Flat roof gradient - firrings or offset joists

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Many thanks all and look forward to oldun's response.

Alastair,I wish I had seen that document a long time ago. From what I can make sense of, For a flat roof, c16 38 by 147 at 500 spacings would meet regs. Am I creating unnecessary overkill by thinking that 47 by 145 at maybe 500 or 600 spacing would be nice and secure.

Would still be keen to hear weather the best way of doing this is using timber plates on either wall and using joist hangers? What size timber plates, recommended bolts and is it worth using a glue on the back of the timbers? The inner skin of the new wall is thermalite shield blocks. Would expanding bolts not crack these? They seem to have good compressive strength but not outwards, I.e. easy to break.

As an aside, if these timbers are more than capable for a ceiling, why can I not avoid having to build a honey comb wall to support the floor joist mid span. I realise these get more traffic and furniture weight, but curious either way.
Here's my take on all this

The whole point of building is to build efficiently and economically. That means doing stuff as easy as possible, with the least amount of work, and using as few materials as possible

So there is no point in paying for any timber wall bearers, joist hangers and fixing bolts when the joists can just be built in, or at least just hung off hangers. It wont be any better

And a joist and a furring is enough for a flat roof. No need for sloping joists and then messing cutting them or messing about with other joists to level off a ceiling

You should be sizing joists so that you use as few as possible. If 5 deeper joists at greater spacing cost less than 8 less deep joists closer together and with a sleeper wall, then its clear which is best. Likewise for the roof

A joist used in a ceiling bears less load than when used in a floor, so the joist can not span as far unaided when in a floor, so may need mid-span support

And 500mm is not a good spacing. Work to units of 400, 450 or 600mm ... which correspond to plasterboard sizes
Woody, I appreciate your advice and perhaps reality check. I do tend to get carried away, like removing all render and ceilings from upstairs, instead of just skimming over. I have a tendency of wanting things done 'properly', which in hind sight is pointless if we won't live here forever.

If I can avoid timber wall bearers and bolts like you say, I will. Just looking at masonary to timber hangers. What are my options if the walls are complete, I.e. the hangers weren't built into the mortar beds? Can I simply angle grind slots into the thermalite blocks and push in with a glue of some sort or cement? My levels might not be exact to mortar beds.

Sorry for all the questions, but you lot are too helpful.


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By having a parapet, you are wasting a lot of dosh and causing yourself a lot of work due to the fact that your falls have to fall two ways to drain down to hopper.
There are two ways of doing it .
Firrings from existing house draining into box gutter cut into and across the joists adjacent to rear wall of extension and draining in to hopper in corner of flank wall.
Or reducing rips draining cross ways from flank to flank into box gutter built into joists adjacent to flank wall. And falling into hopper at corner of flank wall.
Before telling you what way to go and how to do it, answer following questions.
Overall length and width of building.
Is your parapet built and finished with coping, and if so height of parapet above top of plate height, i.e ceiling height.
Let us know and we will come straight back to you.
Regards oldun.

Thanks for that. Too late to turn back now. Parapet it is and the main reason is appearance. To me, nothing looks worse than a sloping flat roof where you can see the roof felt.

The parapet will have a copping on it and the top of the block work is roughly 22 inches higher than the finished ceiling height. As a priority, e brickies, who finish today have asked if I want the inner thermalite skin to the same height as the outer skin. I have said yes, as I don't have the joist hangers etc. getting a bit panicky about it now though. I guess if I use timber for my joist hangers, at is easier. If not, can I chop out holes into the inner thermalite to push the joists into and cement around?

Many anks
Sorry, I forgot that part. The total width of outer skin to outer skin is 5.7m minus 300 on each side for blocks and cavity. The depth from house is 3m to outside wall. Hope this is enough?
No worries, anything helps. Would be good to know that I'm okay telling the brickies that they can do both outer dense in inner thermalite skins up to top of parapet level. I assume that one way or another the joists can be hung.attached to the wall.

