Topic: Greenhouse Construction

[skypilot]

I am still figgering what it is thats holding my greenhouse up?  I thought that the struts  (the sloping bits) were supported by the ridge bar (the bit running along the top of the green house) and ran from there to be supported at their other end by the walls. They were then further strengthened by a tie which ran between them. A friend tells me that this is not so and that the struts merely rest on the ridge bar.

Can anyone throw light on this subject?

[Clayhithe]

The vertical bits hold the roof up.

But the ridged roof would push the walls out except for the horizontal ties.

The ridge is held up by the end walls and the vertical bits from the ties to the ridge.

(Did that make sense?   I wish I could draw)

[skypilot]

The vertical bits hold the roof up.

But the ridged roof would push the walls out except for the horizontal ties.

The ridge is held up by the end walls and the vertical bits from the ties to the ridge.

(Did that make sense?   I wish I could draw)

I think I get the general idea, thanks, Clayhithe. However, perhaps you can see my problem more clearly, if you look at the following picture page of my allotment:

http://community.webshots.com/album/286599773CoPRDK

The greenhouse is 17ft in length and 7ft wide, and more than thirty years old. The ridge is not continuous being joined after about 3/4 of its length. It is as you suggested supported at each end but is not supported by a vertical member from any of the horizontal ties.

As far as I can see, the only things supporting the roof are the ties and the walls.

What do you think ?

[philcooper]

Skypilot,

This is an engineering question.

The roof is supported by the wall (side and end). The angle of the roof is such that the weight (of roof bars, ridge bar and glass) is transferred as vertically as possible. (A flat roof tends to sag in the middle)

All the elements of the greenhouse are mutually supportive in that they provide resiliance (ie don't twist in the wind).

The 4 walls can twist without the roof. The roof being a fairly rigid rectangle prevents this twisting.

The roof is kept stable by
a. the walls
b. the weight of the bars and glass pushing against each other at the ridge.

Horizontal ties help in a lightweight green house but are not essential. especially if the roof slope is 45 degrees or more

The ridge bar:
a.  is an aid to construction - it's easier to butt and fix each roof side to the bar, having fixed it to the top of the 2 ends, than it would be to bolt them to each other
b. provides addition lengthwise rigidity to the roof
c. provides a way of preventing water getting between the 2 sides of the roof. Otherwise water would rest where the 2 roof elements meet (simple coving would solve this problem but not the rigidity  or construction problems)

That's all I can remember from my civil engineering - I rapidly moved to electronic engineering because all the work is indoors!!

Phil

[Clayhithe]

Skypilot,

I see your problem.
The weight of the ridge is tending push the eaves outward,  and they are held in only by those wooden cross-ties,  which are too high to be really efficient.   They would be more efficient at the level of the eaves.

I might be tempted to fasten strong wires to the joint between roof and wall,  at the eaves and across the house,  with tensioners to pull them tight.   Would you have headroom under these wires?

If not,  then one or two vertical supports down the centre of the house to hold the ridge up.

Don't be tempted to hang anything from the roof,  will you?

[Clayhithe]

I've read your posts again.

The ridge rests on the end walls and on the sloping bits.
Without ties the centre of the ridge would move down,  pushing the eaves out.
The ties pull the sloping bits together.   This A-frame then rests on the walls and exerts only downward thrust.
Your ties are too high to be efficient.

The ties are wooden and thirty years old.   Are they held with rusting steel nails?   Is the wood sound?

If the ties were efficient and the walls vertical then the joint in the ridge would not matter.

[skypilot]

Hi John & Phil,

Although the Greenhouse is some thirty or more years old, (according to an old-timer on the allotments)  it has in that time undergone a few modifications.

 (I think the person who installed the division between the shed and the greenhouse must have been at the "Golden Nectar" :))

Despite its somewhat irregular design, for the most part its frame is quite solid with a few patches showing signs of rot, but nothing that can't be managed.

John, the ties are held in position by "new" screws and the wood appears sound enough there wouldn't be enough headroon to allow wires across the greehouse.

My main concern was to understand the "mechanics" of construction, and I am very grateful to you both for clarifying the matter for me.

I can now approach the matter of tidying things up with more confidence.

Thanks to you both

Don

[philcooper]

Skypilot,

All that I said assumes that all the elements of the structure are sound and can bear weight and stress along their length.

A bit of rot could  upset the whole thing

Phil

[skypilot]

A bit of rot could  upset the whole thing

Phil

Phil,

The rot is restricted to a small area on the upper part of one wall and a similar area on the lower part of another wall, nothing that would seriously affect the structure or strength of the building


Don

[philcooper]

Don,

I'm glad to hear that, it would be a problem in any of the roof members

Phil