Workshop build - rafter end design

[Dan0741]

Mick, Looking good, and its an exciting stage you are at. I have no expert knowledge but just a couple of points that may help. I made the edge of the slab thicker than the internal area by filling the internal area with about 100mm of foam insulation. So on a flat base the external 200mm to the slab is 100mm deeper. Keeps your toes warmer also. Just a thought?

Re the roof I did something similar to what you propose, I divided the roof into 3 roughly equal parts - and did plate level rafter ties at one end, raised ties at the other and a gap in the middle with no ties. It just suited my needs and seemed to ensure the shed had wider uses if I needed that. Also makes handling large sheet material easier!

Not a great time of year for it though, everything just seems to slow down when its colder, get your thermals on!

 

[mickthetree]

Many thanks Dan.

Have spoken with quite a few people now who all have differing ways of handling the base. Most common seems 100mm with steel mesh 50mm up and away from the edges. Thickening the edges does make sense, but also complicates calculating the amount of concrete required.

The roof is definitely going to be a balance between useful storage space and headroom.

It certainly is going to be a cold build. Spoke to a Paslode user today who suggests keeping the gas in your pants when its chilly! :shock:

I didn't ask him where he sticks it when it gets REALLY cold.

 

[Mike G]

........Have spoken with quite a few people now who all have differing ways of handling the base. Most common seems 100mm with steel mesh 50mm up and away from the edges........

That's the one place where steel is just a waste of money......right bang in the middle of the slab. Steel works in tension, and concrete doesn't. It is there to strengthen the side which would stretch if it moved. Imagine bending a piece of balsa wood with your hands, such that it bends downwards (sags). The top of the wood is being compressed (because it is being made shorter). The bottom of the wood is being stretched, so is in tension. The mid-line of the wood is neither in compression nor tension: it's neutral (line of neutrality). If you wanted to strengthen the piece of balsa with a piece of cotton to make it more difficult to bend, you would obviously glue the cotton to the underside of the bend I described. If you glued it to the top it would do nothing, and same with the mid-line. now. moving that principle back to the concrete............the reason you don't put steel right on the very bottom is that it would rust, and it also would have poor connection to the concrete. So, you need some cover for it, and 25mm is enough.

Now, if your slab was 150mm thick, which is what I suggest, then 50mm up from the bottom would be fine. 100mm is really too thin for a building that size, but if you are willing to take the risk, then put your reinforcing on 25mm spacers.

 

[mickthetree]

Hi Mike

Ok, points noted. I might not have made that clear enough, but the 50mm was also applying to the distance from the edge. Is that too far? or ok?

I'm still at a stage where I can change things. I was basing some of my build on your original post back on UKWS which suggested 100-150mm hardcore and 100mm of concrete thickened at the edges.

Could I go to 150mm all over without thickening the edges anymore? then I could place the mesh 50mm up as you suggest.

Thanks again

 

[Mike G]

Yes, you could. That would be fine........and the 50mm cover at the edges is spot on. I didn't comment on that because you've got it right.

 

[mickthetree]

I'm trying to get the roof right before I place the order for my materials.

Span of the roof is 4.8m I plan on using a 35 degree pitch and putting collar ties at 2.4m from the floor. This would allow me to use 3.6m joists.

So the question is should the rafters and joists all be 150mm x 50mm?

Also could I get away with mineral wool insulation in the roof instead of celotex? seems a big difference in price. Just picking the right bits to save money on.

 

[9fingers]

Your rafter size will depend on the roof materials and spacing.

I make them about 2.8m long plus eaves. Unless you have a very light roof, then poss 150x50 would be better and perhaps 600 centres. For a heavy tiled roof then 400 centres.

If there is no storage in the roof bearing on the collar ties then 100 x 50 should be fine as they are in tension otherwise use 150 x 50.

Mike will be along in a bit with the right answer no doubt :lol:

Bob

 

[mickthetree]

:lol: Cheers bob, points noted though.

I do intend to store some timbers and other bits above the joists.

I plan on lining the roof with 18mm OSB and felt.

 

[9fingers]

:lol: Cheers bob, points noted though.

I do intend to store some timbers and other bits above the joists.

I plan on lining the roof with 18mm OSB and felt.

Gut feel with the storage would be 150 x 50 all round and 600 centres due to the light weight roof cladding.

I strongly suggest you look at EPDM rubber for the roof. Typically the material has 30 yr warranty. Terry has used it on his workshop and can no doubt advise and I've done a small shed with the stuff.
Felt can have poor life and lifts off in the wind etc.

