Building a Garden Office Advice Required

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Graham

Thanks for the reply and compliments.

The outer OSB sheathing is nailed using Galvanised ring shank 75mm nails at 150mm spacing.

The roof is going to be a 20 degree dual pitched roof with the gable front and back. I was going to leave the roof space open inside and so there would be no ceiling timbers. Looking at the roof design pdf you sent me my open span on the rafters would be acceptable without any bracing between the rafters. I assume the OSB boarding that I am planning on using on the roof will tie the two gable ends in to the structure giving me a said "closed box".

I was unsure what you meant by the trada publications? Sorry for the naivety. Do I need more studs??

I will attempt to measure the wind deflection but I feel I am preparing for a 10 year wind. 2009 we were hit by 80mph winds all over the south west. I am not sure how to ensure my structure does not disappear over the hill?

Again let me thank you for all your help during this project.

Mark
 
Hi Mark,

I was confused because I thought you might use my numbers when answering the 5 questions but I think I have joined your answers to my questions now. The following replies are in the order of your last post:

OSB sheathing is nailed at 150mm spacing

That was my question 4. Nailing at 150 mm centres is ok. If you needed to increase the resistance to racking of the panel nailing at closer centres, as close as 50mm, around the panel edges can achieve that. I would only do this if racking resistance proves to be a critical factor.

The roof is going to be a 20 degree dual pitched roof with the gable front and back. I was going to leave the roof space open inside and so there would be no ceiling timbers

MarkM150.jpg


Is this sketch how it will be? I have only shown one purlin although obviously there will be more! If so, the purlins plus roof sheathing and ceiling lining together can make an adequate tie between the gables provided all the roof elements are the correct size and spacing etc. and fixed together soundly.

I was thrown by your photos because there were no gable ends, that’s why I asked about a shallow mono pitch falling to the rear. I would have extended the wall studs up into the gable frames from the walls below to avoid a joint between gable panels and walls which could be a weak point

I was unsure what you meant by the trada publications?

I read item 3 of your post of March 19th
The centres are 2'.
as a response to my question 4 about nailing centres. My “TRADA” quote doesn’t relate to stud centres, its all about nailing the sheathing to the frame.

I will attempt to measure the wind deflection…

We are coming out of the windy weather now so you may not get the chance to measure the deflection under the strongest wind events until the autumn or beyond. However, I don’t believe measurement would prove much until either the roof is in place or you put in good strong temporary bracing in lieu of the roof. To give you some numbers for how wind varies through the year and where it blows from see the average wind data and wind rose at the bottom of this web page:
http://www.metoffice.gov.uk/climate/uk/sw/print.html

I am not sure how to ensure my structure does not disappear over the hill?

If you were inland in a normal location you wouldn’t need to worry as your structure is potentially much stronger than most timber garden buildings but I share your concern about 80mph winds. Being close to the sea and in the south west of the country puts you in a higher risk category. If you are also near the top of a hill your exposure and risk is much increased. Is your site location something like this?

MarkMSite250.jpg


There is some information in part A, Section 2C of the building Regulations which is generally informative (wind speed map and diargrams page 18) but only gives answers for small masonry structures so will not give you an answer for your timber frame.

The comments I made on March 3rd still stand. If you can’t find some existing local timber buildings in equally exposed positions which have withstood wind loads to emulate you may have to pay someone like a structural engineer to check your proposal by calculation.

It appears you have neighbours close by so if a bit of the building did take off there could be damage to persons or property which could cost you a fortune.

On the positive side while there is no internal lining or floor boarding in place it would not be difficult to reinforce the structure if that were needed.

Graham
 
Graham

I have opted for traditional roof construction with rafters instead of purlins and a ridge board. I used the councils roofing tables to find the timber size and type. The gables are correct in your drawing.

The layout plan is similar. We are quite well protected by trees. The shed is been built in a small copse with a number of trees around. So even though it is elevated and has quite a view it is surrounded by trees. To the south west there is a lot of houses and very large trees. The only exposed elevation is the front elevation which is north west.

Lets assume I wanted to over engineer the structure to cover most eventualities except tornado alley. What additions would you recommend to my structure?

Hope I can repay all your help in the future.

