corrugated garage roof ventilation...

[rich080487]

Morning,

I have moved into a new-to-me 1950s bungalow about 8 weeks ago. It has a garage, which is best considered detatched.
The garage has an asbestos roof, brick walls and minimal joists to support the roof, and is 6m x 3.5m.

I will need to replace and improve the joist/supporting roof structure.

Over the last few weeks I have noticed that the roof is actually leaking (I had replaced the guttering as I thought that was the problem).
This means I would like to add a new roof as cheaply as possible, so I have been looking at corrugated bitumen. Any experience or thoughts?
I am not a builder, and have not built a roof before. I am practical and a Mechanical Engineer by profession, and happy to learn...

Please see photos of existing garage https://photos.app.goo.gl/Xd94qa4haf57amrV9

The roof of the house over hangs part of the roof of the garage (it is "semi-attached" on a single corner). So building a roof with joists resting on wall plates isn't an option. So I have been looking at using joist hangers, and therefore positioning the joists between the walls.
I have a few questions...

 

1. It is a flat roof with less than 10 degrees slope, is it ok to have the joist not horizontal? Or should I make them horizontal and then create angle in another way? I'm concerned at the use of joist hangers not perpendicular.
2. Should I attach OSB to the top of the joists to make it easier to add the corrugated sheets? Or would battens be easier?
3. Should I seal where the corrugated sheets meet the top of the wall plate? This would stop animals and insects getting in, but would remove any ventilation. I presume condensation will be an issue if I did this?
4. Would I be better to OSB the bottom of the joists to seal the garage space, and then ventilate above by not sealing the corrugated sheets to the top of the wall? Would I need a vapour barrier? How would I protect against insects and birds?
    

Most important question; am I thinking about this too much? I don't want a wet, cold, draughty garage to work in...

Any help appreciated,
Rich

[scottishjohn]

I think it will be how much do you want to spend ?

can see daylight between roof in some of those pictures...

single skin tin roof will get condensation on inside it its warmer in that out.

and then there is the floor --does it have a dpc below it?

how much heating you going to put in it .

 

 

 

 

 

Edited December 28, 2018 by scottishjohn

[Onoff]

I used corrugated bitumen sheets from Wickes. Biggest waste of time and money EVER! I'd even put in extra roof joists to halve the span. It all just sagged like washing up bowls between the joists. I tried inserting extra ali tubes to help but it was just crap. Ruined the car stored in there.

 

Stable where I am now has an asbestos roof that leaks. I guess over many years it just breaks down?

 

 

I'd go EPDM as it's quick and easy. Fibreglass maybe if you fancy the challenge. Lets face it they are what they are these old garages and draughty isn't a bad thing as it can keep things dry.

[ProDave]

16 minutes ago, Onoff said:

I used corrugated bitumen sheets from Wickes. Biggest waste of time and money EVER! I'd even put in extra roof joists to halve the span. It all just sagged like washing up bowls between the joists. I tried inserting extra ali tubes to help but it was just crap. Ruined the car stored in there.

I have had the opposite experience.  

 

I bought my butumen roof sheets from B&Q. They have been on my wood shed roof for 10 years now with no sag and no leaks. 

 

They do need supporting every 2 feet / 60cm in both directions. Give them that support and they are fine, so your roof structure will need a lot of additions.

[Nickfromwales]

Hi.

Assume you mean to remove and dispose of the asbestos sheets and start from scratch?

[Russell griffiths]

Budget?

£1500, new joists at 400mm 18 osb, 

torch on felt or my choice would be grp or corrugated steel sheets with no joins 

minimal fall needed, 

my last choice would be those bitumen sheets, keep them for the wood shed. 

 

Why can you not put a wall plate on top of walls with 150mm x50mm joists on top strapped down to internal wall face with steel strap, 

this way would create a 200mm deep facia that you could trim with the same metal trim and provide a nice finish. 

[ToughButterCup]

1 hour ago, rich080487 said:

[...]
I have moved into a new-to-me 1950s bungalow about 8 weeks ago. It has a garage, which is best considered detached.
The garage has an asbestos roof,

[...]

 

My reading of your post skidded to a halt here.......

You need to get a grip of the asbestos. Find out how much that will cost to dispose of - properly.