Bob

 

[Mike G]

Your rafter size will depend on the roof materials and spacing.

I make them about 2.8m long plus eaves. Unless you have a very light roof, then poss 150x50 would be better and perhaps 600 centres. For a heavy tiled roof then 400 centres.

If there is no storage in the roof bearing on the collar ties then 100 x 50 should be fine as they are in tension otherwise use 150 x 50.

Mike will be along in a bit with the right answer no doubt :lol:

Bob

This could be one of those where rafter depth is determined not by the load on the roof but by the depth of insulation. With mineral wool I would be looking for 150 depth, minimum. You need a 50mm airgap above that under the OSB (don't forget your insect mesh at the eaves. So I'd suggest 200x50s for the rafters.

I would suggest 150x50 for the ties, because even if you might not use this area for storage, someone will.

 

[Tusses]

haha :lol: .. don't read this !!!

I use 6x2" on 1m spacing on a 6m span for my tin roof

and I have racks strapped underneath with oak and ash waney boards, and all sorts stored :shock:

...

carry on

 

[Harv-53]

Hi mick
I've just read your workshop build thread and realised your not to far away from me, if you get a paslode framing nailer I have a full box of 90mm galv nails your welcome to have, I was recently given 2 boxes theres no gas with them but that is easily purchased separately, also I have a few off cuts of rigid insulation and about 1/2 a pack of 150mm loft roll.
PM me if your interested
Harvey

 

[mickthetree]

Ha Cheers Tusses. Out of interest is it still standing ok?

That is very kind Harvey. I think I have decided on screws. I can get them at a good price from work and means I can "undo" any minor mistakes along the way. If I get caught short on the insulation I'll tap you up, but I'll be ordering so much of the stuff I will probably be left with over too :lol:

Ahrg! The insulation dilema. I wont be spending all day everyday in there, but I do want it to be warm when I do. The foil foam board is much more than the stuff I'm using in the walls, but the wider timbers will cost more! Hmmmm :eusa-think:

 

[Tusses]

yep .. , been in this one 12 years .. built 2 others the same in the past .. all fine ..

I'm not saying you should do as I do .. just don't worry about it so much


My only problem is the tin roof .. where one sheet overlaps another it's not air tight.
It's fine all year, except winter .. I get frost on the underside (above the insulation)
and when the sun comes out the next day, it all melts and drips :-( ..
My vapour barrier is on the inside and between the beams .. so the water can get from outside to inside where the barrier is tacked to the beams .

I'll have to take the tin off, and put a full plastic cover on under it. .. one day !
(not the best solution) .. I guess I could seal the overlaps with sticky stuff etc ...

It only happens a few times each winter .. so I've put up with it so far.

 

[mickthetree]

I am wondering how to manage the end of the roof. I am using OSB with felt but to get the best usage out of the sheets they will end in the centre of the last rafter.

I reckon I'll need about a 100mm overhang to sit over the battons and featheredge.

If I overhang the boards by this I will end up needing far more sheets and having watage.

Is there another way to achieve this? I could cut a strip of OSB off and start it in the centre of the last rafter. It could get some additional support by the batterns.

Any ideas on how to do this? Or do I just need to bite the bullet and by more OSB?

Oh and one supplier is quoting 2400x1200 and another is 2440x1220. :eusa-doh:

Many thanks

 

[9fingers]

Looking back you wanted a 7m long building. 3 x 2440 is 7.32m so enough for overhang at both ends.
it still works out just about OK with 2.4 sheets

Therefore layout your rafters from the centre outwards to match the sheet sizes and used a reduced spacing for the ends ones according to the amount of your overhang and how you propose to support them and your fascia design.

Bob

 

[mickthetree]

Hi Bob
working from the centre out and redue the spacing. I'll give that a go and see what I can come up with.

I had to change the dimensions slightly so now looking at 4.5 x 6.5. I'll do the maths tonight and see what I can come up with.

Many thanks!

 

[mickthetree]

Afternoon all

progress has been going well, slab is down, two long walls are ready to go up, just starting on the end walls.

I'll try and get some photos, but when progress is going well I don't think to stop and go inside to get the camera, also means taking my muddy boots off which takes an age!

I have a roof question if someone would be kind enough to help with:

I plan to make the roof in 6x2s with full span joists for the most part and higher collar ties for the rest. roofing bolts will join the rafters to the joists, rafters will be birds mouthed (is that a verb?? ) around the wall plate.

Q.) How should I connect these trusses to the wall plate? one of those galvanised plates? or just screws / nails?