Mark
 
Hi Mark,

info about your site and exposure is reassuring.

When you say rafters and ridge board will you have ceiling joists spanning from wall to wall parallel to the rafters?

Graham
 
Hi Mark,

Info to help you choose a roof structure.

MarkMGardenOfficeroof001Red.jpg


MarkMGardenOfficeroof002Red.jpg


MarkMGardenOfficeroof006Red.jpg


MarkMGardenOfficeroof007Red.jpg


I would avoid the couple roof (7.4c) for anything but the smallest low value structure.

I would also avoid collar roofs unless you are driven to it by lack of headroom.

Graham
 
Hi Graham

No problem with headroom. The open span of the rafters is 2.7m or thereabouts.

Putting ceiling joists in would make a clean but rather box like design and I keep adding to my already stretched budget. The wood yard charges a premium for timber over 4.8m. Saying all that it the difference between doing it right or not. Another issue for me to consider is that the close coupled (e) would require ceiling timbers of a span of 5190mm which I would assume would need additional vertical support in the middle?

Also with a cold roof space I am assuming I would need to ventilate it using soffit ventilation??

Could I get away with one ceiling joist (Restraint) every other rafter??

Mark
 
Hi Mark,

I saw you were ok with headroom, it was a general comment about collar roofs.

I understand what you say about budget as your first post called for a 10’ x 14’ garden office but it is now near 60% bigger on plan :)

Have you decided what the roof finish will be? Your final design can be adjusted to carry whatever you decide to use but I have assumed that you will use 18mm sheathing covered with felt which would come in below 0.5kN/sm

You could put in ties without a ceiling. Ties could be smaller as they don’t have to carry the weight of a ceiling but would need hangers to stop them from sagging. Ties could be tension wires although if you bought the smart looking stuff and its hardware it would cost much more than timber.

You could consider using common rafters with purlin support at the centre of each slope plus a purlin as the ridge. Table A6 in Part A of the Building Regs 1992 (now withdrawn) doesn’t stretch to a 4m span purlin but by a bit of extrapolation I expect a 300 x 63mm purlin should be ok. You would need about 8 pairs of rafters (38 x 89 is the smallest in table A5 but you would probably be ok with 50 x 75) plus the 3 purlins. Because the purlins are long they should be braced to prevent winding. Add 2 more 2.7m lengths to cut for solid bracing between purlins. The purlins would be built into the gable end framing set vertically and the rafters birdsmouthed to the purlins and wall head.

However if you were willing to consider this scheme why not use 9 purlins to support the roof sheathing directly without any rafters. Extrapolation from Table A23 of Part A of the Building Regs 1992 suggests 63 x 200 should be ok at 600mm ccs.

Neither of the above include timber to build your eaves/verge ladder frames.

As discussed earlier there probably will not be so much water vapour in a garden office as in a dwelling. However, I would vent the roof to let water vapour escape.

Hope that is some help

Graham
 
Ok Status Update

I used a Roofing table and went for 8 Pairs of Rafters at 50 x 125 PAS Tanalised. I have cut out the birds mouths and done the plumb cut. I installed them using a 50 x 120 x 4100 Ridge Beam. All C24 I hasten to add.

As I have never installed rafters on this scale I was quite surprised to feel first hand the forces especially on the side walls. It took quite a bit of balancing to hold each rafter in place as I skew screwed each to the roof plate.

Graham I feel I need to brace these rafters. Do you think 50 x 150 x 5100 will work? What sheathing should I use as I am going to use the rest of the Tyvek Housewrap as a breathable membrane on the roof over 18mm OSB/PLY and then fitting felt shingles. I think the shingles need fixing with 18mm hot dipped galvanised collated nails?

Thanks for the advice in the last post. We are out of sync at the moment as the missus is pushing me to get it completed for May.

I have included some updated photos below.
5570120681_8c295ae28e.jpg


5570711210_761553dacc.jpg


5570119349_09fbe9f993.jpg



Mark
 
Hi Mark,

When you say brace, are the 50 x 150 x 5100 timbers to be used to tie the feet of the rafters to resist outward thrust or bracing in some other way?
If as ties, are you putting a ceiling in?

