it, venting into the loft space.
At least, it’s an easily identifiable issue in this loft snagging checklist as you can often already tell from the smell as soon as you open the loft hatch.
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 does not meet these requirements.
There are a huge number of conditions surrounding the correct installation of soil stack vents in new build homes. So, it is important they are fitted by a professional plumber.
Specifications regarding their geometry, their connection to the roof, the fitment of AAVs (Air Admittance Valves) and more are tightly controlled and must be adhered to if you want to avoid problems 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. Soil Vent Pipe with Flexible Ducting
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. But it’s a common sight in Scottish lofts.
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 for fitting flexible ducting like this. Choosing and fitting the correct rigid pipe parts takes time, so it’s very common to see cheap, unsuitable flexible pipe used like this.
Please note: In some cases, ducting from extractor fans can be flexible. But it must be done in accordance with regulations. Which takes us to the next item on our loft snagging checklist…
3. Poorly Designed/Fitted Extractor Fan Ducting
Flexible ducting is generally quite flimsy and easily squashed. That’s why we often see it collapsed under insulation or held too tightly by brackets. This can prevent good extraction of moisture from your bathroom and allow dampness or mould to occur.
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 flexible 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 the installation of mechanical ventilation to 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)
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, we had to add this to our loft snagging checklist. 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 (supposed to be) fixing.
4. Uninsulated Extractor Fan Ducting
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:
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:
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. So make sure to tick this off your loft snagging checklist!
5. Duct Tape Connections (or similar)
Duct tape is rarely suitable construction material and, in some documents, is explicitly forbidden. The NHBC Standards (8.1.12 Installation) specify 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. These give more detailed advice on how to comply with all requirements, like 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:
Arguably, failure to comply with the guidelines above and the resulting damp issues could pose a long-term threat to your home and breach this mandatory Regulation.
6. No Access for Maintenance
Do you have permanent equipment in your loft that needs regular maintenance? Then make sure you have suitable access to your loft as required by the NHBC in Chapter 7.2 Pitched Roofs. Section 7.2.12 (Access).
According to this, access should:
- Be provided to the main roof space and voids which contain cisterns and tanks etc. though it is not required to roof spaces which contain only water pipes.
- Permit the removal of permanent equipment (e.g. heating and ventilation plant) located in the roof space.
- Have minimum opening width of 520mm in each direction.
- Not be located directly over stairs or in other hazardous locations.
- Include securely fixed boarded walkways between the opening and the cistern or other permanent equipment; boarding should be securely fixed without compressing the insulation; at each piece of permanent equipment or cistern, a minimum 1m2 platform should be provided to facilitate maintenance.
Anything less than that needs to go on your snagging list.
7. Insufficient Loft Insulation
The requirements for loft insulation set out by building regulations in Scotland and the rest of the UK have steadily increased over the years.
According to comparison site uSwitch, the current recommendation for blanket style loft insulation is 250-270mm. This should be fairly consistent throughout the whole loft area. So, look for any low points where insulation needs to be topped up.
The actual regulations do not specify the thickness of the insulation layer, but rather specify overall thermal efficiency values permissible for walls and ceilings. Find out more in Scotland’s Building Standards Technical Handbook.
8. Lack of Ventilation
Another issue we often encounter on lofts is too much insulation. More specifically – insulation restricting the ventilation path from under the eaves into the loft space. To maintain a gap between the roof and the top of the loft insulation, a spacer is required to ensure adequate ventilation. This is essential to prevent condensation and mould.
NHBC Standards Section 7.2.15 states that:
A spacer in the eaves should be used to allow insulation to be installed over and beyond the wall plate to minimise the cold bridge without blocking the ventilation path (the spacer should be of sufficient length to maintain ventilation above the insulation).
So, when going through your loft snagging list, don’t forget to check if your insulation gets enough ventilation. Otherwise, any condensation that forms naturally, especially during the colder months, won’t evaporate. This will lead to mould and subsequently render your loft insulation useless.
9. Modified/Damaged Trussed Rafters
We see this all the time: Plasterers need to reach the apex of the roof, so an extra beam is fitted when creating the partition wall between two properties. The problem is that your roof trusses have not been designed to support a load in this way.
In this case, they were most likely not affected by one individual standing on the beam. But I doubt that any calculations were done prior to fitting the beam to verify this.
Aside from that, it simply is not right and, most of all, there is no reason to leave the beam after work has finished.
Another issue we often encounter that should be on your loft snagging checklist is damage to trusses. The split seen in the image above is a good example of that.
Damage like that can weaken the mechanical properties of your roof trusses. So, make sure to get advice from a structural engineer and get it repaired before the snagging period is over. This will most likely involve bolting a section to either side of this timber section to reinforce it.
These days, trusses are made offsite at specialist manufacturers. As such, they may travel hundreds of miles before being installed in your new house. So they will have been picked up, put down, moved around and stored in different locations, mostly outdoors.
There are strict requirements defined by the NHBC (Chapter 7.2.4) for the protection of trusses whilst on their journey, and if damaged prior to installation should be rejected and not repaired.
10. Upside-Down Roof Vents (yes, really!)
Do you have an unexplained source of damp directly below a pipe which vents out of your roof? Or is there water getting into your extractor fan somehow? Then check if the cause is an upside-down roof vent.
We’ve seen this (and similar things) often enough to include it in our loft snagging checklist. So, it’s definitely worth double-checking and not dismissing some of the more curious explanations right away.
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