stone plinths under timber frame uprights

hi folks,

I'm going to make a green woodworking space in my woods in Hampshire, southern England (zone 9). It will be a monopitch roof on a timber frame, no walls. The uprights are straight chestnut coppice poles (10-13cm diameter, barkless).

My question is about how to prevent the bottoms of the uprights rotting. I don't want to bury them in the soil. In England it's common to see timber frame uprights perched on top of stone plinths called 'saddle stones'.

I don't have any nice pieces of granite lying around so I'm planning to make some plinths out of concrete, using old yogurt pots as a mould.

Will my yogurt-pot idea work? See picture. The moulds are 13cm high, 13cm diameter at base (which will be buried) and 10cm at top (which will be exposed). I plan to burry most of the concrete, leaving only about 3 or 4cm exposed. Are they big enough? Deep enough? (I have lots of these pots so it would be an easy zero-cost solution)

The soil is heavy clay and the climate is wet in winter, dryish in summer.

If you have experience of stuff like this, I'd welcome your comments, observations, warnings, etc.

Thanks!
Paul



example saddle stones:
https://www.google.co.uk/search?q=timber+frame+saddle+stone&client=firefox-a&hs=Bpd&rls=org.mozilla:en-GB:official&channel=fflb&source=lnms&tbm=isch&sa=X&ei=ARgcVOC4CImp7Abll4CIAw&ved=0CAgQ_AUoAQ&biw=1280&bih=672

yogurt pot mould:
https://www.dropbox.com/s/34zr7vzjafhfu85/IMG_20140919_122907.jpg?dl=0

Hi Paul,

I build with plinth stone often under my timber frames. If you do a search of my posting here with the word "plinth," you are apt to get more than you wanted to know....

Here are two of perhaps some value...here and here

As for a small "pavillion frame" with open walls this may be doable on "concrete plinth" the issue will be the actually strength of the OPC, and the tendency of OPC to hold moisture like a sponge...unlike stone and even brick. I would place a copper or lead "shield" between the OPC plinth and the post to perhaps negate this "wetting" effect.

Charring the "contact zone," oiling, and perhaps a bit of a "borate" treatment wouldn't hurt either.

With what you have described for a structure...I believe you would be better served just finding a few real stones instead of trying to move outside of the natural/traditional methods of plinting a foundaiton.

Good luck, and let me know if I may help further...

j

Thanks for your response Jay C. White Cloud. I see from your website you are a craftsman timber framer, so thanks for taking the time to post. I am at the other end of the scale, having never done if before.

I take your point about Ordinary Portland Cement being porous. Unfortunately, I don't have access to stones of the appropriate size so I will have to make do. Your suggestion of including an impermeable layer (copper or lead) sounds like a very good one and I will include some copper under my posts.

Attached is a picture of the site so far. At this stage all I have done is thought about it, cleared all the weeds and underbrush, marked out the location of the posts and dug my holes for the sand/cement plinths. The poles will be on top of the plinths, above ground level. The roof outline will be larger than the post outline.

(The tarp in the background of the picture is just a temporary shelter for me and my bench while I get the workshop built).

finally got round to putting up the frame on Friday

it's surprisingly sturdy (and I haven't added the corner braces yet)

all the posts are the same height, I'm relying on the slope of the land to give me drainage off the flat roof
(the slope is 15cm in 4.8m ...so that's 3.1% or 1/32 - I hope that's enough!)


I have 4"x2" rafters to add, then some recycled bits of 5mm plywood I found lying around, then the plan is to proof it with a pond liner, then throw up some soil and brash and let it become a green roof ...
(this will keep me going for a while, as it's a side-project from a side-project

Nice looking project! I'd love to be able to have an open-air workshop.
You say the frame seems very sturdy, and it may well be rigid once you put corner braces on, but with an essentially dead flat green roof you will be getting a large mass of material on top. With all the posts the same height and basically pinned at top and bottom there will be no real resistance to lateral motion aside from the braces. If you get a freak rainstorm with 6" of rain in an hour, that will add tremendously to the load, and a strong wind or something like a small tree falling against the frame could tip it just enough for gravity to take over. I would not want to depend on one brace in compression to withstand that load in any one direction; I would want redundancy with most or all of the posts braced.

