Building a Garden Office Advice Required

[Dibs-h]

Mark

Obviously we aren't discussing aerofoil design here, as you don't want your building lifting off.

Framing issues - over your openings you are missing the doubled up headers. The wall plate where your rafters sit are undersized. Are the nailing patterns for the studs, noggins, etc. as per TRADA's recommendations? Have you cut birdmouth's in the ends of your rafters? What is the depth of the vertical cut of the birdsmouth as a ratio of the rafter depth? Have you skew nailed them from both sides?

"Using a ratchet strap to close up the gap where the rafters meet the ridge board" - it appears the length of the rafters is marginally short. How well do you think the fixings will cope with the tension that will be created once you release the straps and over time under the tension?

How do you propose to contain the outward thrust at the base of the rafters? What's the depth and thickness of your ridge board?

Having created 3 openings in one wall - a set of french doors (IIRC from one of your earlier posts) what negative affect does that create in terms of wind loading? How do you propose to deal with that? In my own build putting 3 openings in a masonry wall caused the structure to fail wind load calculations with a traditional purlin and rafter roof, until the design was modified to include 2 large King Post trusses and their large tie beams. That's in a sheltered location compared to yours. So how is your current design going to fair - worse, comparatively speaking.

Is you nailing pattern for the OSB as per TRADA's recommendations, in terms of nailing pattern\density and nail spec?

What are your primary requirements from a roof? Ease of build? Headroom? Cost? Aesthetics? As these will dictate the options you have to choose from.

As for taking your comments the wrong way - don't worry about that, after all it's your shed. My comments earlier regarding pausing for thoughts still stand. I appreciate you might not see it, but to me this build, i.e. planning on the hoof, isn't the best way forward. You may pull it off or you may not. You may disagree - in which case we can agree to disagree.

Regards

Dibs

 

[Markymark]

Thanks Dibs

That's exactly what I need. Possibly a little bit more on how to rectify said issues.
Positive criticism. With regards to nailing yes it is in excess of Trada's specification. I have birdmouthed all the rafters and they are skew nailed. They are bird mouthed to 1/3 of the rafter depth. As I am going to turn them in to Trusses shortly or their equivalent shortly I feel I am addressing the wind load on the roof. The roof itself is having sterling boards fitted with felt shingles or something similar so the roof has very little weight.

The front elevation is a concern for me. but I feel some binders should provide a little support as the collars will be running the other direction.

With regards to the rafter length they are all exactly the same length of the Pattern rafter. This leaves two options either the ridge which is 50 x 125 is not straight but then the rafters on the opposite side would be high, or the walls are deflecting under the load and that when I correct this by installing collars they should meet accurately at the ridge beam.

I read your Workshop project and I am very impressed with your king beam trusses. This is something I would love to work on in the future.

Apologies but I am getting a lot of pressure from her indoors regarding the build and feel I just need answers not problems so late on in the build.

I hope I did not offend with my comments and I will attempt to address the issues you have so kindly highlighted.

P.S. A wing is very similar to a roof, the areodynamics cause low pressure above causing the suction effect you pointed out which in a plane induces lift. :wink:

Mark

 

[Dibs-h]

... I am getting a lot of pressure from her indoors regarding the build and feel I just need answers not problems so late on in the build.

You aren't the only one in that boat - that's day to day life for a lot of us.

 

[Markymark]

( I assume you have worked to metric dims therefore you should get metric 11mm boards 2400x1200x11mm they are available)...bosshogg

I have worked to Imperial measurements because my local wood yard and Builders merchant stocked 2440 x 1220 x 11mm so I stuck to 2' centres. I have followed those centres to the roof so each rafter is sat on a stud. Should I not use 18mm Sterling Board to cover the roof as I am using felt shingles which are attached using 18mm clout nails ( I think).

Still a little unsure on the size of the timber for the lower chord (Collar).?