Q.) Do I need a ridge board? I had planned on it, but can I just butt the rafters together like this?

Or are there clear advantage to using a ridge board?

Is the OSB on the roof (sandwich above and below enough to tie it all together and prevent any racking?

If you are around Mike, I see that you added your insulation from the top. Is this just because its floppy wool type? I have 100mm celotex which I could install from the inside. I'd rather get the OSB then felt on and make it watertight then carry on from inside.

Out of interest it doesnt look like there is a 50mm air gap above your mineral wool. IS that because there is a gap under the tiles where the battens sit?

Many thanks

 

[Mike G]

........If you are around Mike, I see that you added your insulation from the top. Is this just because its floppy wool type? ....

Mainly because my OSB is on the inside, and acts as perfect support for the mineral wool. Do it in whatever order suits you. I'll just say that doing Celotex into rafters above your head is quite an unpleasant experience.

Out of interest it doesnt look like there is a 50mm air gap above your mineral wool. IS that because there is a gap under the tiles where the battens sit?

There is a notional 25mm gap, I think. The 50mm gap is a requirement if you have an impervious roof covering, as you do with OSB & felt. My roof doesn't have such a covering, and so doesn't need airflow from eaves to ridge, as your will.

 

[Mike G]

......Or are there clear advantage to using a ridge board?......

It makes site work much easier. You have a fixed point to which you fasten your rafters, rather than waving around a pair of bits of wood in the air. If you have a small enough roof that you can make trusses up on the ground and erect them pre-made, then fine, you can do without. Don't forget, my ridge beam is structural (in other words, it is holding the top end of the rafters up in the air, and thus preventing the bottom ends from pushing the walls outwards). If you don't have roof ties (joists), then you can't just omit the ridge purlin.

 

[mickthetree]

Many thanks Mike!

I must say I'm very grateful for all of the information you have given on this subject over the course of yours and others builds. It has enabled me, and I'm sure others, to build something to a much higher standard than if left to our own devices!

Cant wait to get in and watertight!

:eusa-clap:

 

[Malc2098]

Go for it, Mick!

 

[Mike G]

..........I must say I'm very grateful for all of the information you have given on this subject over the course of yours and others builds.......

My pleasure Mick. I'm chuffed to bits that there are some really excellent workshops being built all over the country partly as a result of a bit of typing on my keypad.

We're promised a new sub-forum specifically for workshop builds very soon. It will be great to have all this stuff in one easily accessible place.

 

[TrimTheKing]

...We're promised a new sub-forum specifically for workshop builds very soon. It will be great to have all this stuff in one easily accessible place...

Ta-Daaaaaaa! :eusa-whistle:

Cheers
Mark

 

[mickthetree]

Well done that man :eusa-clap: :eusa-clap:

I have my walls up! Again apologies for not getting any shots, just cracking on whenever I get a second. I have my truss template all marked up and I have next week off work so hopefully I can get water tight.

Do I need to line all window and door openings so the featheredge has something to butt up against? Do these linings not then become horizontal surfaces where water could penetrate?

Also is there a specific corner profile that is used at the corners? It would seem I need to plant a square piece on the corner so I dont see how I will fix that in place.

I also need to find a breathable membrane for the walls. My building supplier only seems to supply a bitumen non breathable paper. Any recommendations for a brand and supplier online?

Perhaps this?

http://www.wickes.co.uk/Easy-Trim-Maste ... m/p/144022

 

[Malc2098]

Well done that man!! :eusa-clap:

You're in front of me! I've either got sub-zero or non-stop rain and only a few bricks and blocks of the plinth left to do!

 

[mickthetree]

Cheers Malcolm

I've been well delayed by weather so far and family commitments. Frustrating when all the timber has been delivered and I'm itching to crack on!!

Trying not to cut any corners as I get closer to completion. I know I'll only look at something in a years time and think "I should have done that better"

 

[Malc2098]

Trying not to cut any corners as I get closer to completion. I know I'll only look at something in a years time and think "I should have done that better"

You betchyer!

 

[timothyedoran]

Well done that man :eusa-clap: :eusa-clap:

I have my walls up! Again apologies for not getting any shots, just cracking on whenever I get a second. I have my truss template all marked up and I have next week off work so hopefully I can get water tight.

Do I need to line all window and door openings so the featheredge has something to butt up against? Do these linings not then become horizontal surfaces where water could penetrate?

Also is there a specific corner profile that is used at the corners? It would seem I need to plant a square piece on the corner so I dont see how I will fix that in place.