Graham
 
Hi Mark...bosshogg here...I wonder if you might need a second opinion on some matters of your build. I assure you I don't want to step on anybodies toes, so to speak, I only come upon your posts today. I am a semi retired Project Manager in the construction industry, to my credit, and have been involved timber frames for a number of years...bosshogg
 
Hi Graham

Yes, I do mean to tie the feet of the rafters together to resist outward thrust. Do they have a ceiling spanning tables info sheet??

Hi Bosshogg - Your feedback is greatly welcomed as is all feedback.

I feel I have created a monster, I now have the scary task of boarding the roof and assuming it is safe to climb on secure 8' x 4' 18mm OSB 3 on. Then fit a membrane and fit and nail the Felt shingles. All that when I have a fear of heights. :lol:

On a different note I have just been to the local sawmills. I was comparing there buildings, Stables, Sheds etc. They build all of them at 16" centres. I have to say they look very substantial , I feel they need to be as they originate from making Stables and agricultural buildings. I took a look at Waney edge Douglas fir at 33p a Foot for 8"-10" Wide boards. Look Very nice!!!

Mark
 
Bosshogg

No Not yet. I need to calculate what size timber to use. The span is 5m. The rafters are 50 x 125 with a open span of 2.8m. I was going to use 50 x 150 C24. I agree I wont attempt climbing the roof timbers until it is fully coupled.

Alternately I could fit the collars higher up the rafter span this would mean that I could have more attractive open roof.

Is there such a thing as ceiling joist tables???

Mark
 
Mark...Honestly you don't need a reference table, this only a large outhouse for which you have already stated doesn't require planning permission. The crucial thing is to build a building that you will, both be proud of and will stand against the elements for the foreseeable future - yes???
OK some pointers, obviously you take all and any advice given freely, as non binding, that is you quantify - qualify - and assess said information as a means of making up your own mind, thereby removing the onus of any liability on others, should something go wrong! I will assume your compliance unless you inform otherwise[/colour]

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OK... 600mm centres is fine, the 600 v 400 centre scenario has nothing to do with what a buildings final use is to be, but is about stud dimensions used i.e. 100 x50 and above 600 is fine, 75x50 or less should be 400 centres. You say you are using 125x50 Tanalised batons for the rafters, best use same for your braces (correct terminology - collars) as you have fitted the rafters you should cut the the collars the exact length (designate) between opposing walls, fix all collars one side only with five 90mm galvs, oh are you using a nail gun or a good old fashioned pelty (hammer) and swear words??? with collars all fixed down the one side only (choose which side you are going to fix them too) then match them all to the same side (comprende) get back to me if unsure, I'll do a sketch, now line up the same side wall fix a string line along the collar ends ensuring all ends line up, using a temp brace, brace this wall at in position...Now the other side...assuming you have cut the collars, all to the same length, they are now in parallel to the first side (I speak of the collars only, not the second wall per-say) nail up this side same as you did the first, only this time pull/push the wall until it lines up with the collar ends...you now have a parallel building. Cut the ends of the collars to match the angle of the rafters and nail on, with the correct twist nails, truss clips to all the collars and rafters where they intersect with the inside of the walls (the ends of these will be covered by your plasterboard lining) this ties the whole roof structure together. Now you need to tie your roof structure to your shell, to do this you need to get some hold down straps (bat straps are as typical) like this once nailed you have a monolithic stricture, in some sense which will counteract any wind lift...I'm a bit puzzled about you fitting insulation in the wall structure but wanting to retain the lofted roof space open, I assume you have some other means of insulating the roof???...bosshogg
 
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
 
Mark...I have just read Graham's latest post...all good advice...we appear to be on the same track...now looking at the photos you posted and referring back in posts, I think you expressed a wish to have eaves extending all round the building your front/back eaves effectively achieve this trough their projection from the shell. Also in the photos the gable walls are terminated with a header (stud on it's side) can I ask if these are in line with the bottom of the rafters to allow ladders (frames built same height as rafters, but built from say 125x32mm Tanalised - 125 being the width of the batons you used for the rafters) to be built and fitted to extend the roof projection past the gable walls (as per the front and back elevations)...bosshogg
 
Thanks Both Graham and Bosshogg

Great information from some very experienced people is valuable to us Novices.

