Roof Ventilation
This page deals with roof ventilation and roof ventilation problems, or the fitting of roof ventilation to existing cold roofs. A cold roof is a traditional loft space as seen in the vast majority of UK homes, and other parts of the world. Commonly they are usually insulated at ceiling level with fibreglass insulation laying across the ceiling joists.
There are many different ways of installing ventilation to the roof of a house. Some are easier to fit during large maintenance projects such as new fascias or a dry ridge conversion, whilst retrofit soffit vents, ridge vents, tile vents or lap vents can be fast, effective, and low cost by comparison.
So where to start? How about with these quick on page links, or by just scrolling down the page if you prefer.
- Do I need roof ventilation – Common signs are condensation, excess heat & smells
- How much roof ventilation – Why it’s not always straight forward
- Roof tile vents & slate vents – Easy to fit & can be used for bathroom extractor fans
- Lap vents – The easiest DIY solution to ventilation
- Soffits vents – Eaves ventilation for new installs and into existing soffits
- Over fascia ventilation – My preferred type of low level soffit ventilation
- Dry ridge for ventilation – As well as ventilation for traditional mortar ridge tiles too
Do I need roof vents
Some degree of roof ventilation is always a good idea, but not always a necessity. Certainly if you have any of the problems discussed later on in the article, such as condensation in the loft, you should either fit ventilation yourself, or seek out a roofer to install some ventilation for you. Often older houses were built without any real form of roof ventilation and do not exhibit any sort of problem at all, in which case adding ventilation is probably not a necessity.
Attic ventilation – This short video takes us through some of the basic benefits of roof ventilation, what it does, and why you might need it.
Sometimes though problems manifest themselves later on, like condensation or stifling heat either in the home, or loft area during the summer months. Sometimes there is what I call a tipping event that is responsible. This is where a recent modification or change in the house creates a problem that didn’t exist before tipping you from just OK into the problem category. Most often this is noticed with damp in the attic during winter and it seems to coincide with the following…
- Recently fitted new double glazing
- Topping up, or changing loft insulation
- External or internal wall insulation
- Additional members of the family like babies, relatives or guests
- Rising damp or drainage problems
These are all common factors that can turn an absolutely fine unventilated roof into a damp environment for rot and mould.
The basic concept of roof ventilation
Think of being in a roof like being in a car with the windows shut. During winter the windows will steam up if you are in there long enough, or during summer it can become stiflingly hot. So what do you do? Roll the windows down a little. It’s how much you roll the windows down that determines the level of ventilation, all the way in summer is fine, all the way in winter is not. Simple concept.
During the planning stage of a house the general idea is to ventilate the roof at a low level for fresh air entering, and a high level for stale air exiting, and you can find a page on planning regs for new roofs linked to an article about this at the bottom of the page if you need it. On new build houses this is planned for at concept when every aspect of design is taken into account by the architect, and it all comes together as a well thought out final package (mostly).
Roof ventilation – This picture represents what is considered to be the best way to ventilate a roof space. Different forms of high and low level ventilation are used as well, apart from the soffit vents and ridge vents pictured. Fairly simple stuff, fresh air in, old stale air out.
There are some problems that can occur occasionally when applying these same modern standards to older properties though, as their design and build may not always be in sync with a particular type of modern ventilation practice. An example of this in some circumstances can be soffit vents, and that is covered further down in more detail.
Condensation in the loft
Normally, there are two main symptoms that alert a homeowner to the possibility that they may need some type of roof ventilation fitting. Let’s briefly look at both scenarios to try and find out the answer to ‘ do I need roof vents or loft ventilation ‘.
Condensation in the loft or roof space is nearly always noticed late autumn through to early spring, with the worst time being November to February. Sometimes referred to as roof sweating, it can show itself in a very obvious manner, and you may notice small droplets of moisture forming on the underside of your roof undersarking (felt, membrane, timber), slates or tiles. I find these problems are normally noticed on the run up to Christmas as people rummage through the attic space for decorations looking for a christmas tree full of dead spiders. These small beads of water can vary in size and amount, from large beads of water all over the underside of the roof, to small sporadic patches here or there.
Roof condensation – This is a classic example of moisture beading on the underside of the roofs undersarking, the undersarking in this case is traditional slaters or tilers felt. Water beads (shiny spots in picture) can be larger, smaller, like a mist or sporadic. Notice the wet ‘condensation drip back’ onto the roof purlin at the bottom of the picture.
