Advice on flat roof construction.

[wakey77]

Hi guys im looking for a bit of advice on flat roof construction.
Im building a 6m x 6.1m flat roof bedroom extension on the back of my house using a warm deck so my intention is this.

Im dropping an rsj half way through so as to split the span of my joists to around 3m lengths.
space joists at 400mm centres
lay firring strips 75mm-0mm
board out with a 9mm shutterung ply
lay a suitable vapour barrier on top of ply (sugestions on this welcome)
insulate with 150mm kingspan
ply board on top of insulation then either firestone rubber or 3 layer felt it.

I would like to use firestone rubber as in my opinion it will stay waterproof a lot longer than felt HOWEVER
Having been to look at various firestone rubber jobs initially it looks great but after time it starts to look a bit like a patchwork quilt with the shape of the boards underneath showing through etc and as its directly under our bedroom window i dont really want to be seeing that.
With 3 layer felt although im told it doesnt usually last as long it does at least look more pleasing to the eye over time.
Can i get round this by using a tongue and groove osb3 boards then hopefully i would lose the patchwork quilt effect and use the firestone rubber.

Also due to the depth of the roof deck 2 x 9mm ply + 300mm joist + 150mm insulation + roof covering = A deep unsightly fascia board which i am hoping to reduce by ending my joists which will be sat on a 75mm bedded wallplate at the cavity side of inner blockwork then continuing my brickwork 4 coarse to top of joist possibly somethig detailed like dental toothing corbelled out 25mm - 50mm then reducing my fascia to around 175mm
does this sound do,able.

Also any advice on ventilation of the joist cavity is welcome although im told i dont need this with a warm deck i do however intend to insulate the joist cavity with loft insulation.
ceiling plasterboard will be foil backed and i will be strapping down the wallplate every 2m.

any suggestions are welcome

 

[nickjb]

Only a DIYer so take it with a pinch but here's what we did:

Fibreglass for the top. Should last a while and the plan this year is to put a green roof over the top. Should look nice and be good for wildlife.

We finished the joists inside the inner cavity on hangers and block up to top of the joists. No insulation between the joists so no need to vent. Cavity insulation then meets the roof insulation to minimise bridging. Not sure adding insulation between the joists will help much, if you really want more then add more to the 150mm.

 

[ban-all-sheds]

Only a DIYer so take it with a pinch but here's what we did:

Fibreglass for the top.

//www.diynot.com/forums/roofin...s-coming-from-new-flat-roof-extension.363718/

Should last a while and the plan this year is to put a green roof over the top. Should look nice and be good for wildlife.

I assume you allowed for the weight of the green roof when doing the design?

What/how are you planning to plant? I quite fancy the idea of a green roof, but most, if not all, of the ones I've seen actually look like waste ground with scrubby weeds growing on it.

We finished the joists inside the inner cavity on hangers and block up to top of the joists. No insulation between the joists so no need to vent. Cavity insulation then meets the roof insulation to minimise bridging. Not sure adding insulation between the joists will help much, if you really want more then add more to the 150mm.[/quote]

 

[nickjb]

Calcs were based on an extra 100kg/m2 then rounded up a bit. More than enough for the sedum we are planning. I suppose the look is a personal thing. It'll never be a lush lawn but it should have decent biodiversity and be a bit more interesting from the bedroom window.

 

[ban-all-sheds]

It'll never be a lush lawn

It would be a b*gger to mow...

Unless.....

 

[ban-all-sheds]

This one is quite impressive:

 

[theoldun]

wakey77 Hi guys im looking for a bit of advice on flat roof construction.

You are part way there.
47x150 C16 reguiised to 145 will span 3.07 for maintenance and repair/. Included in this size is 0.75 KG/m2 snow load and 0.75 KG/m2 dead load. This will be more than adequate for you.
If your joists are running with fall then use firrings But cut them 95 to 20. Cutting down to zero leaves a flat spot at bottom.
If joists are running across the fall then use set of reducing rips.
12 mm shuttering ply will be okay, but lay good face to top.
Either 1000g or 500g polythene VCL.
We never use Kingspan. always Celotex. To comply with L1B the u value required is 0.18w/m2k. 120mm Celotex TD4000 will achieve this.
You can felt over the top of this, but if you want single ply membrane covering then you need an additional layer of 12mm ply on the top.
Use Instafast fixings for your celotex.
Never yet seen a rubber roof that after time you can not see the outline of the boards.
Facias on warm deck are always a problem. We cut ends of joist down to 75mm where they go through outer wall and before firrings and celotex we fix a 47 x 125 with the 125mm face standing up to collect facia fixings near the top. It is over the cavity so there is no heat loss. Do not insulate joists as could cause interstitial condensation. No ventilation required to joists providing they are not insulated. Do not use wall plates sit joists on top of block work, beam fill and fix twisted holding down strap every fourth joist
That’s about it, which should be no problem for ex ballerina.
Regards oldun

 

[wakey77]

Hi the oldun many thanks for the advice.

instead of using 12mm shuttering ply would 11mm osb3 be better to use as im told its made for the job for the bottom deck and for covering the insulation prior to laying the rubber.

Also is there any structural reason for extending the joists through to the brickwork face at a depth of 75mm or could i not take my brickwork right up to the underside of deck board / top of joist reducing the depth of the fascia by a further 75mm and then fix my fascia to the 125mm upstand you advised on the deck board.

am i right in thinking the 1200g polythene i use under my pads is the same stuff i can use as a vapour barrier over the top of my 1st deck board.

thanks.

 

[theoldun]

Sorry for delay in coming back to you.
1200 polythene okay.
11mm OSB okay
Facias. Difficult to explain this.
Cut joists to size flush with external brickwork
Mark down end of joist 75mm and 200 back (if 300mm wide cav wall) Cut this out so joist sits on internal wall and 75mm end of joist is flush with external wall. This will give you a noggin for fixing bottom of facia. Beam fill to joists Deepest depth of facia including the above 75mm and 25mm tilt fillet at top will be 239mm or there about. We use a 22x250 UVPC facia. As said previously clunk a 47x whatever depth you need on the end of the 75mm portion of joist to give you a centre and top fixing for facia. Stick your tilt fillets with Stick likes Sh*t.
Hope you understand.
Regards oldun

 

[wakey77]

thanks again for the info very helpful

 

[ban-all-sheds]

Facias. Difficult to explain this.

Do a drawing?