[Russell griffiths]

Grp roof on our current place, was trying to avoid doing this as I want to knock it down, but it decided to let go in a spectacular fashion on Boxing Day two years ago??

i did this with only the wife helping in very far from ideal conditions in January 

i think grp and all trims came to about £700. 

[Onoff]

I'm not saying to use this at all but was interested to see there is asbestos roof renovation paint available.

 

https://www.kingfisheruk.com/asbestos-roof-coating-item-55arcb

 

Surely though the existing roof would need cleaning first and that process would disturb fibres etc?

 

Edit: There is a guide:

 

asbestoshowto.pdf

 

Edited December 28, 2018 by Onoff

[Nickfromwales]

Nothing worse than putting lipstick on a pig  

Pay for disposal and start afresh. I wouldn't want to spend a lot of time in there tbh with its ( the asbestos' ) integrity unknown.

[rich080487]

Thanks for all the very quick replies...

 

41 minutes ago, scottishjohn said:

I think it will be how much do you want to spend ?

can see daylight between roof in some of those pictures...

single skin tin roof will get condensation on inside it its warmer in that out.

and then there is the floor --does it have a dpc below it?

how much heating you going to put in it .

 

I don't want to spend a huge amount, as I may need to relocate the garage depending upon the long term plan... (my wife is already thinking about extensions)

 

7 minutes ago, Nickfromwales said:

Hi.

Assume you mean to remove and dispose of the asbestos sheets and start from scratch?

 

Yes. Remove the asbestos roof, and rotten joists. Then rebuild the roof structure from scratch.

[rich080487]

Just seen a few other replies. Once again, thanks.

 

I have already got a few quotes for the asbestos removal. Don't worry, that  will be the first thing to go!

 

44 minutes ago, Russell griffiths said:

Budget?

£1500, new joists at 400mm 18 osb, 

torch on felt or my choice would be grp or corrugated steel sheets with no joins 

minimal fall needed, 

my last choice would be those bitumen sheets, keep them for the wood shed. 

 

Why can you not put a wall plate on top of walls with 150mm x50mm joists on top strapped down to internal wall face with steel strap, 

this way would create a 200mm deep facia that you could trim with the same metal trim and provide a nice finish. 

 

Unfortunately, the main roof of the rest of the property over hangs the garage by a small amount, which limits the height I can go to.

This makes me think the long term plan should be to re locate the garage and make it properly detached.

I just need to the roof to be replaced so that I can work in my garage safely.

 

Once again, thanks for all the replies.

[Mr Punter]

I had a firm replace an asbestos garage roof with a double skin metal insulated roof.  It was en-bloc and staggered up to the one neighbour and down to the other.  They measured, removed existing, installed, provided extra bearing where needed £1,600 plus VAT with an asbestos disposal ticket.  They had to move some of the crap in the garage.  New roof is safe to walk on and no condensation.

 

With the floor I just put down DPC and laid 18mm chipboard flooring over £105 plus VAT diy.  New Hormann garage door £435 plus VAT diy fitted.

 

Overall, £2,140 plus VAT for the upgrade, but now I can store everything in the dry and it is secure.

 

Single garage

Edited December 28, 2018 by Mr Punter

[Oz07]

20 hours ago, Russell griffiths said:

Budget?

£1500, new joists at 400mm 18 osb, 

torch on felt or my choice would be grp or corrugated steel sheets with no joins 

minimal fall needed, 

my last choice would be those bitumen sheets, keep them for the wood shed. 

 

Why can you not put a wall plate on top of walls with 150mm x50mm joists on top strapped down to internal wall face with steel strap, 

this way would create a 200mm deep facia that you could trim with the same metal trim and provide a nice finish. 

Would there be a condensation risk between osb and tin roof or would felt protect this?

[Crofter]

1 hour ago, Oz07 said:

Would there be a condensation risk between osb and tin roof or would felt protect this?

 

There is some risk if no breather membrane, although a garage is likely to be very well ventilated anyway.

 

To the OP- I would get a price for corrugated steel, it may not cost any more than bitumen sheets and you will save money on the rafters because it needs considerably less support.

[joe90]

I used steel box section for my garage roof, I asked about the coated stuff (on the inside) that is supposed to stop condensation but was told unless there is a lot of airflow , like a barn, it does not work well, I was told (by the suppler) that using a roofing felt under the steel was a better solution so that is what I have done and it has worked very well. I think steel is a much better product than bitumen sheets IMO.

