How do roofs work?

[vicky whitworth]

Hello again, please could someone explain to me the basic principles of how roofs work in terms of condensation inside? We have a modern (done in 1990) roof on our 18th century rubble stone cottage. We are getting damp patches on our bedroom ceilings where the sloping roof (the bedrooms are in the loft space) joins the vertical wall. We assume this is condensation and that water is running down the rafters and collecting at the angle. I don't think the pantiled roof leaks as the patches are in multiple places in a number of rooms.

The insulation in the roof isn't great, the loft hatches (there are four) are ill-fitting and the roofing felt looks old-fashioned.

I have asked a roofing company to come out and talk to me about it and I want to appear like I know what I'm talking about.

Should the roof void be kept cold (well insulated so that the heat from the house doesn't get into it)? Should the roofing felt be replaced with a modern breathable felt and how would you do that without taking the tiles off? Should the roof space breathe? And how can this be achieved. There's a horrid grilly thing on the timber fascias which someone said was a 'fly mesh'? What's that? I would like to get rid of it as it's grim.

Any advice gratefully received. Many thanks.

 

[Feltwell]

Non-expert reply here....

Condensation forms when the warm, moist air from inside your home gets into your loft and hits the cold surfaces. Yes, broadly speaking you want a well-insulated layer that keeps your top floor warm, with a well-ventilated cold space above it. It does sound like you've got an ideal receipe for condensation there, but firstly....

Are you sure your roof doesn't leak? Have you got lead flashing sealing your roof around a chimney stack, rooflights or against a wall? Is it in good condition, no splits or cracks? Have you got a roof "valley" anywhere, which is where 2 roof slopes meet at an internal angle so that water runs down that angle? If so does it all look to be in good condition? Are any of your pantiles cracked? Are your ridge tiles all complete and well mortared in place? Don't forget water can travel a fair way from it's point of entry before it appears.

Assuming for a moment that your roof doesn't leak, one thing from your description shouts out to me - 4 ill-fitting loft hatches. The warm moist air from inside your house will be rising up and the ideal way for it to get into any loft space is via an ill-fitting hatch, let alone 4 of them! First thing I would suggest is sorting those out so they seal well and are insulated. That alone might be enough to sort out your problem, and would be quick and cheap to do.

"Old fashioned" felt - not a problem, many houses are built or have been refurbished with that. You wouldn't use it now, you would use a breathable membrane, but the only way to change it is to strip the entire roof of tiles and re-do the whole thing which would be overkill, there are other ways of stopping your condensation that are much cheaper and less intrusive.

You want insulation in your loft to keep your house warm of course. For the sloping sections, this may mean putting a solid foam insulation board such as Kingspan or Celotex in between the roof rafters BUT (and this is vital) leaving a well-ventilated gap of at least 2" between the top of the insulation and the felt. But, access to this area may well be a problem without taking the ceilings down - is "undercladding" the ceilings with insulation boards an option? It does mean you cover up your original ceilings of course (if they are original) and you lose head room.

But - has it already been done, and did they leave a ventilation gap? If not that could be the problem.

For the horizontal sections of ceilings where presumably you have the hatches, ordinary loft insulation (or sheep's wool insulation if you want to be greener) laid on top of your ceilings, inbetween the joists on the first layer and across the joists for a second layer, will do fine.

You must make sure that the "cold" area above the insulation is well ventilated, so that any moisture that does get through can escape. Your "horrid grilly thing" will be trying to do this, but there are other ways. If you have sloping ceilings it may well be that the grill is there to let air flow over the top of insulation that is already there though, so check out carefully why it was installed and if it was correctly installed before doing anything with it.

Has the house got gable end walls? If so, you could put air bricks in to ensure a cross-flow of ventilation across your loft area. You could also look at putting tile vents in, these ensure moisture can escape past your impermeable roof felt - this sort of thing (with due thanks to whoever it was on here (Mdb??) who reminded me of the brand name recently):-

http://www.lapvent.co.uk/

If you get a roofer in they may want to put a modern tile vent in, which may mean an ugly plastic or new concrete tile going in - the product above does away with this and looks far better in my opinion.

