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Started by Believer, March 27, 2012, 12:44:50 PM

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Believer

Hello all.  This is a wonderful forum.  I'm about to embark on my first timber frame project after having taken a great class at Northern Lights.  I have a limestone walled spring house about 10'x12' with a 2/12 pitched roof on it that's about gone.  I'd like redo the roof with a living roof (green roof).  This will be practice for building our retirement home a couple years down the road (I think I can, I think I can).  Anyway, I'm hoping to get some advice.

I found a product that will stabilize a pitched green roof so I want to have a 4/12 pitch.  My first thought is to simply use a corner half lap sill joint (maybe in this case it's a plate).  The rafters will sit directly on these sills/plates.  Should I tie the long sills together in the middle of the 12' dimension?  Will the half lapped joints resist the lateral forces over time?  Should I plan a different joint?

To the rafters--could someone suggest for me the best approach for a 4/12 pitch?  Thanks.

Jim_Rogers

The first thing you need to do is figure out the roof load per square foot for the living roof.

After that we can talk.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Believer

I believe the live load is 20 lb per square foot and the dead load is 30 lb per square foot.

Jim_Rogers

Sounds way to light to me.

I did a design for a fellow who was going to do a living roof and his load was 100 lbs per sqft.

Are you sure?

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

witterbound

According to this document
http://ts.nist.gov/ts/htdocs/230/235/weightcart/historyof105-8.pdf
at the US NIST, the average density of dirt is 120 lbs per cubic foot

So if your dirt is 4" deep, you'd be looking at 40 lbs of just dirt, right?

Jim_Rogers

Sounds right to me, that's why I asked.

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

losttheplot

Rob Roy has a couple of pages about living roofs, in his timber framing for the rest of us book.
He recommends 8 inches of soil (120lbs/cu ft) over 2 inches of crushed rock, for temperate climate with moderate rainfall.
He also states "drainage is the better part of waterproofing"

If in doubt build it stout  ;)
DON'T BELIEVE EVERYTHING YOU THINK !

Jim_Rogers

From Believer
In answer to some of the questions about loading, modern green roofs do not utilize field soil--it is too heavy and its fine particles will migrate.  Materials for extensive green roofs usually comprise at least 80% lightweight coarse mineral aggregate.  Examples are expanded clay, slate, or shale; they are heated to very high temperatures in a kiln expanding into a lightweight, popcorn-like form.  The expanded material is sponge-like and it's voids hold air and organic material and increase the particle's surface area, slowing the movement of water.

An extensive roof that will sustain succulents and some grasses is only 3"-4" deep.  When fully saturated with mature plant cover, this load (the growing medium and plants) is only 20-30 lbs per square foot.  This load is for a flat roof; a pitched roof projects more mass of saturated growing medium to horizontal plane but it also drains much faster.  I'm not sure how to account for the pitch.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

I split out Farmer's question into it's own thread.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

Quote from: Believer on March 27, 2012, 12:44:50 PM
My first thought is to simply use a corner half lap sill joint (maybe in this case it's a plate).
You may need to secure these two timbers together at the corner so if they are on the same elevation you could use a standard mortise and tenon joint that could be pegged.

Like this:



It will just not have any post above it and no joist pockets. The plate that has the rafters on it should have the mortise and the connecting one has the tenon.

QuoteShould I tie the long sills together in the middle of the 12' dimension?

At that pitch you will have some thrust to deal with, so I'd say yes, you should/could tie them together at the middle.

Quote
Will the half lapped joints resist the lateral forces over time?  Should I plan a different joint?

Answered above.

QuoteTo the rafters--could someone suggest for me the best approach for a 4/12 pitch?  Thanks.

Do you mean size and location or what?

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Believer

Well, I'm wondering how to join rafters at the peak for a 4/12 pitch and what kind of seats to put in the plates that rest on the limestone wall.  Thanks.

Jim_Rogers

Quote from: Believer on March 29, 2012, 01:44:12 PM
Well, I'm wondering how to join rafters at the peak for a 4/12 pitch
There are three basic ways to join the rafters.
One is to just butt them to each other and nail them.
The second is to do a half lap.
And the third is to do a mortise and tenon with a peg.

I would peg or screw together the hap lap as well.



Quote
and what kind of seats to put in the plates that rest on the limestone wall.

As far as seats go it depends on whether or not the rafters are going to overhang the plates or not.

There are several ways to do it depending on what you want.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

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