Need flat roof advice

[will_uk]

Hi everyone,

Im currently making a small workshop/shed in my garden, it's been built on a concrete base using pre-fab concrete panels (just like a concrete sectional garage, but smaller).

It's about 8x10ft (2.4x3m) in size. Basically 5 panels long, 4 panels wide.

I'm trying to build a flat roof to go on it.

Basically, I'm thinking of using some 4x2" joists at about 2ft intervals, spanning the width with some noggins between. Then covering it in some 18mm ply that I already have.

The panels are slightly staggered, meaning that along it's length there are 3 panels which are 215cm tall and 2 panels which are 210cm tall. I'm thinking of making some slanted supports out of 3x3" timber for the joists to sit on and cutting the notches so that the joists will be at the correct 'tilt' for the ply to sit flush on top?

Does the roof need to tilt slightly to one side as well as having a fall over it's length? I'd like to have a water butt in one corner so it would help if I can direct the water to a drain pipe that way? Or should I keep it level and just use a gutter along the front?

Going on my rough working out at the moment, it would have about a 3" fall over it's length - does this sound okay?

Is there any serious problems with my plan or am I along the right track?

How should I attach the joists to the supports other than them sitting in the notches? Should I just nail through at an angle or something else (eg strapping or something?)

Any other advice much appreciated

Thanks,

Will

 

[will_uk]

Hi again,

Right well I've started to build the roof as per the diagram above.

I've created a 3" fall from back to front over 10ft. Basically, 15mm every 2ft/600mm.

Have cut the supports for the joists by hand out of some leftover 3" fence posts (hard work - took ages! ), these are bolted to the top of the concrete panels. Was quite time consuming measuring/cutting/then measuring again/chiseling out the notches for the 4x2" joists but I'm pleased now it's all done

Now all I need to do is nail/screw the 'deck'/boards down to the joists after cutting them to size.

Could anyone please let me know if this final part sounds okay:

1) Skew-nail the joists in place to the supports that they're sitting on (they seem pretty solid as it is anyway )

2) Cut the joists down so that there is a 1" overhang all round the edge.

3) Cut the ply to cover the main roof, screw or nail (?) to the joists.

4) Make an edge/'upstand' on top of the roof board along the back and sides out of 3x3 Arris rail and maybe a 3x2 around the edge as well.

5) Fit some boards around the edges screwed to the joists (fascia boards?). On the back and the front where the joists aren't sticking out (along the flats of the joists/edges), use some timber to space out about 1" along?

6) Then finally felt the roof and put a gutter up along the front etc.

I'd really appreciate any help or advice you can give. I've been working on this as best I can and I'm certainly no 'pro', just trying to get this all waterproof before the bad weather sets in again...

Many thanks in advance

Will

 

[Xenon]

Looks like a decent effort to me Bit of facia round the sides to cover the opening, job done.

 

[will_uk]

Cheers for that Xenon, it makes it seem more worthwhile somehow hearing that

Do you think that the rest of my steps sounds correct - ie nailing the joists in, putting the deck on and fitting the fascia boards?

Is 1" about right for the joists to stick out? What do most people do flat roofs to?

Any pointers or tips gratefully received

Will

 

[Deluks]

After your first posting, I would've suggested making up some mini trusses and going for a shallow pitched roof, less likely to leak and would give you more headroom/overhear storage inside.
Still you seem to be doing ok with your current plan.

Your proposed 1" overhang should be at least 4"

skew nail is fine. Screw or nails for the roof deck, but use ring shanks if nailing.

Arris rail good for the edges, but cut off the top edge of the 'triangle' so there's less of a steep sharp angle where the felt goes over. A plane, circ saw or table saw would be best for this, or get the edges ripped off from the place you get your rails from (proper timber merch, not a DIY shed)


If cutting 3" fence posts is hard work then you need a new saw

 

[will_uk]

After your first posting, I would've suggested making up some mini trusses and going for a shallow pitched roof, less likely to leak and would give you more headroom/overhear storage inside.
Still you seem to be doing ok with your current plan.

Your proposed 1" overhang should be at least 4"

skew nail is fine. Screw or nails for the roof deck, but use ring shanks if nailing.

Arris rail good for the edges, but cut off the top edge of the 'triangle' so there's less of a steep sharp angle where the felt goes over. A plane, circ saw or table saw would be best for this, or get the edges ripped off from the place you get your rails from (proper timber merch, not a DIY shed)


If cutting 3" fence posts is hard work then you need a new saw

Cheers Deluks,

I did think about a pitched roof, but to be honest it's quite tall for a shed as it is (over 7 feet exc. the roof on top) and I'm trying to limit it's impact on the garden/keep the neighbours happy, etc

Nice tip for the Arris rail, I'd not thought of ripping the edge of the rail off (would be too difficult for me to do myself), was going to use say a 50x50 arris rail next to a piece of 2x2/50x50mm timber - same effect I guess but your method sounds more simple

When I said about cutting the 3" fence posts, what I meant was along their length. Ie, nearly 1 metre of sawing against a 3" cut (don't ask, it was late at night after work and I couldn't use power tools...) This was so I could make one of the joist supports as in the photo

I've got two whole sheets of 8x4 18mm ply here, and some more of the same 18mm ply that's already been cut. Is it best to use the two 8x4 sheets right in the centre so that where it's most likely to 'bow' (not that it should with 4x2 joists?) will be made of the whole sheets. Then do a 8x1 piece at the front and the same again at the back (shed is 8x10 in total)

Thanks for the pointer on the overhang - so joists 4" out either side from the outer panel and then screw the fascia onto that?

Should I fill the gap between the deck boards and the joist supports with something to stop the inevitable spiders etc from coming in to make home

Thanks again for your help

Will

 

[will_uk]

Okay guys,

Currently trying to work out how much overhang to leave on the joist ends.

I was originally going to leave a 1" overhang along the sides, and battern the front/rear supports out by 1 " (well only the rear as the front has a 1" overhang already). Then fit the fascia boards to the joist ends.

Probably put a small former right on the top of the fascia to bring the felt edge out a bit to drip off. I guess if I use 6x1" for the fascia, and maybe 2x1" for the drip edge it would then come out about 3" or so in total?

Does this look about right? (I've left off the arris for the roof top in this picture)

Is this too small, does it need to be 4" even for a shed?

Had a quick glimpse at the small flat roof on the front porch and that only seems to come out about an inch or so.

 

[Xenon]

At the end of the day, its a shed, cut the joist ends flush with the concrete panel, and fix the facia so it is against the panel. Then a 2x1 batten round the top for your drip detail. It doesn't warrant or need a boxed soffit detail.