Damp in roof space – As a general rule of thumb slated or tiled roofs without underslaters felt tend to breathe and ventilate through natural gaps in the slate. Unfortunately these slates are exceptionally thin and flat, and the mortar torching (pointing) prevents any air beneficial movement. Visible damp can be seen on the rafters, laths and in the mortar torching.
Another cold weather symptom and giveaway of roof condensation is sometimes a musty smell or black mould growing in or on stored clothes, or cardboard boxes used for storage.
Although roof ventilation is obviously a very good idea, loft condensation is not just caused by a lack of ventilation on its own, it can also be a warning sign of excess humidity in the home. And if you haven’t seen my article on how to stop condensation it may well be worth a look in case you are suffering from that problem too (that doesn’t mean that roof ventilation is unnecessary however).
What causes damp and condensation in the loft ?
Warm moist air from everyday living like cooking, showering and drying clothes rises and can find itself trapped in a loft space that is either unventilated, or simply cannot ventilate at a fast enough rate. This in turn condensates on cold spots inside your loft, and turns from being warm moist air, back to its liquid state, forming small or large drops of water on cold surfaces depending on how bad the problem is.
I find these days roof space condensation has started to become more of a problem thanks to the insulation of both cavity walls, double glazing, and external wall insulation schemes to older properties that were simply not designed with that level of modern insulation in mind. Often it’s this insulation, or top up of insulation that creates a tipping point within the home. In the rush to insulate and eradicate cold spots, there is often little thought given to where any moisture created within the home is supposed to go. Warm moist air rises, often the final destination is the roof space.
Strange smell in the loft
At the other end of the scale to winter we have summer. Although condensation and damp isn’t usually noticed during the summer months some people do complain of either a stiflingly hot loft space, or a strange “lofty smell”, and these are normally linked. In most cases the lofty smell they are referring to is the smell of the black bitumen based roofing felts that get very warm and emit a bituminous smell that most people aren’t used to. Unfortunately I am used to this because I crawl through lofts for a living, so for me it’s very easy to identify. If smell is strong it indicates a lack of fresh air through air movement.
Another common symptom is an extremely hot house or hot upstairs bedrooms in the summer months. Although we all suffer from this on very warm summer nights, it can be particularly bad in homes without adequate, or indeed any roof ventilation. When the sun heats the roof and loft space during the day it creates ‘solar gain’, think of solar gain as sitting in a car with the windows closed on a hot day. Large volumes of hot trapped stale air inside the loft space effectively stops any heat from rising upwards and away from the rooms below. This lack of fresh air movement in the loft means it effectively insulates your home super efficiently, just when you don’t need it.
How much roof ventilation do I need ?
This is a tricky question to answer when ventilating older roof spaces, and I’ve seen lots of people have a crack at a one size fits all approach over the years, and I’ve seen them get it right and wrong a lot too, so what’s the problem?
The simple answer is that every house, construction and location is different, not only by shape and size but by geographical location and positioning, even in the same street. This makes an off the shelf one size fits all mathematical solution or a roof ventilation calculator very difficult to achieve with any degree of accuracy. Let me give you a quick example.
- House number one – A standard semi detached house in a slight hollow on a housing estate. This sits 500 meters down the road from the example below, and nestles in a row of other semi detached houses. It has continuous soffit vents all round, and two roof vents front and back. And it has lovely gentle ventilation in nearly all weather conditions, cool in summer but warm in winter. Winner.
- House number two – Another semi detached house straight off the same drawing board as the one above, and identical in every way. This house though is an end semi detached and is situated 500 meters up the road on a slight hill. This time though the wind channels through the estate (unlucky), up the road and blows like hell into the vents and soffit vents. So much so that it’s prone to blowing back the insulation in very strong winds, and causing heat loss from the insulation near the edges of the roof in winter. This in turn causes black stains and mold around the edges of the ceilings internally. Not a winner.
So what does this mean in real terms?
If you are unsure call in an experienced roofer or architect to evaluate. My personal default position is to err on the side of caution slightly, a little extra ventilation is generally easier to add later than it is to remove afterwards. Generally speaking you don’t want to make your roof void too cold, warm air can hold more moisture than cold air, and this will evaporate moisture from your insulation better as a result, and reduce heating bills. Yes ventilate, but don’t go mental.
For more specific advice on different types of roof ventilation see below.
