roof advice needed

triathleterob

My new breezeblock workshop is 5m x 4m.

I originally intended to use felt on the roof but I'm now considering bitumen corrugated sheets.

I've fitted the rafters at suitable points to lay 8' x 4' plywood sheets but they won't align with the corrugated sheets which are only 950mm wide. I have rafters every 45-50cm, can I still get a good fix if I screw into the 18mm boards where the corrugated sheets overlap?

My other concern is that my builder said a 3" drop would be enough for water to drain to gutter fitted at bottom of roof. I've since read that 5% is minimum, I have only 2%.

Any advice appreciated.

Thanks

Furts

Personal opinion, not based on anything technical, but I would be aiming at no less than 15 degrees to give a fall on the roof. I accept it is a workshop, so it is all a matter of how much flatter you can go till a problem arises. It also depends on how exposed it is and how wet your area is. Basically you may take a chance on it.

Have you read the technical data sheets, or manufacturers recommendations, for your corrugated sheets?

triathleterob

Thanks for replying Furts.

According to manufacturers info, sheets should overlap above a rafter for fixing. They also suggest a 5% minimum fall.

I guess I was hoping for someone with roofing experience to tell me I could ignore all that and use them anyway! It was a long shot I know!

I feel I could fit the corrugated sheets myself but I'm not confident enough to attempt either torch-on felt or the cold application stuff. It's all the edges and corner detail that bother me.

My budget is tight so I'm trying to avoid paying anyone at this point but it may come to that.

Cheers

aurion22

Not sure if this is the right place: I searched for "roof problem" and this thread came up. Anyway, I've just noticed that a couple of chunks of cement have fallen off from under the ridge tiles on the extension (built in October 2010) and some other bits of the cement are cracking. Is this the builder's responsibility? Trouble is, I'm afraid he'd "bodge" it .. the workers were okay but they said he kept harassing them to do the job as quickly as possible!
I don't want to get someone else in if he's still responsible for it but I expect I'd be better off doing that anyway. Just thought I'd check out the lovely people here first.
Many thanks.
Loretta

triathleterob

Hi Loretta

I think that's known as thread hijacking! How very rude! :rotfl:

If you google How long is building work usually guaranteed for? it will take you to another thread on this site which may help you.

I hope you get it sorted.

Cheers

aurion22

Thanks for the re-direction. Humble & profuse apologies for the "hi-jack" .. but the thread you've sent me to doesn't address the issue.
Any other ideas?

triathleterob

"and in any case most builders only guarantee their workmanship for 12 months."

It was the above comment that caught my eye.

My builder was recommended by work colleagues. They'd had work done by him and were very pleased. I'm also very pleased. My roof problem is due to me changing my mind on roof construction!

Ask around, I'm sure someone will know someone!
I'm not a builder but I don't think it will cost much to put your issue right.

Good luck!

capeverde

With the bitumen sheets (coroline or onduline) they dont fix to rafters, you need purlins or at the very least tile battens running horizontally. The shallower the pitch, the closer the battens. These sheets will sag at the minimum stated roof pitch (5 deg) even with the battens at 300 centres. With that pitch, all you can do is close board your roof with marine ply first, or batten at maximum 150mm centres. You dont need to fix on every batten though.

triathleterob

Hi capeverde

Thanks for replying.

During the course of my workshop build I've noticed how different people use the same terms but apply them to different parts of the roof structure, which I've found very confusing.

Here's the current state of my roof as I know it-

I have one 5" x 2" purlin running across the 5m width, which supports eleven sloping 4" x 2" rafters running across the 4m width.

I have 18mm plywood boards ready to go on when weather permits to provide additional security.

Can I fit bitumen sheets to this structure (2% drop)? If not, what do I need to do to make it possible? Or should I stick with my original plan to torch-on felt it?

Many thanks

Rob

ian103

Simple answer is yes but not recommended as your roof pitch is too shallow.

Why not go the single ply flat roof route? plenty of manufacturer choices out there.

Or a grp roof but that will require a company to install.

vaio

The recommended min pitch will be to deal with the worst of all worlds and to avoid sagging/water penetration in windy conditions at a very exposed location with purlins/battens at max spacing etc etc.

You have close boarding so sagging isn't going to be an issue, I suspect you'll be fine with the pitch you have but if you do happen to live in an exposed position then maybe increase the overlap between adjacent sheets