What is the purpose of this bamboo purlin?

canopy · May 02, 2011, 06:35:30 AM

I am trying to figure out what this purlin might be for. See the green purlin at mid height on the right. It is bound to the bottom of the rafters with bamboo tie straps. Bamboo builders seem to favor it so if you have a guess as to its purpose structurally or otherwise I would be interested in your take.

icolquhoun · May 02, 2011, 01:41:41 PM

since it doesn't appear to be supported I'll go out on a limb (hehe pun intended) and say it's real purpose is to distribute point loads from the roof rafters to each other, as well as keeping their spacing.

It honestly can't do much more than that.

tyb525 · May 02, 2011, 01:43:47 PM

It probably helps keep the roof even, in case some of the rafters sag more than others, so the roof doesn't look so "wavy".

Tom · May 02, 2011, 02:46:47 PM

My guess would be the "keeping their spacing" suggestion. But, we have to keep in mind that all things are not structural. It might be a favored place for the society using the building to hang their cured meats or cooking utensils. :-\ :D

Thehardway · May 02, 2011, 03:08:41 PM

The building style is more reflective of panelized or plank construction than timberframe. This type of arrangement can be found in some boat building construction and concrete forming and it would be called a "strongback". As previously mentioned I believe it serves to distribute weight across members uniformly and maintains alignment as well as preventing individual deflection of members. It would not surprise me if it served a purpose in the erection process or maintenance functionality as well. I don't believe it would be classified as a purlin as there is no supporting framework under it.

ely · May 02, 2011, 03:17:21 PM

maybe it keeps the wind from blowing the whole roof inward.

LeeB · May 03, 2011, 02:37:56 AM

Interesting that it is green while everything else is dried. I would think it would not be as stiff green and therefore maybe not for load sharing.

thecfarm · May 03, 2011, 07:42:39 AM

Might be put on to stand on when building the roof. Maybe just to keep the roof from moving from side to side.

dukndog · May 03, 2011, 09:38:18 AM

Hammock or clothes hangar!! :D :D :D

witterbound · May 03, 2011, 01:35:44 PM

I'd guess that it helps to keep the rafters "connected" so that the wind doesn't loosen the ties that attach the roofing material to the rafters.

Tom · May 03, 2011, 08:58:42 PM

Some fellow said to the house owner. I've got this piece of bamboo left over, do you want it?

The fellow said, "Sure! I don't know what I'll do with it, but it'll come in handy one day."

Then he took it into the house, looked up and said, "There's a place to keep it. " ;D

canopy · May 03, 2011, 09:48:33 PM

lol, I have seen that happen before and was kind of wondering if that might be its purpose. But a lot of the comments here make justifications for increased durability, so it seems a useful member though non-essential. On the other hand, it isn't useful for non structural purposes like scaffolding and is too high up for a hammock or hanging stuff. The bamboo is green because it is newly cut as I had recently done some renovations as a preventative measure to the structure due to some minor design and bamboo selection issues. The old purlins were smaller and actually fine but were reused on the ridge for an improved water tight system.

jamesamd · May 04, 2011, 10:25:43 PM

has to be a wind brace,green or not.
Jim

jim king · May 05, 2011, 01:28:12 AM

I would say it is to spread the possibility of the bamboo flexing in the wind and just general stability. I have built many of those type of buildings but with wood poles that are not flexible like bamboo. That is not a strong method if there is much wind.

canopy · May 08, 2011, 11:29:57 PM

Hi Jim, nice looking roof and the thatch looks like very tough stuff. Actually the area is windy to the extent a wind farm was chosen to be made in this area. This structure gets a lot of wind and has a very big roof (10M long, 40 degree pitch). I would not expect a bamboo frame to be as strong as a timber frame, but I am looking for methods to make them sufficiently durable and maximize longevity each of which usually takes a back seat to speed of completion by bamboo builders. I appreciate your experience and knowledge in this area so please share any specifics on things you noted do or don't work well. My goal is to develop pure bamboo frames with bamboo joinery; no nails, wire, bolts, metal, wood, or other materials of any sort. I have learned a lot through reading, observation and experience and am always looking for improvements anywhere I can find them. If in the end I find a design that withstands the trials of time and weather then one day I would like to publish a concise document on getting the most out of bamboo joinery.