Roof tile vent or roof slate vent
Tile vents or slate vents are by far the easiest professional solution to roof ventilation problems. They are super easy to fit either during the process of installing a new roof, or retrofitting to older existing roofs. There are various designs on the market from visible humped versions to almost unseen flat profile varieties… Here’s a link to the type of roof vent I fit in the video.
It’s normally just a matter of matching your roofs tile profile to a matching tile vent. The main types are Marley, Redland and Sandtoft.
Roof tiles – These are common types of roof tile profiles. Often manufacturers make vented tiles that fit the same shape tile even when it’s made by different companies.
In my opinion one of the best places to fit roof tile vents is normally the bottom third of a roof just above insulation level. This allows air to circulate over the insulation taking away any moisture evaporating upwards, or trapped in the insulation fibres. Positioning a vent too low in the insulation may in some circumstances chill the edges of internal ceilings, or create cold spots resulting in condensation dripping back onto the insulation or ceiling below. It can also create black marks on the ceiling internally near outside walls as mould starts to grow. Heat loss from insulation can also be an aspect as it simply blows the heat out of the fibres removing valuable trapped heat.
Roof ventilation tile – This video shows how to install a roof vent tile and a bathroom extractor fan vent, all in one video.
When installing roof tile vents on the front and rear sides of a roof, some staggering in height and horizontal positioning should ideally take place to prevent wind tunneling.
What is wind tunnelling?
Well, imagine one vent to the front roof elevation, and one vent to the rear as an example, installed in exactly the same height and location so that they line up. Any wind blowing through the vent on the front may pass straight through to the rear vent without disturbing or circulating any air as it does so efficiently. This may not particularly matter if you’re adding roof ventilation simply to provide clean air to the loft space, or for air circulation systems like PIV (positive input ventilation), but if you are suffering from condensation in the loft it’s a missed opportunity to circulate stale air thoroughly.
Tile vents fitted lower down a roof will also work very nicely in conjunction with roof tile vents fitted higher up the roof, lap vents, vented dry ridge, or ridge tile vents for better air circulation.
How many roof tile vents do I need ?
This is a bit like how long is a piece of string. If you have ventilation by other means like soffit or eaves vents, no undersarking, or no condensation problems, the answer may be none. On the other hand if you have bad condensation, a semi detatched could need 2-3 per elevation, or 2-3 front and back as a randomly plucked example, and in very bad cases maybe some ridge ventilation too.
For most situations a simple 2 vents front and back seems to be a good starting point, some roofers favour two low and one high per elevation, making a total of 3 front and 3 back for example. I have found this combination usually sorts out most roof space condensation.
Roof vent test – This roof is a low level detached bungalow with only 1 roof vent per elevation. It had no roof condensation and vents were fitted for fresh air only, but you can see how effective they are.
Bathroom vent
As a bonus, bathroom vents for extractor fans or bathroom exhaust fans can be attached to most types of roof tile or slate vents, usually through the purchase of a separate adaptor plate. These are then simply coupled to the 100mm ducting hose supplied with the bathroom extractor fan. I normally fit the vent as level as possible with the surface insulation, placing a vent high on a roof can mean the ducting will fall backwards, potentially carrying condensated bathroom moisture downhill towards your bathroom extractor.
Roof vent for bathroom extractor – This is a standard low profile roof vent with the optional 100mm (4″) extractor fan ducting adapter. This is a hell of a lot easier than cutting holes through walls with a diamond cutter, or through fascias and soffits.
The video for installing a bathroom extractor and ducting to a roof tile vent can be seen here or above in the section about installing a roof tile vent.
Felt Lap vent
Often called a felt lap vent because they are mostly fitted to older roofs with bitumen ‘felt’ undersarking, a lap vent will provide the easiest DIY fit solution to ventilation problems and despite their name they can also be fitted to roof membranes too if needed. These super easy Manthorpe lap vents simply slip in between an overlap in the horizontal undersarking inside the loft providing an air path for the wind blowing up the face of the roof outside. Simple, easy, effective, and with no external access or ladders to worry about.
Lap vents – The easiest DIY fit on the market as long as you don’t bang your head or plunge through your ceiling that is. Follow the fitting instructions and be careful not to rip anything, as mentioned below.
One snag to look out for is accidental ripping of the undersarking if it has aged badly, it’s not always a disaster though and poorly aged felt can be seen in my how to fit a roof vent video if you haven’t seen it already. If you are considering fitting lap vents it is important to be gentle separating overlap joins in the felt undersarking, often hot summers stick the two surfaces of the bitumen together and a thin blunt tool like a butter knife or pointing trowel can prise them apart without damage.