[Oz07]

36 minutes ago, joe90 said:

I used steel box section for my garage roof, I asked about the coated stuff (on the inside) that is supposed to stop condensation but was told unless there is a lot of airflow , like a barn, it does not work well, I was told (by the suppler) that using a roofing felt under the steel was a better solution so that is what I have done and it has worked very well. I think steel is a much better product than bitumen sheets IMO.

So you've got plain (non insulated) corrugated metal and no condensation? Do you ventilate the garage or just natural airflow from gaps?

[Crofter]

As Joe90 says, roofing felt is a better way of preventing condensation. You may still get water condensing on the underside of the sheets, but the breather felt will prevent it from dripping back down into the garage.

[joe90]

7 hours ago, Oz07 said:

So you've got plain (non insulated) corrugated metal and no condensation? Do you ventilate the garage or just natural airflow from gaps?

 

Yes, non insulated roofing with a membrane under the buttons, any condensation (if any) will run down the membrane and into the gutter. The workshop is draughty and Not heated.

[Tosh]

@rich080487 are you certain it's asbestos? A lot of these fibre based roof sheets are difficult to assess even by the experts hence why they 'assume' asbestos. If you're approaching companies to remove it then may be worth asking if they undertake testing or if they now anyone. A quick google search may turn up trumps as well. For the sake of approx. £15 you would know for certain. That's what we intend to do and I know an architect down the road did just that. He was converting his agri shed and that had fibre roof sheets but wasn't sure if they were asbestos - got it tested and had a certificate to say it was ok so he sold it to a local farmer. Would've cost him £300/ton to dispose of if it had proved positive asbestos!

 

I do appreciate though that the scale is different and your area looks pretty small so may not be worth the research however if you start getting silly quotes then something to fall back on maybe.

 

[Ferdinand]

On 28/12/2018 at 11:16, rich080487 said:

Over the last few weeks I have noticed that the roof is actually leaking (I had replaced the guttering as I thought that was the problem).
This means I would like to add a new roof as cheaply as possible, so I have been looking at corrugated bitumen. Any experience or thoughts?
I am not a builder, and have not built a roof before. I am practical and a Mechanical Engineer by profession, and happy to learn...

 

I have done 2 separate projects very similar to this ... essentially leantos. 

 

One was built with a box section corrugated roof ventilated beneath with a membrane and insulated subroof, and became a 4 season lounge type room but North Facing. Span 4.2m. Roof angle was under 5 degrees.

 

The other was a covered way with a polyprop roof and no insulation ... think car port style ... with a roof angle of under 10 degrees and a span of up to 3m.

 

My approach has been to put an 8x4 or 6x3 wall plate attached to the wall with thunderbolts, and run joists to that with (I think mini) joist hangers.

 

I then flash properly into the wall above, and that has worked. SPec your joists to be strong enough to walk on and spacing to match the roofing. THen insulate and ventilate as necessary.

 

To keep beasties out just use normal insect netting.

 

 I would say that for a new roof all your materials should come in at very roughly £500-600 more or less for plastisol coated box section corrugated.

 

FErdinand

 

Edited December 31, 2018 by Ferdinand

[rich080487]

Thanks for all the replies everyone.

 

This week a few of my asbestos removal quotes have come back. Most of them are around £500. This was more than I was hoping, but as most of you have said, I don't really have a choice.

 

Thanks for all the comments about bitumen corrugated sheet. Even if I ignore the material differences you have all mentioned, the fact that I can get 3m+ lengths of corrugated steel to span the garage, means installation will be a lot simpler and probably even cheaper than the bitumen version.

 

This has got me thinking about the roof structure...

As stated before, I can't rest any joists directly on top of the wall/wall plates because I have restricted height. So I have been looking into a similar approach to @Ferdinand

But I have been thinking why would I not rest the joist (purple in picture) on the wall plates (pink in picture) and then secure with small 90 degree brackets. I don't know why, but I would feel more comfortable with this setup. Any thoughts or objections?

 

I think the plan would then be to cover the top with OSB/ply, then membrane, then corrugated sheets.

Is this a sensible plan? DO I need the OSB/ply?