Bosshogg - I feel I understand. I have looked at where the plumb line of the rafter intersects with ridge board and some of the rafters look about 10mm too short. This formas a pattern as I have not fixed the rafters to the ridge board yet. When I look at the rafters on one side the ones closest to the gable are fine but the rafters increse their gap as they reach the centre of the ridge and so I have deduced, as they were all produced with the same pattern rafter that the centre of the side wall is deflecting by 10mm which would not be hard at all.

I have decided to use a ratchet cramps to remove this deflection in the side wall and so close up the connection betwwen the rafter ends and the ridge. At this point I shall attach the said Ties (Ceiling Joists) to one end of the rafter where it intersects with the roof plate.

I WAS insulating between the timber fram using celotex or the like but that was when I planned to have a open dual pitched roof. Now I am needing to put a ceiling in I will insulate the roof void with rockwool and ventilate as required.

Points I am not sure on is the use of fixings to the end of each tie to attach it to the rafter. Should I use bolts or Nails?
Or possibly a bit of both.

Should the timber for the 5M Span between the two rafters be the same as the rafters? If so will it not sag in the middle and if so sould I use some kind of timber brace between the rafter and the ceiling joists?

When you say tie the roof structure down to the wall frames using straps. Do you mean the twisted galvanized straps to hold the roof on to the wall framing?

On the theory of this. If the ridge board is freely supported by the gables and the rafters attached to this ridge board, would that not remove the outward force that pushes the walls outwards?? Would it not be like taping two playing cards together and lifting them in the middle with say a pencil of pen??? If you lift high enough the cards come together.

I am learning off the cuff at the moment. Its a great process and I feel I would be lost without your help.

Graham - With the above comments also. I feel I should invest in some Coach bolts. At the moment the structure is held together with 6" screws top middle and bottom. Where the walls intersect I placed three 50 x 100 studs and so it would be difficult to keep a bolt within the stucture.

Apologies if I have not covered eveyhing here but I feel a little on the information overload. I feel I need to read your valuable posts again to soak up all that they contain.

Again Thanks for all this great help. Yes in this crazy world it pays to ensure that the advice given is given without and obligation.
If the structure collapses it will be only me to blame hence why this information is so valuable.

Please dont ever feel that you should not share your great experince with others for risk of legal liablity this I feel would silence great forums like this.


Kind REgards

Mark Howarth

PS BossHogg do I need to keep calling you Bosshogg or just Boss. :lol:
 
bosshogg":3vhl8w74 said:
Mark...I have just read Graham's latest post...all good advice...we appear to be on the same track...now looking at the photos you posted and referring back in posts, I think you expressed a wish to have eaves extending all round the building your front/back eaves effectively achieve this trough their projection from the shell. Also in the photos the gable walls are terminated with a header (stud on it's side) can I ask if these are in line with the bottom of the rafters to allow ladders (frames built same height as rafters, but built from say 125x32mm Tanalised - 125 being the width of the batons you used for the rafters) to be built and fitted to extend the roof projection past the gable walls (as per the front and back elevations)...bosshogg


Due to my lack of experience I designed the Gable frames in four parts and used a post in the centre to support the ridge beam. In doing this the roofing calcs made the end of the timber flush with the end of the front and rear walls. I was not aware at the time that this would be lower than the Rafters as they are only partially sitting on the edge with a birds mouth. The difference is not a full rafter size but the projected line from the top of the birds mouth and the end of the rafter or less I assume it is. I am not sure if that give enough play to let me make a small ladder frame??

P.S. I am using a Nail gun with 90mm ring shanked nails Galvaniszed of course. (hammer)

Mark
 
Mark I'm going to answer your points one at a time as I read them, some I may ask for some clarity should I not quite understand...OK...first rafter alignment, I was trying to ascertain the alignment of your rafters to the gabled (angled) head plates of four gable frames...i.e. if you placed a straight edge across the rafters and overshoot the gable frames, would the rafters be above the frame by 125mm (upper 50mm edge of rafters) leaving a 125mm dimension, in which to lay your ladder extensions...bosshogg
 
Bosshogg - No the top of the rafter is about 84mm above the gable wall. I was thinking of making a small ladder frame. Do you think that is possible??

Mark
 

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