felting and flat roofing

[Fred]

Hi,

I've a detached garage with a flat roof. The decking was chipboard and the water has got in and destroyed it.

I imagine removing the old roof could be fun. Do I cut the old felt with a knife. Most the chipboard will just fall away and I can use a crowbar for anything else?

Getting some 8'x4' boards up there could be interesting but I would put either plywood or osb3 up there. Interestingly some merchants recommend ply others osb3, they are split 50:50. Some merchants have stressed that they sell structural ply and that the cheaper boards are not structural. OTOH other people have said shuttering ply will be enough.

What is structural ply? Or rather what is non-structural ply? Is it that non-structural ply is ply that does not pass whatever structural test and is therefore inferior or is it exactly the same stuff but the manufacturer has saved money by not paying for a certification?

I've seen some boards (non-structural) with green edges and structural with red edges. Is the colour an indicator or is that just coincidence?

Back to the roof. It is 2.7m wide. That means a join is inevitable. Would it be better to lay the board as a 2.4m board and a 0.3m board so that there is only one join, rather than lay two 1.2m boards and a

0.3m board, which involve two joins?

I'm guessing joins are bad things and should be limited or don't they matter?

I assume it is best to support all joints by placing noggins underneath? Skew nailed into the joists or with joist hangers? The joins in the other direction would be over the joists.

The firring strips need replacing. Over the 2.7m span, the drop is only one inch. Should I aim for a two inch drop or three inches or more? Is there a rule of thumb: span in feet divided by x or something?

Or am I thinking of the rule about joist sizes, doesn't that have something to do with half the span?

I guess the best way to felt a roof is to use hot bitumen but I am also guessing that is beyond the capability of a first-timer (i.e. me). So that would leave me with what, applying cold bitumen adhesive? Is this a summer job? I'm guessing it should not be applied below 10C?

I know not to buy the cheap, thin, shed felt but I am confused about all the grades of felt out there and underfelts. Is there a guide to how to felt a roof out there somewhere?

I know onduline has been recommended here before but I have two concerns:

1. I think it would be easier to remove or break through onduline rather than osb3, so I think a timber deck would be more secure.

1. It is currently a flat roof and for onduline I would need to make a pitched roof. How to go about that? Would you use some 4x2 to make a triangular frame? I suppose I could fit the osb3 deck and then fit a frame on top of that. Who knows, it might work out cheaper than the felt?

TIA

[Tabby]

Yes, knife, crowbar, hammer.

single story flat roof not really hard

You can, but to be honest it doesnt add a lot onto the life expectancy over chip. The main difference is with chip you've got to take care over detailing, it wont tolerate the odd drip.

unless you get 2.7m sheets, they're available but not common

I'd avoid a 0.3m width, its going to be very weak unless you support it all along with noggings.

theyre definite weak points on felt roofs.

I cant imagine any reason to use a joist hanger

the more the better

yes, bitumen in solvent.

if you nail the felt down all round the edges it doesnt matter how long the glue takes to dry.

time for a wiki article on roof felts.

Felt is fibre plus bitumen. life expectancy is primarily down to fibre content: short to long: mixed rag, polyester, fibreglass workability in cold weather: bitumen isnt, modified bitumen good

NT

[Fred]

But the boards are heavy!

If they are rare, then I guess they are also expensive?

I think it's back to the drawing board then because I only thought of laying the boards as 2.4+0.3=2.7 or 1.2+1.2+0.3=2.7, either way involves a 0.3 wide strip.

Perhaps I need to think about cutting a board in (approximately) half and laying a 1.0m wide board with a 1.7m wide board. I'll have to work out what widths I could use without wasting too much material.

I can't skew nail very well!

Thanks for all the information.