Everything You Need to Know About Roof & Loft Ventilation

By a former roofing contractor with over 10 years experience

Written by Daniel Woodley. Fact checked by Paul Farley. Published to Prices. Updated: 8th March 2023.

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Warm moist air created by the occupants of a property will always rise to the highest point in the home, which is a loft or roof void, and in winter it may come into contact with cold timbers and roofing felt, where it condensates.

I used to work as a roofing contractor and every December I would get calls from customers with this issue – they had gone into their loft to get out the Christmas decorations, only to find the entire loft soaked. I even saw a few cases where there were huge puddles of water on top of the insulation.

The main cause of this moisture is:

The occupant’s lifestyle: Cooking, showering, sweating and even breathing causes moisture to build up in the home.

Roof leaks: These can allow water into the roof or loft space and even minor leaks can cause issues if the roof isn’t vented as the moisture will have nowhere to escape.

Newly built homes: New build homes will have moisture trapped in the timber and other materials; this can take a year or two to fully dry out, hence why ventilation is crucial.

I’ve also seen several cases where the occupant experienced a loft condensation problem after replacing their double-glazed windows – the old windows were drafty and vented the property, but the new ones were airtight, this led to an increase in the amount of moisture rising into the loft.

The Solution

The solution is to install ventilation so the moisture can escape rather than build up and cause problems.

If your property is experiencing condensation, mould, damp or associated issues in the loft or roof space, please read this guide, which is based on my 10+ years of experience.

DIY Option: Solve Loft Condensation Yourself

Retrofitting ventilation to meet the current Building Regulations is costly, time-consuming and the work must be done from the outside of the property, so the workers may need to erect scaffold.

For most homes in the UK that have an attic with insulation placed on the loft floor, that will mean installing vents at the top and bottom of the roof to create airflow.

There is a cheap alternative that takes minutes to install, is DIY friendly and works just as well:

Loft vents

Loft lap vents only cost a few pounds each and fit snugly between the roof rafters where they open up the flat lap just enough to create airflow in the loft.

I’ve found that a pack of ten is enough to get rid of condensation, mould and dampness in a typical small loft.

I’ve installed these dozens of times, and anyone who can safely get into a loft should be able to fit them, but they must be careful not to tear the felt or fall through the ceiling. The feedback I had from my past customers was very good, and I tested them in my loft too – I could actually feel a slight breeze after fitting them.

Professional Option 1: Exterior Vents

Ideally, homes should have ventilation at the lowest part of the roof (near the gutters) and also at the highest part (near the ridge).

Unfortunately, plenty of homes have been constructed with no or inadequate ventilation.

One option is to instruct a roofing firm to install ridge vents and eaves vents. I’ve fitted hundreds of these in the past and they work very well:

Eaves felt overhanging into gutter

Eaves ventilation strips just above the gutter

Roof ridge vent

Ridge vents

Tile vents

Vent tiles

Soffit vents

Circular soffit vents

Professional Option 2: Cost to Replace Felt in The Loft

I don’t recommend this option as it’s rather drastic – replace non-breathable roofing felt with a breathable membrane felt.

This will allow moisture to escape from the loft, directly through the roofing felt.

Unfortunately, the only way to replace the felt in a loft is to replace the entire roof. This will involve removing all the roof tiles, replacing the felt and relaying the tiles – a costly exercise.

One cannot replace the felt from within the loft as all work is external.

I feel that breathable felt membranes are best installed on roofs as they’re being built rather than retrofitted.

Flat Roof Ventilation and Condensation

Condensation doesn’t just affect pitched roofs but can also affect flat roofs.

From what I’ve seen it usually affects “cold deck” flat roofs – these are flat roofs where the insulation is placed directly above the ceiling and there is a cold area between the insulation and the roofing material above.

“Cold deck” flat roofs should be vented at each end of the roof to prevent moisture buildup. I’ve seen condensation form when these vents were missing.

“Warm deck” flat roofs have insulation placed directly under the roofing material, and the void and timbers underneath are the same temperature as the room below.

Thus “Warm deck” flat roofs should not be vented.

(Not sure if your flat roof is a warm or cold deck? Tendring District Council’s Building Control Department published this quick guide.)

Flat Roof Condensation Solutions

If the flat roof is a “warm deck” it should be constructed with a vapour control barrier and should be sealed with no airflow. Any windows and doors in the property should have trickle vents fitted to prevent moisture buildup inside the home.

If the flat roof is a “cold deck” and is experiencing issues with condensation, consider asking a professional to:

  1. Make sure there is a 50mm air gap above the insulation.
  2. Install ventilation in the form of circular soffit vents, fascia vents or similar at one end of the roof.
  3. Install a 25mm continuous vent at the opposite side of the roof.
  4. Consider mushroom vents as a last resort.

If a “cold deck” flat roof cannot be adequately ventilated, you should consider changing it to a “warm deck” flat roof, although this won’t be cheap.

Edit: You may find this video by London Roofer insightful.

Cost to Ventilate a Roof

Below is a price guide for retrofitting ventilation products.

The figures exclude scaffold or specialist access equipment and are based on the cost of the materials and how long the project will take.

These are estimates, just what I would charge if I were still working in the roofing industry, and they assume a best-case scenario, more complex projects will be more expensive.