Do I need battons if I have OSB/ply?

Is this the right place for the membrane?

I have highlighted a few areas in RED, am I being too picky or will these small angle differences cause me problems? Or are the wall plate on top (green in picture) blocking my ventilation and getting in the way of the OSB/ply?

 

Finally, if I go for the approach in the picture below, there is no ventilation INTO the roof space. There would be a small amount between the OSB/ply and corrugated sheets.

How could I introduce ventilation? Do I need to? Could I fill the voids between the joists with insulation?

 

As always, any advice appreciated.

Thanks,
Rich

[Ferdinand]

14 hours ago, rich080487 said:

As stated before, I can't rest any joists directly on top of the wall/wall plates because I have restricted height. So I have been looking into a similar approach to @Ferdinand

But I have been thinking why would I not rest the joist (purple in picture) on the wall plates (pink in picture) and then secure with small 90 degree brackets. I don't know why, but I would feel more comfortable with this setup. Any thoughts or objections?

 

I think the plan would then be to cover the top with OSB/ply, then membrane, then corrugated sheets.

Is this a sensible plan? DO I need the OSB/ply?

Do I need battons if I have OSB/ply?

Is this the right place for the membrane?

I have highlighted a few areas in RED, am I being too picky or will these small angle differences cause me problems? Or are the wall plate on top (green in picture) blocking my ventilation and getting in the way of the OSB/ply?

 

Finally, if I go for the approach in the picture below, there is no ventilation INTO the roof space. There would be a small amount between the OSB/ply and corrugated sheets.

How could I introduce ventilation? Do I need to? Could I fill the voids between the joists with insulation?

 

For the insulated one my approach was to notch the (obv. tanalised, probably 200mm x 47mm in your case?) joists, leaving them some way proud of the wall plate. That lets them sit whilst you fix them. I also used mini joist hangers that were only a couple of ££ each (Wickes?) to make it rock solid.

 

This means you can fix your box-section directly to the joists. It is important to match your joist spacings to where you will want to walk, but also to the down bits in the corrugated - box section is more forgiving that trad.

 

I would use the OSB for a ceiling finish or just not have any. I think full sarking is gold-plating for a possibly temporary garage, unless you want to walk absolutely everywhere on it - and as you say you lose your ventilation that way and any moisture that gets in could condense.

 

Ventilation I left the corrugated approx 25-50mm short of the wall for ventilation, and expansion if necessary, then flashed over that with wide enough Ubiflex (now I would use the middle spec as it is more malleable), leaving the ends open with insect grilles. I arranged a slot outlet at the bottom end of the roof.

 

Then I insulated between joists leaving a 50mm gap (I used 100mm rockwool and a staple gun), membrane beneath that attached to the bottoms of the joists, then any finish (I used hollow plastic fascia board). 

 

I used a combination of lights on the walls and joists, and external lights wired on the joists. I made my finish removable where needed - any fixings will be in lengthways tension, so you want screws anyway.

 

This gave me a conservatory - lounge type space that is happily used for dogs and people all year. The only limitation is that it will overheat in very high summer when the sun comes in - that was for 2-3 weeks last year, so I would have provision for a lot of ventilation (2 sets of doors each end?) if required or close it up and use a portable aircon for those rare occasions. This one is North-West facing and fine all winter; it does, however, have insulation (75mm Celotex) under the floor over the slab.

 

I used mini shed guttering from (I think) B&Q.

 

I would also consider leaving an overhang at the lower end if you can - perhaps 0.6-0.8m - for standing or leaning things with or without tarpaulins that need to be kept out of most of the weather. That does leave you with a question about mounting guttering though.

 

If you put a silicone nipple or part-bead under the end of each corrugated trough it helps control random drain-off or track back and keep moisture off your timber. In my case I could not find a happy way to mount the gutter on my roof, so I did not do an overhang.

 

Ferdinand

 

Edited January 3, 2019 by Ferdinand

[rich080487]

Thanks for the reply.

I am getting my asbestos removed today, so that is the first step in the right direction!

 

I am getting a better idea of what I need to do, and this evening I will be able to inspect the condition of what is left after the asbestos has gone.

 

@Ferdinand

Do you have any photos I could look at please. Particularly your flashing and insect netting at the end of your roof sheets please...