It's realy hard to say without seeing your house as roof spaces vary a lot, but hopefully the above helps. I really would start with the loft hatches though - could be a quick, cheap and easy fix.

 

[MdB]

Condensation occurs when warm moist air cools. Remember that and the rest follows.

With reference to your roof, lets assume that outside is very cold, the roof void is cold and your room is warm. Lets also say that it is raining outside and the outside air is carrying the most amount of moisture it possibly can (99% humidity). If this very cold moist air enters your roof space it will warm up a bit which will enable it to carry additional moisture i.e. its humidity level will fall. If your roof space is damp, this drop in humidity enables the external air to pick up some of this moisture. When this now moist air leaves the roof space and goes outside, its temperature will fall and the moisture from the roof space will fall as rain in the garden. This is considered "a good thing" (TM) and why a lot of people have advised here that having a howling gale passing through your roof space is a good idea.

In older properties with non breathable roofing felt, to get this air flow through the loft space required air vents in the fascia boards (your grilly things). Ideally they should be on either side of the building such that air can be blown in by the wind on one side and then come out of the other.

Now that breathable roofing felt has been invented, the air vents are no longer necessary as the moist air can pass through the membrane.

I don't see that there is anything particularly wrong with the old system so I wouldn't be in too much of a hurry to replace your old felt unless it is leaking. It is important that the air vents are clear though so as to allow air into and out of the roof space.

Now lets move on to the air inside of your house. This will be warm and most likely moist. If this air escapes into the roof space, it will cool and the cooler air will no longer be able to contain all of its moisture and it will condense out. If there is little insulation above the ceiling then the underside of the ceiling may be cold enough for the condensation to form there. If there is insulation, the moist air can condense in the middle of the insulation making it damp and in some cases lose its insulation properties. In some modern construction, this problem is dealt with by having a vapour barrier (think sheet of plastic) on the warm side of the insulation. It is important that it is on the warm side as this stops the warm moist air from cooling but stills allows air movement through the insulation from the cold side.

Having said all of that, a leak in your roof could be the cause of the problems too.

 

[Flyfisher]

Damp patches on attic ceilings would surely require a really extreme case of condensation?

I agree with the comments about the four loose-fitting loft hatches but, even so, do we really think enough water vapour could get up there and condense out in sufficient volumes to run down rafters formings pools of water and thus soak through the ceiling material? If so, I would expect condensation to be all over the attic insides and easily detectable with a wipe of the finger almost anywhere.

I think I would be looking for external problems and my experience of pantiles is that they are notoriously difficult to align perfectly for a truly waterproof layer. Wind-driven rain (and snow!) can easily get under pantiles, which should not be a problem if the membrane layer is sound, but a big problem if it's not. Flashings (or lack of them!) were a big problem on our roof and resulted in a lot of localised water damage, but even so, it took many years of exposure to a lot of water and almost none of it was condensation because the attics were so draughty.

I find it hard to imagine basic condensation causing such 'hard' damage, but I freely admit my experience is limited.

 

[vicky whitworth]

Thank you for your really helpful replies. I think I understand the issues a bit better now and I feel much more confident about talking to the builder. One of my problems is I'm squeamish about attics (they're creepy somehow) so I won't go up there myself to look and I think I will have to overcome this. Those lap tiles look great. I'll point the roofer in that direction if we think vents are needed.
Many thanks.

 

[Flyfisher]

No need to actually venture into the attic Vicky. Just poke your head up through one or more of your hatches and have a look around with a good torch. You might be able to spot something and I'm sure you'd get a feel for any serious condensation problem. I don't think anything will jump out at you but don't fall off the step-ladder at the first sight of a spider!

 

[MdB]

Thinking about it, I agree with Flyfisher. It would have to be a truly bad case of condensation to cause damp patches.

(And my wife used to have a phobia of going into lofts but has managed to get over it now. Having it well lit helped a lot.)

 

[vicky whitworth]

Thank you for the reassurances that the bogeyman doesn't actually live in my loft. Are you sure? Don't know why it's scary since the roof structure only dates to the 1990s - perhaps that's why it's scary!