jim king · May 09, 2011, 09:08:49 AM

Canopy: That purlin should have braces to the center wall height beam to keep the wind from pushing the roof in. If you have good winds you should also use 45 degree bracing on the walls as in the photos below, You can see my wife standing in the patio when a freak storm pushed the patio to a nice slant,
That is why I suggest the bracing.
For me the thatch in your photo is to far apart and will wear out in half the time as if it was overlapped closer together. Changing a thatched roof is not pleasant as when the leaves are old and dry they create a dust storm in the removal process and cleaning it up with no electricity is a problem.
If possible spray or dip the leaves in Borax to keep the termites out or better yet a dip for lumber if possible. This will also help with the rats that are every where in the tropics that like to live under the woven roof cap.
I have built a couple of dozen of these with the largest being about 40 feet by 120 feet that housed a sawmill in the middle of the jungle.
You can lash everything together with vines but I did not have the patience and used nails .

canopy · May 10, 2011, 09:20:54 PM

> That purlin should have braces to the center wall height beam to keep the wind from pushing the roof in.

What I was thinking of doing for structures with larger spans than this is sort of a queen post that runs from purlin all the way into the ground. Then run a tie beam between these queens. Finally a brace from ridge to plate that holds the queen and completes the triangle. See illustration.

> If you have good winds you should also use 45 degree bracing on the walls

This is an area I need more advice on. The timber framer in me really wants to button up everything with knee braces so I have come up with 4 mounting options:

top left: run a brace through the post & beam. I feel this will create too much of a bending force and blow out the post/beam mortise.

top right: parallel braces on either side and secured with pegs.

bottom left: fit brace inside post & beam and secured with pegs.

bottom right: lash a brace to post and beam. This would be ideal since it doesn't weaken the post or beam but can it be fastened securely enough?

> For me the thatch in your photo is to far apart and will wear out in half the time as if it was overlapped closer together.

You are right about that. I did 20cm layers which for this area is actually thicker than the norm believe it or not. Now each time you double the thickness it does increase longevity but it also doubles cost, hauling, and installation time so that becomes a deterrent. The other thing I have read that impacts thatch longevity is pitch. Pitches should be high (some say 40, 45, even 50 degrees) to get water to exit quickly.

> If possible spray or dip the leaves in Borax to keep the termites out or better yet a dip for lumber if possible.

The thatch grass seems to be pretty resilient and long lasting stuff. The problem with the thatch panels I do have is the boring insects go after the bamboo splines in them. I don't know how many years it would take for them to totally gut them out but they gradually weaken. I am not sure the best solution to this but it seems it will mean I will need to make my own panels. I wonder if seasoned bamboo splines would do the trick.

> I have built a couple of dozen of these with the largest being about 40 feet by 120 feet that housed a sawmill in the middle of the jungle.

That's must have been awesome.

jim king · May 11, 2011, 01:00:20 PM

canopy:

The problem with the termites and insects is that the dust from them eating the wood will make you scratch your skin off. If it is like here the spline will outlast the thatch even tho it has bugs in it.

The pitch of the roof is also important here. The 45 degree idea is good but I lowerd my roof on the house due to winds and storms . I got tired of sleeping in a moving house.

I still think you need a center beam ceiling height to put the braces to the purlin or your roof is going to flex in the wind.

I have worked very little with bamboo but I did find it brittle so I dont think running your braces thru it would be a good idea. I would lash one to each side as you have shown.

This method of construction is in reality from the stone age so I did not do mine as a purist.

canopy · May 13, 2011, 12:17:27 AM

Very good feedback. Just one more follow up. You've mentioned a bit about wall cross braces and purlin bracing. Do you have any closeups or can you describe joinery you would recommend for angular bracing for those type applications?