Timber framing advice

[angelboy]

I'm building an outbuilding and have been given conflicting advice with regards the timber upright sizes. I do have a formula to work out the sizes but it's beyond me to be honest.

The building is about 4m x 3m. I've built up 8 courses of brick, single skin so 100mm wide, which I was intending to use 100x50 uprights at 400 centres which would be about 1200 in length. The problem might be with the weigh of the roof, which is clay Rosemary tiles on a 25degree pitched roof.

I've been told to use 150x50 timbers so the sole plate would have a 50mm overhang on the brick, which I'd offset each side anyway.

Should I use the bigger timbers or would the 100x50s do?

I was going to use the same 150x50 at 400 centres for the rafters and although Barry's construction book says that a building with a span less than 3500mm doesn't need collars on the rafters, I had intended to put maybe 3 in total along the 4000m length rather than every one and use. 200x50 ridge board.

Does this sound ok or Should the rafters be deeper?


I'm sure I'll have other questions as I'm getting on with it.

I've seen mikes shed design post but there's nothing about clay roofs.

 

[angelboy]

Bump......before it goes onto page 2

 

[Steve Maskery]

Hi Angelboy
I'm an expert in the making...
An enormous percentage of homes in the US are built using 2x4s on 16" centers (sic). They survive more severe weather than we ever get.
However, there is a movement in the US away from 2x4 on 16" towards 2x6 on 24". The advantage is that you need only have a single top plate, because your trusses are also on 24" centres, so the top plate is no longer load-bearing. Also the increased thickness means more insulation can be used. Bigger timbers, yes, but fewer pieces, so fewer joints to make and therefore quicker building.
That is how I am building my workshop.
So yes, 6x2s are probably better, but on a 4" wall? Not sure about that, maybe you should consult a structural engineer. 4m x 3m is not a huge roof, so the weight isn't that great, certainly not compared with a proper house, as it were.
I recommend a book called Graphic Guide to Frame Construction, by Rob Thallon.
S

 

[EBH1]

In modern timber framing 145 x 45 C16 timbers tend to be used at 600 centres for the levels of insulation needed. The old part of my 70's house is constructed with 100 x 50 and a lightweight trussed roof with concrete tiles. All the weight is held on the timber kit and a brick "rainscreen" on the outer leaf carries no weight.

There is no question 145 x 45 (Reg'd 6x2) timbers give a much more solid feel to a wall and much stronger. You could however use 100 x 50 and double sheath the wall with 9mm osb and nail the perimeters at roughly 250mm centres to give a very strong structure.

Personally I would put collar ties on the rafters for 150 x 50 or increase the timbers to 175 x 45 at 600 centres with no ties. I would also put in a head binder once the walls are up - it's amazing the difference it makes the super structure. In Scotland we also like to put Sarking boards on a roof to further increase its strength. Again OSB is a good solution. I've used 12mm on my extension roof to stop racking. We have a 4.6m span with no collar ties on the rafters, but we do have a large 305mm UB in the ridge.

It may also be prudent to put metal hold down straps onto the kit which fix to the brick course.

I'm by no means an expert, but i've built a few modern timber extensions including a large one on my house just now. TRADA also have a great website for details regarding timber framing.

Cheers,
Euan

 

[angelboy]

The main issue was to determine if the clay roof was going to be too much for the smaller timbers.

Some positive advice, thanks.

I'm not too bothered about the overhang. Even if it were brick on brick under building regs I could safely have 25mm per side overhanging so a 150mm sole plate on the 100mm brick work is ok.

I think I'll draw up a couple of plans and see what you all think before I order the timber.

 

[busy builder]

For a building that size I would use 6x2 and also dwangs/noggins. For the sake of a happorth o tar etc etc......

 

[angelboy]

How about this.

150x50 sole plate
150x50 uprights & noggins & gable ends @ 600c - corners doubled with 50mm gap to be filled top and bottom (to act as a spacer rather than using another timber as I doubt the corners will need three timbers)
150x50 header (doubled up)
175x50 rafters @ 600c (doubled up at each end)
200x50 ridge board

Does anyone have anything to add?

 

[Steve Maskery]

If that is to scale I would say that 2x6 is overkill. You could easily do that with 2x4s.
Also if the rafters line up with the studs, you don't need a double top plate (although, of course, you may choose to have one).

 

[angelboy]

The rafters would sit above the timbers on one side but the front, the side with the windows and doors, wouldn't line up. As the windows are right up to the header I'd probably want to double up above them so that the weight of the roof didn't transfer directly through the window. I was also concerned that the sash window would have a direct load from the ridge beam above it.

If I chose 4x2s then I'd have to have 400 centres.

Does this make sense or am I over-egging it?

 

[Steve Maskery]

I'm probably over-stepping my remit here, TBH, but IIUTC there are parts of the USA where they do 2x4 on 2ft anyway. It's a tiny building, no upper storey, the rafters would transfer directly through the joists... over-egging was the word I was trying to avoid using.
I've seen greenhouses bigger than that on much smaller timbers.
Good point about the top plate though.
What skin are you planning? 12mm? Perhaps consider using 2x4 on 2ft but beefing up the skin to 16mm (or whatever the 5/8" equivalent is)?
S

 

[kostello]

4*2 @ 2' will be fine.......

it's only a shed....

more important is to brace it so that the frame stays square

 

[angelboy]

4*2 @ 2' will be fine.......

it's only a shed....

So 4x2s are ok even with a clay tiled roof? There's probably about 1.2t of weight over around 1000 tiles.

 

[Steve Maskery]

Doesn't that work out at about 50kg per stud? If so, it doesn't seem so great to me. OK you have to factor in snow, but as I say, entire houses are built on less in the USA.
S

 

[angelboy]

Wouldn't there be more load on the gable supports at each end as the ridge board transfers the load from both pitches onto two point. At the roof plate there would be less weight.

Am I getting that wrong? I think I'm trying to say that the load wouldn't be equal through the entire roof would it?

 

[Steve Maskery]

More? Less, surely? The weight of the end of the roof is spread over the whole of the end wall, rather than just two opposite studs.
I'm getting out of my depth, I'm afraid, I think you need a structural engineer.
S

 

[angelboy]

Yes, you're right. Silly me!!!

I'll dig out that calculation I have for the timber uprights and see if anyone here can make sense of it.

 

[Steve Maskery]

One thing I notice, AB, is that you don't have much tying the opposing walls together. Trusses would do that. OK It's not a big build but AFAICS there is nothing at all tying the centres.
S

 

[EBH1]

From a structural point of view taking the load from the ridge beam directly down to the wall head would be the preference. You could achieve this above the window by making a timber lintel of two pieces of timber well nailed together then cripple studs at either side.

My extension roof is a clear 7.6m span with no ties in the rafters. The structural engineer sized the timbers to 195 x 45 C16 to stop the walls splaying with a dirty great steel beam in the ridge. The head binders will also help stop this. As Steve said sheathing the whole structure in a a material such as OSB will hold it all together.

Euan

 

[kostello]

4*2 @ 2' will be fine.......

it's only a shed....

So 4x2s are ok even with a clay tiled roof? There's probably about 1.2t of weight over around 1000 tiles.

4*2 for the studs will be fine......the timber is in compression straight down the grain and resists bending very well

 

[kostello]

if you use a structural ridge you don't need any ties in the roof......