Hi Mark,
Good to see that you are pressing on but before fixing your roof deck you should, as bosshogg says, ensure that the rafters are tied. The roof structure as a whole must be adequately tied and braced so that all the components are in their correct positions and acting together to make a stiff and safe platform to work on. You may need temporary props and battens pinned to your rafters. I would stagger the end joints of your sheathing so they are not all in line.
Do have in mind that once the roof is on and closed your structure will be subject to the full wind loading. I would make sure that you are satisfied with all the following:
Ties to the foundations to resist wind loads
Solid connections between the floor and the wall panels. The minimum I would look for is a bolted connection at centre and each end of each panel plus a bolted connection at either side of the door opening.
Solid connections between adjacent panels at each corner close to the top and bottom of the panels plus one near the centre as a minimum.
Good bolted connections between the front and back wall panels and their separate gable panels.
All bolted connections should have large washers both sides, I would use large square plate washers.
Pressed metal plate connectors to strengthen and stiffen the connections between roof structure components, see below.
I am sure you have all this covered and hope you don’t mind me reminding you.
Back to your questions;
Fixing of your shingles should be as the maker’s instructions. As far as I know “collated nails” are used in nail guns, ok if you have one but loose nails would do the same. Never used felt shingles but plain felt fixing is usually with galvanised “clout” i.e. large headed nails.
There are tables showing ceiling joist spans in the old archived version of the Building Regs Part A which can be downloaded free at:
http://www.planningportal.gov.uk/upload ... A_1992.pdf See tables A3 & 4. Remember that all these tables are for buildings that fall under the control of building regs and the sizes will be calculated to restrict deflection to 1/360th of the span. If you are happy to accept more deflection you may be able to reduce the timber sections.
I would not have used rafters on this roof without purlins and birdsmouthed connections at all rafter supports. Working from where you are now I would add hangers to minimise the deflection of the ties/ceiling joists and I would add binders to stiffen the structure, to add some lateral support in the short direction of the building and to prevent winding of the ties. Before you fix the hangers temporarily prop or lift the ties so that the deflection is taken out. You can then add the binder(s) across the top of the ties making sure they are well fixed to the walls at the ends.
I would use pressed metal plates to add strength to the timber connections. They are easily found at decent builder’s merchants. You can see the sort of things at:
http://www.allmat-online.co.uk/download ... alwork.pdf
http://www.batmetalwork.com/bat.pdf
http://www.strongtie.co.uk/catalogue/Straps.pdf
Through nailing or screwing can be ok although it may not provide as strong a connection as metal plates and is prone to split the timber especially at ends of small sections. If the timber is split the strength of connection is lost. Metal plates rely on the use of many smaller nails, commonly square twisted nails, or suitable screws or bolts where appropriate.
Using common rafters on your shallow pitch roof means there will be a lot of outward thrust at the rafter feet. You will need to connect the ties securely to the rafter feet and the head of the wall. I looked up the safe load on a 12mm bolt in 47mm timber and found you would need three at each end. The bolt holes should be a snug fit, not oversized, and the washers (two per bolt) diameter 3 times the bolt diameter. Bolt holes should be 7 diameters from the end and 4 diameters from the side of the members. Toothed timber connectors would increase the load carrying ability of the connection but I don’t have any figures for them. I would consider adding them between the timbers but they do need pressing in to place. Just tightening up the bolt can overstress the threads. You can fit them to low density softwood without the correct tools by using strong G clamps to squeeze the timbers together whilst doing up the bolts, equal pressure on each as you gently tighten the bolt. You may think they are not worth the trouble.
As before you should temporarily take out the deflection in the tie before making the fixings at the ends.
Raised collars will not resist the outward thrust of the rafters. For your span and low pitch I would have looked at purlins or engineered trusses. Purlins would have given you an open roof. Maybe you could redesign your roof to support the rafters by purlins.
What size timbers were they using in the stables etc at your local sawmills? The ones I am familiar with use ex 3” x 2” planed softwood at 24” ccs. The roof purlins of the newer ones are 6” x 2” sawn whereas the older boxes (at least40, probably 50 years old) have ex 3” x 2” purlins at graduated centres: 36” – 24” – 12” top to bottom. The old ones have t&g boarding covered with onduline on the roof. Both buildings are adequate for their uses and don’t show obvious signs of failure but they are significantly smaller than your garden office, 3.1m square for the older ones and 3.1 x 3.7 for the newer ones.
Keeping a simple rule of thumb in mind may be helpful, stress generally increases as the square of the span so if you make your span 25% bigger the stress will increase by 1.25 x 1.25, that is 1.56 times more stress. The same applies to wind loading so a small increase in basic wind speed due to proximity to the coast and being in an elevated country location can make a significant increase in stress, e.g. 50% higher basic wind speed works out as 1.5 x 1.5 = 2.25 times the stress.
I am not trying to make this more complicated only pointing you to all the information you need to make your decisions. When you have a windy day your shed walls, floor and roof need to work together as each element on its own is not particularly stiff in all directions. Imagine the front panel standing on its own, not much wind would be needed to blow it over. So wind load on the front panel needs to be transferred or “shared” with the side walls and the roof. At 50mph the wind load on your walls could be 1.8 tonnes horizontally and 1.2 tonnes of lift on the roof. The mass alone of your shed is probably a bit less than is required to resist this so you do need these good connections from foundation to roof which I commented on at the start of this post.
As an aside, to see a failure in wind is really scary. A few years ago we had a site with a temporary roof up in December. One night about six roof sheets blew off and landed in the car park the other side of the road from the site :shock: . Fortunately the loss of those few sheets allowed the wind to blow through relieving the load on the rest and very much more fortunately the pre Christmas party had ended a few hours earlier and the car park was empty.
PS just seen what bosshogg wrote today and agree with what he said. Apologies for any duplication. The building is simple and will be trouble free if you do it right. There are some important principles in building that are not obvious until someone tells you about them. When you start a project with not so much prior knowledge there is a lot to pick up quickly so I do admire what you have achieved and how you have pressed on.
Graham
