Help: Posts sizes for an Oak framed garage

Help: Posts sizes for an Oak framed garage

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Barreti

Original Poster:

6,680 posts

236 months

Wednesday 29th October 2014
quotequote all
Hi chaps,

Anyone bought an oak 'kit' garage who can advise me on the spec ?

I've finally got planning permission for a timber garage and gravel drive, which we initially asked the builder to do from scratch.
But we got looking at pre-manufactured oak kits and the builder is happy for us to buy in an oak kit rather than him sub it out to a joiner to do, and in fact the price for an oak kit is favourable compared to his softwood quote.

The kits come in 2 sizes - 150mm square posts and associated beams etc and 175mm posts etc.

But the builder said 150mm is nowhere near good enough to hold the roof we will be putting on it - concrete pantiles to match the house - and said we need 200mm x 200mm posts with 200mm x 50mm rafters.

The price for this is nearly double the 150mm kit price. The kit supplier says the builder is massively overkilling it and 175mm uprights in oak would hold up a block of flats.

To my mind, if the kit people say 175mm is that good then a good compromise is 175mm posts etc and 200x50 rafters.

Can anyone give me reassurance/advice?

This is a 6m x 6m 'carriage house' (ie it has no doors) and will be clad in 38mm oak.
I'm looking at the website for Rockingham Oak as they are relatively close.


Oh, I should add I did have an architect for this build and 6 months in he made absolutely no progress other than to get a structural engineer to the site who advised we needed 24 piles putting in. So we parted company, and we submitted a planning application based on a floating concrete base with no piles, which the builder is more than happy with. So its just us and the builder - and maybe an oak kit supplier.

Edited by Barreti on Wednesday 29th October 14:04

DKL

4,480 posts

221 months

Wednesday 29th October 2014
quotequote all
150mm sq is fairly standard. We are talking about solid oak posts that are 6 inches sq.
8" sounds quite alot.
Check with the kit supplier

gifdy

2,065 posts

240 months

Wednesday 29th October 2014
quotequote all
Mine are 210mm square but there is an upper floor and tiled roof. Not a direct comparison I know, but another data point.

Barreti

Original Poster:

6,680 posts

236 months

Wednesday 29th October 2014
quotequote all
For reference the price difference between the kits looks like this

150mm posts = £4600 +VAT (£5,520)
175mm posts = £5850 +VAT (£7,020)
200mm posts = £7360 +VAT (£8,832)

So the step up from 175mm to 200mm costs £1,800 to me, which is why I want to be sure I need to do it.

roofer

5,136 posts

210 months

Wednesday 29th October 2014
quotequote all
150 sounds weedy to me, green oak? They'll need lateral restraint imo.

I am using oak to hold up 2 steels at the back of my house, and they're 250 square air dried qp1 !

EggsBenedict

1,770 posts

173 months

Wednesday 29th October 2014
quotequote all
The kit manufacturer has to know - what are their approved roofing materials? If you need building regs (don't know if you do), then they may ask for an engineer's calcs that show the structure can handle the load. Again, the kit supplier must have those for all their standard sizes if it's the case.

The only other way is to get a structural engineer in.

Us armchair experts can't see what you can, and don't have access to the data that the kit manufacturer has.

MintSprint

335 posts

113 months

Wednesday 29th October 2014
quotequote all
Barreti said:
But the builder said 150mm is nowhere near good enough to hold the roof we will be putting on it
The builder is talking bks.

HTH

Steve H

5,224 posts

194 months

Wednesday 29th October 2014
quotequote all
MintSprint said:
The builder is talking bks.

HTH
This sounds about right.

I built this -









out of green oak, the bulk of the main frame was 6x6, supports were 4x4. The purlins were bigger (8x8 I think) as I wanted to do it in a single unsupported span.

Doing the main frame from 6x6 sounds good to me.

MintSprint

335 posts

113 months

Wednesday 29th October 2014
quotequote all
Steve H said:
Doing the main frame from 6x6 sounds good to me.
We do a range of oak framed garages, and our standard specification is 150 x 150mm posts and 100 x 220mm beams at eaves level. This is fine even for 'room in roof' garages, where the posts and beams are having to take a floor load as well as the roof, and we have the engineering calculations to prove it.

We use 220 x 45mm rafters @ 400 centres, which are fine to span a 6 metre deep garage without purlins, with all but the heaviest roof coverings (again, proven by engineering calcs).

OP, I can guarantee that your builder wouldn't even be able to interpret a full set of engineering calculations for an oak framed garage, so it's laughable to consider his opinion above that of your kit builder.


roofer said:
150 sounds weedy to me, green oak?
And I can show you plenty of properly engineered timber frame designs that have steels bearing on nothing more than a couple of 38x89mm studs either end. It depends on the load the steel is taking, of course, but you could balance a battleship on a 250 square air-dried oak post! wink

You'd be surprised how much compressive load a timber post can take - it's bending deflections on beams that are usually the bigger problem.

ATTAK Z

10,586 posts

188 months

Wednesday 29th October 2014
quotequote all
^^^^^^^^^^ This

eps

6,272 posts

268 months

Wednesday 29th October 2014
quotequote all
Sorry, is the roofing finish, concrete pan tiles or clay pan tiles? There's quite a difference..