Project:Price inc VAT:
Install 10 lap vents in a loft£2-£3 each installed DIY or £20-£30 + 1 hour labour
Install 10 circular vents to a soffit on roof eaves or flat roof edge£350 - £400
Install 10 metres of 12mm fascia ventilation to a roof eaves£400 - £500
Install 4 universal tile vents£500 - £750
Install 4 ridge vents£400 - £500
Install 4 mushroom vents to a flat roof£400 - £500
Install 25mm vent to abutment wall on a flat roof and install new flashing£250 - £500 per linear metre (depending on overall length, complexity of project and roof covering type)

The prices above exclude scaffold or specialist access equipment which can add a significant cost to the project.

(Explore our guide to scaffolding hire prices here)

Get Your Custom Roof or Loft Ventilation Quotation Today

I hope you found these insights into roof and loft ventilation helpful.

As every project is different, the next step is to get a no-obligation custom quote for your roof:

  • Submit details of your roof project.
  • Arrange a convenient time for a roof inspection.
  • Get your custom quote.
  • Compare prices.

Roof Ventilation FAQs

Should all roofs and lofts be ventilated?

All cold roofs and lofts – that’s where the insulation is typically placed above the ceiling – should be vented. These spaces are cold and prone to condensation.

Warm roofs – that’s where the loft or roof void is the same temperature as the property below – shouldn’t be vented. Instead, trickle vents should be fitted to the doors and windows to create an airflow in the home to pull out the bad air.

Is loft condensation normal?

Yes, some condensation is normal, especially if the occupants are creating lots of moisture from bathing, cooking and drying clothes on radiators etc, and the loft is very cold.

The condensation should clear after adequate ventilation is installed, assuming there is some wind.

I have seen well-vented lofts suffer from condensation problems, but this was due to:

  • No wind – without adequate breeze, there’s no airflow but this is usually only temporary.
  • Downlights or other holes in the ceiling allowing excess moisture into the loft.
  • The occupants creating excessive amounts of moisture (drying clothes on radiators etc).
  • The occupants making the property below more airtight (no trickle vents on the windows, excessive use of draft excluders etc which increased the amount of moisture in the home, which rose into the loft.)

Is it normal to see condensation in the loft in the summer?

This is very rare, look for:

  • Roof leaks.
  • Pipe leaks.
  • Chimney leaks.
  • Gutter leaks.
  • Anything in the loft that could be producing moisture, like a hot water tank with a broken thermostat.

What's the cheapest way to get rid of loft condensation?

Loft lap vents can be installed DIY and cost £2 – £3 each, I’ve found them easy to install.

Are lap vents suitable for my loft?

Here you can find more information about how to cure loft condensation with lap vents.

Can a dehumidifier help reduce condensation in the loft?

Yes, but they require electricity and most will have a bottle that needs to be emptied, so I don’t feel they are a long-term practical solution.

I see dehumidifiers as a short-term, temporary solution and I think you should only use them until you can find out what the underlying cause is and fix it.

These are the best non-electric dehumidifiers I could find and are perfect for short-term, emergency use.

Will loft condensation damage the roof?

Only if the loft isn’t adequately ventilated and the moisture can’t escape. I have seen condensation lead to mould, damp, odours and eventually the rot and failure of roof timbers, but in every case, this was due to a lack of ventilation.

Why has condensation suddenly appeared in my loft?

Any changes made to the property that reduces ventilation can lead to a buildup of moisture that rises into the loft.

I have seen the following lead to condensation problems:

  • New windows and doors without adequate trickle vents.
  • Door draft excluders.
  • Building work, extensions etc where there’s inadequate ventilation.
  • Some types of insulation will reduce breathability (inc vapour barriers).

Lifestyle changes can also increase the amount of moisture in the property:

  • More occupants.
  • More drying of clothes on radiators.
  • Occupants not opening windows after showering/bathing etc, or not using extractor fans.

I’ve seen condensation problems after tenants taped up and sealed vents to save money on their heating bills.

The weather also affects the amount of condensation in the loft:

  • The colder the loft, the more likely condensation will form.
  • I know from first-hand experience that vents are less effective on windless days.

Where can I find a good roofer?

The National Federation of Roofing Contractors and the Confederation of Roofing Contractors are well-known organisations that maintain databases of approved roofing contractors in the UK. The Competent Roofer scheme promotes roofers that agree to adhere to a high set of standards.

We’ve also partnered with Bark – one of the best places online for finding vetted, reviewed and rated tradespeople.

Why You Can Trust This Guide

I know there’s a lot of nonsense published online, usually by those that have never worked in the industry.

I have 18 years of experience in the home improvement/construction industries and for 10 of those I worked as a roofing contractor.

Loft condensation was and still is a common problem, I have seen it first-hand dozens of times. In the past, we roofers would install roof vents which cost about £60 each and must be fitted from the outside but the lap vents are so cheap and easy to install, I now recommend lap vents to those with condensation problems. In my experience, they are just as effective as tile vents.

To ensure accuracy, I asked Paul Farley to fact-check the important statements I’ve made in this guide.

Author: Daniel Woodley

Daniel has over 18 years of experience in the construction, home improvement, and landscape garden industries, including a spell working as a project manager.

In his spare time, Daniel enjoys gardening, hiking and scrambling. He frequently posts videos to his popular YouTube Channel and is well known in the hiking community.

More About Daniel Woodley.

Daniel Woodley

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