Regards mac
Building the parapet before joisting was one of your bigger mistakes. If your blocks are AAC we would not trust them with though bolts or rawlbolts and masonry to timber joist hangers will be no good due to insufficient courses above hangers plus you are split courses.to achieve a 2.4 storey height with masonry hangers.
Will come back to your joists at the end.
As you have to fall two ways, easiest way to do it is with the primary fall from flank to flank on reducing rips into a box gutter adjacent to flank wall by alley.. Box gutter built into joists, and to either fall front to back or back to front or to centre. Your choice.
For access, maintenance and repair only and a 2700 span, C16 47x150 regularised to 145mm at 600 centres will suffice. These will support 0.75kN/m2 snow load and a 1.0kN/m2 dead load, which is more than enough for your purpose.
To mark out and set out joists.
First joist against flank wall next to neighbours. Fifth joist 2400 centre of off block work same flank wall, then work back with 600 centres on 4, 3, 2,. This will leave smaller centre in 1 and 2, but doing it this way means that your decking and plasterboard will work with no waste or cuts. Joist 9 set 2400 centre from joist 5 and work back same way. Joist 10 set against alley wall and will leave a space of 225mm for box gutter between 9 and 10.
Minimum fall for flat roof is 1in80 but suggest you go 1in60, therefore fall from joist 1 to 9 at 1in 60 = 0166 x4800 = 80mm.cross fall.
In middle of joist number 1 put a packer piece 80mm deep. From the top of this packer string a tight line down and over joist number 9. Measure down from line to every joist and they are your reducing rips to be fixed on top of joists. On the thick rips at top of fall run a bead of Sticks like Shyte along joist before nailing home. You can either cut them your self or order them from a saw mill.
Back gutter. Fall 1in60 = .0125 x2700 = 45mm. Nail a 25x50 batten to the sides of joist 8 and 9. Batten to be level with top of joist at existing house end and 45mm below at back wall of extension. Deck this out on top of battens and will form a dropped back gutter in between joists. Form your aperture for chute and hopper in this corner.
Warm deck to achieve 0.18W/m2K you want celotex TD4000 126mm thick, this includes the 6mm ply bonded to insulation. This will comply for 3 layer felt, but if you want a single ply covering, then you need an additional 12mm ply on top.
Joists either carefully stitch drill pockets in to new existing blockwork and build in or use arbortech saw with plunge blade to form pockets. Hire charge about £70 for week end. Your choice. On existing house wall, plate same size as joists through-bolted to wall at say 600 centres with timber to timber joist hangers
Any thing you do not understand, then sing out.
Regards oldun
Donation to Heroes fund would not go amiss
Oldun, that is some impressive advise. Donation will be arranged tomorrow. Any specific heroes fund?

Your advice will take some digesting, already read it 3 times and it is now becoming clearer. I've just had a roofer over to quote on the finishing and his suggestion was to cut slots with an angle grinder into the aerated blocks and slot timber to masonary hangers into these? Don't think he knew half as much as you, but just checking this is a no no??

My first question is the depth of the pockets into the inner skin. Shall I go the full 100mm depth of the block and run the timber right into the block? Is there a need for a plate under the timber or can I just pack it with cement?

Are you suggesting that the box gully must be inside the perimeter of the parapet? I had assumed the box gully would be attached on the outside of the flank alley wall, With the runoff going through a hole in the parapet.

Only other question is the cutting of the pockets. Just curious, but I wonder if drilling fewer holes and sawing with a plaster board saw wouldn't work?

Massive thanks again for all your help. ;)
Answer your last post.
Heroes Fund is for the boys and girls of the armed forces who have lost limbs, been injured etc in Iran and Iraq. Google Heroes fund to make donation.
Standard masonry to timber joist hanger. To comply, either needs three course of block work or six course of brickwork above, which we do not think you have. Safety fast joist hangers do not need this, but it is not possible to cut safety fast hanger in to block work after built. They must be built in at construction stage.
Your best bet is to cut pockets in as we described in last post, although you may be able to cut them with plaster board saw if AAC blocks. Never tried it.
With AAC blocks joists need to be built in 100mm, and by building in also achieves your lateral restraint.
Roof needs to fall two ways and easiest way is with box gutter between joist.. It can be made to fall three ways without a box gutter, but this would be far to complicated for you.
You need one lateral twisted restraint strap at each flank wall centre of span. Suggest you cut pocket for strap at joist level and notch strap into under side of joist so that it is below box gutter. Put some acoustic insulation between bottom of box gutter and plasterboard.
Regards oldun
Thanks oldun,

Used my laser level last night and looks like the base of joists can sit directly on a mortar bed in the AAC block. This seems like a better base than a thin section of the block, but not sure it really matters? They seem good under compression.

There is something I forgot to mention, which is a 1m x 1.5m dome skylight in the centre. Sorry about that. As it will be 1.5 m a cross it will essentially cover close to 3 joist centres. I will have to do some drawings, but structurally, will it be okay to use noggins and cross braces of timber on either side of the skylight? Just means that 2 joist will not span end to end.

Also, should I get treated joists for the ceiling or is that unecessary? Will def get treated for the floor joists.

Thanks again.

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