MintSprint

335 posts

113 months

Wednesday 29th October 2014
quotequote all
eps said:
Sorry, is the roofing finish, concrete pan tiles or clay pan tiles? There's quite a difference..
There is, but it won't make a blind bit of difference to the post sizes.

The rafters, maybe. The post sizes, no.

Barreti

Original Poster:

6,680 posts

236 months

Wednesday 29th October 2014
quotequote all
Some brilliant answers chaps, and thanks.
And you know what they say about a picture being a thousand words. They're a great help too, because sitting at my desk today staring at the 6" mark on a ruler looked piddling. So thanks, they have helped visualise it in context.

The tiles on the house which we said we would match are concrete panties, though the builder said something about newer panties which are ok with the lower pitch roof on the garage kit of our choice.

ATTAK Z

10,586 posts

188 months

Wednesday 29th October 2014
quotequote all
One of the most important issues to consider in timber design is the joints ... in the past buildings with good joints have survived , been copied and become a standard construction method.

The Timber Designer's Manual has made a good attempt at producing an accepted method of design which allows for justification of designs but there is still an element of 'Black Magic' involved, especially in gang nailed truss design.

The photos posted by Steve H are a good example of 'copied joints' providing stability.

You can have posts and rafters as big as you like but the building will still fail if you don't take account of overall stability and good practice in jointing the members.

eps

6,272 posts

268 months

Thursday 30th October 2014
quotequote all
MintSprint said:
eps said:
Sorry, is the roofing finish, concrete pan tiles or clay pan tiles? There's quite a difference..
There is, but it won't make a blind bit of difference to the post sizes.

The rafters, maybe. The post sizes, no.
Saying it won't make a blind bit of difference is incorrect. However it may not make much difference when choosing between 150 or 200 oak posts. It would have to be calculated otherwise everyone is just guessing, which I'm sure building control would happily accept. Not.

singlecoil

33,310 posts

245 months

Thursday 30th October 2014
quotequote all
Some of the posters here know about oak, and some don't. 150 x 150 will be fine.

Tyre Tread

10,525 posts

215 months

Thursday 30th October 2014
quotequote all
Barreti said:
Some brilliant answers chaps, and thanks.
And you know what they say about a picture being a thousand words. They're a great help too, because sitting at my desk today staring at the 6" mark on a ruler looked piddling. So thanks, they have helped visualise it in context.

The tiles on the house which we said we would match are concrete panties, though the builder said something about newer panties which are ok with the lower pitch roof on the garage kit of our choice.
Genuine LOL!

BTW, I live less than a mile from Rockingham Oak!

FlipFlopGriff

7,144 posts

246 months

Thursday 30th October 2014
quotequote all
Tyre Tread said:
Genuine LOL!

BTW, I live less than a mile from Rockingham Oak!
That's where I got my 3m long 300mm square air dried oak from for our gate posts. In hindsight maybe OTT and they were fooking heavy to get them into the hole.
I'm no expert as you can see.
FFG

MintSprint

335 posts

113 months

Thursday 30th October 2014
quotequote all
eps said:
Saying it won't make a blind bit of difference is incorrect. However it may not make much difference when choosing between 150 or 200 oak posts. It would have to be calculated otherwise everyone is just guessing, which I'm sure building control would happily accept. Not.
I ought to perhaps point out that I'm Design and Technical Manager for a major timber frame manufacturer.

We've done the calculations, for our entire range of standard structures and for countless bespoke designs. They've been accepted by Building Control on hundreds of occasions.

Trust me, there is no calculation in the world that would show you need 200x200 oak posts to take the load of anything short of Cotswold stone slate as a roof finish (and probably not even then). We recently did a room-in-roof garage (so posts taking the load of a floor, as well as the roof) with hand-made Kent Peg clay tiles to match the roof on the customer's house. The rafters needed beefing up a bit from our standard spec.; the posts were just fine at 150 square. From memory, the Kent Peg tiles had a laid weight of about 72kg/m2, and the calcs will have assumed another 150kg/m2 live loads for the floor. For comparison, concrete pantiles, depending on profile and manufacturer, will have a laid weight of around 45-50 kg/m2, and the OP has no floor to contend with....

The OP's kit supplier will, I am sure, provide a copy of their calculations upon request (as we do), to satisfy Building Control.

The OP's builder is talking utter ste. Period.





Edited to add: Strictly speaking, you will need calculations to satisfy Building Control on any post-and-beam timber structure falling within the scope of the Regulations, regardless of how over- or under-engineered it is, as they're not covered by the 'idiots guide' approach of Approved Document A.


Edited by MintSprint on Thursday 30th October 23:30

MintSprint

335 posts

113 months

Thursday 30th October 2014
quotequote all
ATTAK Z said:
You can have posts and rafters as big as you like but the building will still fail if you don't take account of overall stability and good practice in jointing the members.
This is perfectly true. It's invariably racking stiffness that's the biggest problem with post-and-beam frames, and that's only ever going to be as good as the connections (joints).