You mention 2 x 4 rafters, but that is not going to be strong enough to hold the unknown large load of a dead flat green roof with rain saturating it and maybe ponding in it, for the span seen in the picture. I would not trust that for a span greater than 6 or maybe 8 feet with closely spaced rafters. I just don't want to hear about the frame falling over or roof collapsing with you under it...

Thanks for the constructive input Glen.

Yes, I intend to put two corner braces on the top of all uprights. That's my next task.

> with an essentially dead flat green roof you will be getting a large mass of material on top.

You (and from another post by timber-framer J C White Cloud: https://permies.com/t/40305/timber/flat-roof-minimum-slope-green) have changed my mind about the green roof. A quick calculation reveals it was always impossible because the structure is insufficiently strong to support any significant weight. So no green roof



In fact, even the weight of the rain may be an issue if it ever ponds on the roof. The planned roof area is 23 square metres. If 1cm water collects, that's 230kg of extra weight. That makes 23kg per upright, and 11.5kg sheer force per 10mm steel bolt. Those numbers are *per centimetre* of rain on top ...

So I need to make damn sure water doesn't pond on the roof! My slope is 4%. The UK building regulations specify 1:40 as an ideal flat roof slope. That's 2.4%, so I should be OK with my 4%. The green roof idea now seems rather foolish - I will in fact need to remove the leaves every autumn/fall to keep it draining quickly. We get very little snow. (I didn't say: there is a slope two ways across this roof, i.e. the highest and lowest points are diagonally opposite; the gradient across the diagonal is more than 4%.)


By the way, I intend to use the roof to collect some rainwater in a barrel for dish-washing, hand-washing, bathing, etc. So I will put guttering on the lower two edges, which are adjacent.


> a strong wind or something like a small tree falling against the frame could tip it just enough for gravity to take over.

If any of the surrounding trees fall on the structure, they will go through it like it's not there - no question. They are all big enough to take out a brick-built house. There is not much I can do about that, except a build subterranean bunker! My strategy here is hope In the unlikely event of a falling tree hitting it, I'll just have to start again. The site is quite sheltered and I have never experienced any strong winds at ground level.


It's only supposed to be a bit of shelter from the rain, so the standard of building required is not to the same level as for a home. Nevertheless, thanks for making me stop and think about the weight issue - a green roof was never going to work.

I noticed the size of the trees visible in the picture - obviously one of them would take out anything, but a branch falling is actually a feasible hazard. Full angle bracing ought to take care of the structure as well as an open shed like that can be protected.
I would be inclined to add a 2x4 or something to the top of the uphill side beam just to improve the drainage a bit more and make maintenance less critical. It would of course have to be securely fastened to the beam.
Good luck with your shed!

progress as at 18 October last year
(no more work done on it since then - I have been overtaken by the tide of events)


currently thinking about how to make a waterproof roof without spending a fortune (see here https://permies.com/t/43532/rainwater/LDPE-pond-liner-roofing-membrane#342207 )

Hi Paul,

I am following your project here with great interest, as I want to build a couple of similar structures in my garden, one smaller and one (maybe) larger. I'll probably use 2x8 (over 12ft span) to be sure it will be ok for green roof though. Pond liner/butyl roof definitely seems to be the way to go though.

Btw, green with envy too

some progress at last!

I used a pond liner to waterproof it:
http://www.pondkeeper.co.uk/-flexiliner-6x6m-15yr-free-underlay/p2661
Beware: I had to buy the 6m x 6m version for my 4.8m x 4.8m roof because PondKeeper's so-called 5m x 5m product was cut a bit under-sized.

it's not a full green roof - that would be too heavy - it's just got sandbags with mud and weeds around the edges for effect
(drones will be able to see the pond liner)