Mark

 

[bosshogg]

Mark...12mm OSB is standard dims for roofing, even atop seven story buildings, why anyone would want to manhandle 3/4" (18mm) 2'x4' sheets on a roof would puzzle :shock: me...stick with the same timber as rafters...only thing birds-mouthed rafters will do is displace the point of the rafter/collar joint inboard 20mm or so!...wouldn't advise using loft as storage loft from the headroom point of view, at present with the layout suggested you will produce some 2 meters wide along the full length of the building, provide a hatch, and store only that which is easily reachable...as far as advisory organisations go, you might want to approach your local building control office (this is not PLANNING) which you have already stated is not required. Actually I wonder, whilst you are not required to apply for planning permission, you may require to provide a building warrant, best check, if no ask for there advice, see if it comes without cost to you...bosshogg

 

[Markymark]

Thanks

Cembrit say that the felt shingles should be nailed in four places with 3-3.5mm Galvanised clout nails of a minimum of 22mm in length. I suppose I will have to put at least 18mm Sterling board on the roof. There is also a safety issue with climbing on 11mm OSB. I would not feel comfortable climbing on a roof whose rafters are at 2' centres and is covered with 11mm board.
http://www.cembrit.co.uk/Admin/Publ...es/Filer/uk/PDF/Shingles/Bitumen-SHINGLES.pdf

 

[bosshogg]

Mark…I think you’d best qualify from the felt shingle manufacturer what roof substrate they expect these shingles to be fixed too, no roof covering I’ve ever experienced would take 22mm or more clouts without penetrating through the sheet, this would mean that loft spaces would become dangerous places where peoples scalps could get cut. The only thing that I can come up with is that these are expected to be affixed to counter batons, best check…I can understand anyone new to walking on a roof sheeting of only 11mm thick, but I can assure you it is quite safe from that point of view. Health & Safety regs. would not condemn the substrate, but would expect compliance with working at height regs. a minimum of an eaves height scaffold, you won’t fall through the roof but you can fall from it!
2’centres…that leads me to another issue, I don’t know of anywhere that you can get Imperial plasterboard, in my experience you can only source metric, have you figured this out?...bosshogg

 

[bosshogg]

Mark…I think you’d best qualify from the felt shingle manufacturer what roof substrate they expect these shingles to be fixed too, no roof covering I’ve ever experienced would take 22mm or more clouts without penetrating through the sheet, this would mean that loft spaces would become dangerous places where peoples scalps could get cut. The only thing that I can come up with is that these are expected to be affixed to counter batons, best check…I can understand anyone new to walking on a roof sheeting of only 11mm thick, but I can assure you it is quite safe from that point of view. Health & Safety regs. would not condemn the substrate, but would expect compliance with working at height regs. a minimum of an eaves height scaffold, you won’t fall through the roof but you can fall from it!
2’centres…that leads me to another issue, I don’t know of anywhere that you can get Imperial plasterboard, in my experience you can only source metric, have you figured this out?...if you want I could send you a PM detailing the road map to truss roofing, let me know…bosshogg

 

[Markymark]

Hi Bosshogg

Awaiting a reply from Cembrit regarding felt shingle fixings. With regards to 2' Centres, I chose imperial due to the fact that all the sheets of OSB were 1220 x 2440 (8' x 4') With that in mind I could either have it easy on the outside or on the inside not sure why they all dont stock either imperial or metric sheet products? For additional resiliance I was going to clad the internals with either ply or some other imperial sized sheeting product.

Mark

 

[bosshogg]

Mark…just call me boss…no extra resilience required, if I understand your use of the word correctly [able to absorb shock/load] this is not an improvement as such. Better if you can source 40 or so 2440x1220 standard or better still duplex boards (foil backed) which provide both the wallboard and an integral vapour barrier (walls only) ceiling sheets must breath into a vented loft, the roof covering will provide the vapour barrier. Post your reply from the shingles manufacturer for scrutiny, and we can assess the best avenue of approach…bosshogg

 

[Markymark]

Hi Boss

Had a short reply via email from Cembrit. He said " they wood recommend 18mm decking as a minimum."


mark

 

[bosshogg]

Mark...18mm OSB on a roof, better you than me. Wondered if you would care to view this www.dwbgroup.co.uk/.../Gang Nail Tr...ue. shouldn't be hard to put right...bosshogg

 

[Markymark]

Mark...18mm OSB on a roof, better you than me. Wondered if you would care to view this www.dwbgroup.co.uk/.../Gang Nail Tr...rl]http://www.flickr.com/photos/markymark144/

Thanks for the continued help.

Mark

 

[bosshogg]

Mark...having reviewed the pic you posted, I agree with you. Although you are trussing your roof assembly, the ridge board (which has a live load) bears on the gable pinnacle, this load in turn is transferred via partially down these 20 degree top runners, lessening the load at the door & window slaps. In truth it would have been better to beef up these "lintels" but little harm done...bosshogg

 

[Markymark]

The collars are going in. I opted for the first option you specified Boss. I am using M12 Bolts as Graham specified for the connections between the collars and the rafters and I have to say they feel very sturdy.