Lap vents – This video shows my current favourite no hassle lap vent, and how easy it is to install lap vents.
Ironically these are cheaper for me to buy online than in the roofing yard. The best ventilation tactic seems to be scattering them around in various heights and locations from inside, a pack of 10 may not be quite enough for a standard semi detached.
Soffit vents
Soffit vents, sometimes known as eaves vents. You can see these fitted to most modern houses and are normally built into the soffit board to provide a continuous 10 – 25mm of ventilation at low level around the eaves of a house. Usually this is done at the build of a house, or during the installation of new fascias and soffits.
Whilst they are very effective, there are two slight flaws with soffit vents in my opinion. Firstly they attract dirt and cobwebs with age making them look ugly, and because of the design they are difficult to clean properly later on. Secondly is the occasional possibility of too much ventilation in older properties. Not only can an updraught wind blow into them potentially disrupting or cooling the insulation, but they can create black marks around the ceilings inside a property, more on this further down…
Continuous soffit vents – This is what you see when you look upwards towards your guttering. These soffits, fascias and guttering are only 5 years old but you can see how dirt congregates around the slots of the vent.
Why is the loft insulation missing from the edges ?
The edges of the roof space narrow with the pitch of the roof, and the closer you get the tighter the gap becomes. Unfortunately during the insulation of a loft space the very edges cannot alway be accessed effectively by the installer, or the insulation is stopped short to allow some air movement from the fascias or soffits. The common effect of this is quite often poorly insulated or un-insulated ceilings near the outside edges of external walls. As mentioned above, during winter months wind blown through the soffit vents can chill the edges of un-insulated plasterboard internally, they can then become a cold spot inside the home, and be prone to condensation as a result. This can show itself as a shadowy mark or wet patch, and in turn start the growth of black mould.
Close proximity – This picture shows the relationship between sporadic insulation, exposed ceilings and the close proximity of soffit vents on some occasions. Obviously in the majority of cases there is no problem, every construction is different, it doesn’t mean it’s good practice though.
This is one of the reasons I prefer the installation of ‘over fascia’ vents and the gentle ventilation they provide, as long as the fascia is large enough to receive these vents and the later installation of guttering screwed to the fascia.
Generally speaking, better planning on new build houses prevents problems. Loft vent trays or rafter trays are often used to keep an air gap between the underside of the roof and the insulation too. This means insulation can be brought forward to the very edges of the plasterboard or cavity insulation safe in the knowledge an air gap will be maintained, and air will be channeled up and over the insulation, rather than through it.
Fitting soffit vents
If you need soffit vents the good news is that they can be retro fitted afterwards very easily, and the easiest and fastest to fit is a circular soffit vent. These are simplicity itself as long as you don’t have asbestos soffits (never drill or cut asbestos). Simply drill a hole through the underneath of the soffit board with the correct sized hole saw (usually 70mm) and push in a matching circular soffit vent. These can be purchased from DIY chains like B&Q or Wickes or online without much problem, but they are not as easy to install as the lap vents mentioned above.
Round soffit vents – Make sure you fit them between any rafter legs that may be present. This is normally done by tapping the underside of the soffit and listening for audible changes between hard sounding rafters and hollow sounding voids in which to cut the soffit vent hole.
How many soffit vents do you need to install?
Again this is very hard to pin down, but when I have been installing these under guidance from building control or for customers, it has been every other rafter (approx 900mm) or between every rafter (450mm approx), obviously this will depend on the rafter spacing of the roof on your property.
If you don’t like the look of circular soffit vents, continuous vents can be fitted as an after market solution, but it is a complete faff to install and will involve both scaffolding and possibly the removal of roof tiles, laths and undersarking. I would seek to fit over fascia vents instead if possible, as seen below.
Over fascia vents
When installing any sort of replacement fascia this is my absolute favourite form of low level ventilation, and can it be retro fitted too, but it’s not an easy DIY job for most. Over fascia ventilation is exactly what it sounds like, these are small clip together units that can be nailed over the top of a fascia board to provide a continuous 10 – 20mm of low level ventilation. Thanks to the fine mesh grill design not only will this keep out any bugs or insects, but it is totally unseen unlike soffit vents. A a bonus is the ventilation is usually more gentle and controlled as they are hidden from the elements behind any guttering or felt support trays.