I have carried out a better inspection of the problem inside the house. All the damp patches are on the same side of the cottage and are slap on the point where the sloping ceiling meets the vertical walls - right on that angle. The patches are 3-6 inches in size and don't feel wet but they have stained the paint work. They occur in two bedrooms but not in the third. All three bedrooms have loft hatches. We have had them (the damp patches not the loft hatches) on and off for a couple of years. Tactics so far have been to paint over them! Problem solved for a while and then they're back and looking horribly damp patchy.

Roofer says he will pop round tomorrow to take a look.

 

[Flyfisher]

Thank you for the reassurances that the bogeyman doesn't actually live in my loft.

I only said I didn't think anything would jump out at you, I deliberately avoided speculation about what might actually be up there. :wink:

Good luck with your roofer tomorrow.

 

[AMc]

Reducing the amount of moist air going from your house into the roof will reduce the (potential) condensation problem and help your heating bills. It's worth doing that regardless.

Really try and grit your teeth to take a proper long hard look yourself. There are very few things I can't cope with if I've got a powerful torch, gloves and the prospect of a hot shower straight afterwards - that included the live soil pipes last summer.
You are likely to be more careful, spend more time and have more knowledge about your house than anyone coming in - knowing what the problem looks like from above will help you help your roofer to work out what to do.

With the greatest respect - tradespeople will come in, look and see you haven't got a... or need to add a... and give you a price. It's not in a roofers interest to decide you need to open a bathroom window and improve your loft hatch seals.

 

[Lime]

Thank you for the reassurances that the bogeyman doesn't actually live in my loft. Are you sure? Don't know why it's scary since the roof structure only dates to the 1990s - perhaps that's why it's scary!

I have carried out a better inspection of the problem inside the house. All the damp patches are on the same side of the cottage and are slap on the point where the sloping ceiling meets the vertical walls - right on that angle. The patches are 3-6 inches in size and don't feel wet but they have stained the paint work. They occur in two bedrooms but not in the third. All three bedrooms have loft hatches. We have had them (the damp patches not the loft hatches) on and off for a couple of years. Tactics so far have been to paint over them! Problem solved for a while and then they're back and looking horribly damp patchy.

Roofer says he will pop round tomorrow to take a look.

But the problem may not be able to be seen.

If the problem is confined to where the roof meets the house wall then that could be caused by several totally different scenarios.
1. It could be condensation running down the inside of the felt (it doesn't have to be a lot to wet the wall) and soaking directly into the wall where the felt touches/nears the brick.
2. It could be condensation on the outside of the felt running down under the tiles towards the wall, pooling over the wall and seeping through the felt via a rip, joint, nail hole of where it has been cut short.
3. It could be a leak through the tiles or flashing anywhere on that side of the roof, entering beneath the tiles and running down on the felt as 2.
4. It could be the guttering overflowing or it may be cracked, split or just poorly installed.
5. It could be rain entering and running down rafters from the chimney stack or the apex of the roof.
6. It could even be a leaking water pipe somewhere.

 

[vicky whitworth]

Thank you for the recent advice and suggestions. You guys are great. Roofer failed to show up yesterday! So plan now is to carry out a full inspection myself at the weekend - including facing up to the scary bit - using the lists of ideas you have all put forward. if you never hear from me again that will be because I'm still in the loft and can't get back onto the step ladder. Could you then phone the police for me? But if I survive I will report back.

 

[Moo]

The most successful Christmas present I ever bought was a pair (his 'n hers) of loft ladders* for our last house. These, together with proper lighting, had me scampering up and down them like a two-year-old.

You definitely deserve one for a New Year present.



*Just in case anyone begins to compose a clever response - we had two lofts at the time.

 

[MdB]

I don't wish to discourage you Vicky, but please make sure you know what you can put your weight on and what you can't when crawling around the loft. This bit of plasterboard covers the hole I made with my backside when I wasn't being too careful in my loft....

It could have been a lot worse.

Plasterboard / plaster and lath are not strong enough to take your weight.