One thing that has amazed me, I initially skew screwed the rafters down to check them in position. I found a number of the screws had sheared. This has taught me never to use screws in sheer situations as they are just too brittle!!


Upwards and onwards!!

Mark

 

[bosshogg]

Yeh the modern screw differs from those of old, nowadays the emphasis is on hard screws that power in through deep fluted threads, that and the tension to be found in Tanalised timber can stress the screws. One thing I always have to hand when driving in screws is a bit of soap, when the screws are rubbed on this they go in without any stress so much so I have to wind down the clutch/slip setting on my cordless, otherwise the screw goes right through, even on 60mm batons.
Once your collars are up and bolted you can push some scaffold batons onto the upper face giving you a platform to work on the roof from (through the rafters) handy for fixing those 18mm OSB boards you intend to use...good luck...the weather's certainly in your favour...bosshogg

 

[Markymark]

Hi Boss and all

I need to install insulation in the floor space of the home office.

The problem is that I also want to keep insects and small mammals out of the floor space also. As the underside of the floor joists are between 6" and 3' away from a woodland floor covering I expect that it will be damp and humid. I was going to install ring beams in all the cavities and drop some OSB 3 in each cavity and then either fill with rock wool or celotex expanded insulation. My other option and a much faster one is to nail/screw 8 x 4 sheets of OSB 3 11mm to the under side of the flooring frame. I am only concerned that this may sag or rot over time.

Speed and efficiency is my aim as I am trying to seal the whole outside office so I can safely fit the roof.

Any advice or ideas gratefully received.

Mark

 

[bosshogg]

Hi Mark,

Mineral wool insulation is probably best, it is neither attractive to rodents nor vapour permeable nor affected by moisture. To install, simply drape plastic netting over the joists and make pockets to receive the mineral wool. Make sure you leave sufficient space so as the wool is not squashed, but snug to the underside of the flooring, use a stapling machine to fix the netting on top of the joists. :idea: One thing worth consideration for the proximity to ground level, would be to use mineral wool slabs with a foil back, this would deter "beasties" and will reflect cold back to the ground/wind etc. make sure you cut the insulation for a close fit, don't leave any holes or you will get cold spots. The idea is to nullify any thermal transfer between inside and out, keeping all and any air leaks at bay.
You have done well to to this point so far =D> ...bosshogg

 

[Markymark]

Thanks Boss

I like the idea of netting. I would like some rodent proof netting or may be chicken wire would do best.

OK, I have not boarded the roof yet but I feel that as the roofing timbers have been up a while now, they wont be true and level and that when I lay the boards over them certain rafters will be a smidgin lower or higher. I am going to put a string line on to see. If that is the case what is the best course of action. I assume the collars will be the same, well I know they will be as you can see the variance. A couple of binders installed in the loft space should suffice here me thinks?


Mark

 

[bosshogg]

Mark...When you fit your wind boards in the loft space you might need to turn one of them on its edge, thereby giving you some extra beef to pull the collars into alignment, this is only to allow for the ease of fitting of the ceiling P/B. I doubt in the short time span we are speaking about, that Tanalised batons will have warped very much and as you are using 19mm OSB3 as your roof covering this will pull the rafters into alignment as you fully fix them. It could be worth fitting ring shank nails into your nailer so that you achieve a shake proof fix (wind can sometimes set up vibration). If you do find any rafters more than, lets say 19mm, out of alignment you can push them up using the collars (aligned and restrained by now) with suitable stub timbers wedged into place, using a hammer and nails put a restraining nail, in the collar/rafter, at the point of possible slip of these stubs. Once you have fully nailed up your roof cover boards and before putting on the shingles, knock out these stubs which will allow the roof to settle, then fix your shingles.
Assuming you have no tears in the wall outer breather/membrane you should now have a weather proof structure =D> , fit windows and door and your building should stand up to a storm if need be =P~ . If you haven't done so fit the bottom hold down straps to walls and foundation piers to finalise the structural bond to Terra Firma. All that's required now is to fit your chosen cladding, and finishings as you go :mrgreen: ...then it's on to the inside...bosshogg 8)

You can't help a man who doesn't tell you what he wants