I also need to find a breathable membrane for the walls. My building supplier only seems to supply a bitumen non breathable paper. Any recommendations for a brand and supplier online?

Perhaps this?

http://www.wickes.co.uk/Easy-Trim-Maste ... m/p/144022

When I re did the kitchen roof, 3x4m is used some membrane from wickes. I don't think it was exactly the same but it was grey. It was ok to work with and we'll priced.

 

[Mike G]

........Do I need to line all window and door openings so the featheredge has something to butt up against? Do these linings not then become horizontal surfaces where water could penetrate?

Also is there a specific corner profile that is used at the corners? It would seem I need to plant a square piece on the corner so I dont see how I will fix that in place.........

Yes to the first question, and pop over to my workshop build thread to answer the second. It's a much neater solution I show there than what builders normally do: a damn great lump of 4x2 up the corner. Looks awful!

The horizontal above a window or door is sloped down and out so that water runs off, and there is no horizontal under the cill overhang. Again, have a look in my workshop thread. In areas of high exposure, you can dress some lead over the pentice board above the openings, or use a carefully detailed piece of damp course. I suggest you don't use silicon, because you'll be trapping any moisture that gets in.

 

[mickthetree]

thanks chaps. I have ordered some wrap which will be here next week. Borrowed a stapler I can use with my compressor too to make that easy.

I see what you mean about how that can look chunky Mike. I like the look you have achieved.

I'm thinking about putting some 50mm kingspan on the floor with OSB or ply over the top. This should give me some insulation and a flat surface. If I follow Mike's drawing right I should put a membrane under this. I guess this is to prevent any moisture being drawn up?

It looks like the membrane should come from under the floor kingspan, up the face of the blockwork then over the DPC on the blockwork. Is this right? Or does it simply just go under the kindspan?

Mike did you do this in your latest workshop in the end?

Many thanks

 

[Mike G]

I have an uninsulated floor, just a slab of concrete. The DPM is under it.

 

[mickthetree]

Hi All

Trusses are up and the large boards of OSB are on. I'm not sure how to handle the ends of the rafters. I could hang the 18mm osb over the edge then install so the water flows into guttering, or I would add a facia board and wrap the felt over this and back up under the rafters.

Perhaps I should have trimmed the bottom of the rafters back to horizontal before installing them?

I also need to overhang the 18mm osb over the gable ends to sit over the featheredge, but again not sure how to complete this. I could add some 50x50 battens under the osb and lap the felt over that then add a barge board??

Last one! Where the OSB has joined at the ridge it has created a valley. I tried overlapping the boards but as they are not at 45 degrees it didnt sit right. I plan on adding a piece of beading down the valley for the felt to sit over. Is this normal? is there a better solution?

And one general progress shot!!

Many thanks!

 

[Malc2098]

I can't answer your questions, Mick, there are others who will know the answers, but It looks like it's coming along nicely.

 

[DaveL]

Yes good progress, should be a very useable workshop, very soon. [emoji122]

 

[9fingers]

Yes to fitting infill pieces everywhere the felt will go (epdm would be better but dearer)

Essential that there be no sharp transitions and no unsupported areas as the felt will crack there in time.

hth
Bob

 

[Rod]

Epdm well worth the extra and easy to lay

Rod

 

[Mike G]

.......Perhaps I should have trimmed the bottom of the rafters back to horizontal before installing them?

I also need to overhang the 18mm osb over the gable ends to sit over the featheredge, but again not sure how to complete this. I could add some 50x50 battens under the osb and lap the felt over that then add a barge board?? .........

Nice work Mick. Well done.

Yes to the first. That horizontal cut on rafter ends is a bugger to do in situ, and leaving them with a point looks bad.

I would plant something on the face of the gable wall, such as a 50x50, which provides a soffit for your feather-edge boards to come up underneath. Your OSB comes to the outside face of this. Then, once they're all done and finished, you can then plant a barge board over the lot. If this is 100x25 it will lap the boards by about an inch, which is fine, and cover the edge of the OSB as well.

As for the bottom edge............ah..........eaves details!! They're usually the hardest part of a building to design/ detail, believe it or not. You have 2 major considerations here, in my opinion. Firstly, you don't want EPDM flapping. It needs to be fixed firmly to something, and ideally, folded back under itself or trapped by a proprietry trim to form a drip. Secondly, whatever you do must compromise the ventilation to the underside of your OSB. Sketch this until you get it right. It's always easier to make your mistakes on paper.

 

[AndyP]

Looking good Mick.