 

[Flyfisher]

Are the other two holes where your legs went through? :shock:

 

[Schoolmarm]

:lol: :lol:

 

[AMc]

You can often use the line of rafters in the roof to help work out where the joists run if you can see them. Test with gentle pressure with one foot with the other foot on a solid point, holding on to the wood above if you can to help your balance. Before you stick your whole weight on a new foot hold then make sure there is something solid running left to right or forwards and backwards e.g. a joist. Go easy and you'll be fine.

My TV aerial installer was warned to be careful of the reed and plaster ceilings in my loft which are covered by drifts of mineral wool insulation. There are some very big joists and much smaller cross pieces that won't safely take any weight.
"Don't worry, I only ever step on wood" he told me. After he'd gone I had to go round pushing all the downlighters in the bathroom back into their holes - could have been a lot worse!

 

[vicky whitworth]

OK here's where we are.
Firstly I did it - I braved the loft - and do you know there's no skellingtons up there!
Secondly, it stinks of damp. The roof timbers are what I would call sopping, the insulation on the floor is wet as is the timber floor in a few areas where it is boarded over.

The roofer reckons that the gap in the sloping sections ie: between the roof and the sloping ceilings in the bedrooms, is tightly packed with insulation which is preventing the grilly things in the soffits working at all (thus stopping the ventilation through that route), and also the insulation there is pushed against the old roof felt so, as condensation runs down the felt, it's pushed into contact with the timbers. His solution is to replace the fibre glass insulation with solid board insulation leaving a gap to allow the grilly things to work properly. Problem there are only two ways to get to this area - take the sloping ceilings down or take the roof off. I suggested taking the soffits off and pulling the old insulation out through the gap and pushing the new boards. He thought that was funny!

I suggested the lapvent tiles to increase ventilation and better insulation on the floor of the loft and the hatches but has he said the problem with the sloping sections would still exist.

My husband's view (he doesn't really do buildings) is that it will take years for the roof timbers to rot so why spend a great deal of money on it. Wait til they have rotted and then spend the money having a whole new roof structure!

Could anyone advise me of the sloping roof aspect of this? I can't see anyway to change to solid insulation in there and to create the much needed gap. I can improve the insulation and ventilation in the main loft areas but I can't see a way to resolve that sloping roof problem without having the whole roof off.

Thank you. Vicky

 

[Jill]

Vicky is there any way of getting some water directed on to your roof from the outside using a hose pipe whilst one of you checks whether it is coming in inside. This sounds very similar to a problem we had. Very long story but we were told by a roofer that we had a problem with our ridge tiles and they all needed to come off. I didn't believe it so my other half, under my instruction chucked a jug of water on the roof lower down (he was up the ladder at the time) and water came down the wall inside, it ended up being down to some cracked tiles and the membrane having holes in it. During the process the roofer (who incidently just turned up from no where whilst we were investigating it) put his foot through the ceiling into the room below. In the end it cost us £50 for him to take some tiles up, fit some more membrane, replace the tiles (not the broken ones) in the space of about an hour. It lasted us around 3 years but we have since gone down the new roof option.

 

[Feltwell]

Don't panic Vicky!

First off, what the roofer has said sounds sensible. If that area has been packed tight with insulation I think it would cause a problem.

Secondly, you can't just ignore it - your bedrooms will end up stinking of damp eventually as well as your loft, and your house would be hard to sell if you come to move. Any surveyor would spot that problem. Damp rooms aren't healthy.

Thirdly, don't assume that stripping the roof is massively expensive if that really is the only way to get the insulation out. Only the lower section over your sloping ceiling should need stripping - with the original tiles going back on it may be much cheaper than you think. Get some quotes! To put it in context, with myself plus 3 roofers we roofed a large double garage from scratch in 2 days, which included cutting tiles around 8 rooflights. The most expensive bit is likely to be scaffolding, unless the roofers can do it one section at a time from a tower. People tend to be scared of anything to do with roofs and think they are massively complicated - they're not, a basic tiled roof really is very simple.

You could take the ceilings down, but that's very messy and intrusive. The "tiles off" option will have much less effect on you using your home! Unless your ceilings are in poor condition already in which case it might be worth it.

Can you see any way of getting the insulation out from inside the loft? Would a grabber (sort of thing litter pickers use) be able to get down the space? How big is the sloping section?