Roof Revisit

[i_am_fubar]

Asked some basic questions on this a while ago, but we're now in a position to do something about it.

We have a single story flat roof extension on our property aprox 4.5mx4.5m. Planning was granted to have 'iron railings fitted around the perimeter', but no mention was made about making this into a roof terrice. However, someone has, long enough ago to give it PP imunity though it does have issues.

The main one being the 47x150 joists on 400 centers not being man enough for a terrice. Due to a multitude of leaks also present, we are looking to replace the whole roof.

Having viewed the tables here (please advise if anyone knows if these are out of date):

http://www.google.co.uk/url?sa=t&rc...=dQPxtYrfHVTTNnF0ZGghrQ&bvm=bv.48572450,d.Yms

I imagine that 47x220 would be enough in C16, though that is for internal domestic floors and doesn't consider snow load of a roof. Going upto C24 would increase strength. Or in this situation, would going upto 75mm timbers be worth it?

Currently the joists are bedded into the wall (old external skin only, we believe with a suitable lintle over the opening into the living room). There don't appear to be any pad stones or wall plates.

I am torn as to whether to continue with this method and enlarge the holes in the wall for the new timbers or fix a ledger and use joist hangers off of it. The ledger will help distribute any point loading (such as people, chairs and table) across the width of the roof. But will move the failure point to a shear in whatever fixings the ledger & joist hangers are fitted with.

Is this one that is open to weighing up the options, or do building regs stipulate a specific way to tie flat roofs / terrices to the main building?

Cheers,
Fubar.

edit:

Untitled

• i_am_fubar
• 3 Jul 2013
• 1

 

[jeds]

That table is for a domestic floor. You need the table for a flat roof with unlimited access. (the difference being snow loads) You should be able to google one up from somewhere.

 

[i_am_fubar]

Well, I've found the document I need... "TRADA span tables for solid timber members" but I can't justify spending £21 on it for the sake of one table

What about mounting to the wall?

Fubar.

 

[i_am_fubar]

Also, a friend of mine said that if you but insulation tight up against the underside of a cold deck roof, you don't have to worry about condensation and damp. I find this to be contrary to everything else I have read.

Is there any truth to it?

Cheers,
Fubar.

 

[i_am_fubar]

Got the dims wrong. The span is only 4m.

Reading though:
http://www.dfpni.gov.uk/d.pdf

Tables 2.39 & 2.4 (pages 35 & 36) implies that 47x220 is acceptable in either timber grade, even at maximum dead load.

Depending on cost, I'll either stick with C24 timber, or go upto the next size just to be sure.

Fubar.

[/url]

 

[Alastairreid]

Also, a friend of mine said that if you but insulation tight up against the underside of a cold deck roof, you don't have to worry about condensation and damp. I find this to be contrary to everything else I have read.

Is there any truth to it?

Cheers,
Fubar.

Yes! though convincing the bco may be a different matter.

 

[i_am_fubar]

What's one of those BCO things I keep hearing about

So, for an un-vented warm deck, do you need to vabour barrier the ceiling? If you don't have to, is it still worth doing it for piece of mind?

Can you explain the physics behind this at all?

Fubar.

 

[i_am_fubar]

Few questions on the deck now...

OSB or plywood for the deck? 22mm or 25mm ? I know plywood is the ideal, but can OSB suffice for a terrace? Any chance someone could point me in the direction of building codes for this?

For the roof covering, I am torn between these:
- EPDM (single sheet) + slab supports and paving.
- self adheasive EPDM
- fiberglass
- sarnafil

Which will offer the best resistance to traffic at the best price? Also, which are most sutable for DIY?

Cheers,
Fubar.

 

[i_am_fubar]

Untitled

• i_am_fubar
• 3 Jul 2013
• 1

This is a very rough model to show joist orientation and shape of the roof.

I have a problem in that the deck is so close to the bottom of the door. I won't get my 150mm upstand, let alone any insulation above the deck.

How much of an issue will not having the upstand be? I will be EPDMing with appropriate flashing.

This also means it will have to be a cold roof.

As has been suggested I can insulation fill all roof voids and membrane to negate condensation.

However, if I DID want to have a ventilated roof, how could I work it with the current arrangment of joists?

Cheers,
Fubar.