Mounting sheet glass on GH Roof

[Flavio]

I am in need of some advice/ideas, etc. which would help me on mounting glass to the roof of a wooden framed greenhouse. My plan was to install the glass at approx the level of the roof sheathing. The glass would sit on runners attached to the inside of the rafters on each side and to sit on cross pieces on the top and bottom. My problem is that I am concerned about the bottom acting as a dam and causing wood rot from rain or melting snow. Any input would be appreciated.

[Bill]

Not sure how commercial ones are built but I would not have the bottom at all I would use Aluminuim or simiuler L shaped pieces to allow the glass to overhang the frame. Will do a picture when I get home, if you want.

[Chris43]

All Alton greenhouses are built from a frame of rot proof cedar, with grooves cut into the sides and the top bar, so a full piece of glass can slide right in and up into the top slot. The glass is then secured to the bottom bar, which is not as thick as the others. This lets the glass lie flat on the bottom bar, and is simply secured by a couple of clips.

I have had these greenhouses for many years, and at the end of the life of my previous greenhouse a couple of these bottom bars had rotted. But that was 35 years old, so I don't reckon that was too bad. The only reason I replaced it was to get a bigger one, as it would have been relativley easy to but some more cedar and replace the bar.

You can see the way it works in Figure 8 of the Installation booklet on page 16 at Alton Installation but note it is a big pdf at 4.65MB.

[DaveW]

Traditional method on softwood wooden greenhouses was for glass to overhang the bottom rail to form a drip and be held with copper slating clips. You should be able to obtain these from a roofing or plumbers supply merchant.

[attachment 17061 glass.jpg]

Sorry it's not a brilliant drawing, but it is hard to draw in Windows Paint with a mouse!

DaveW

[Bill]

Yep that's the sort of thing I meant Dave.

They're not bad for paint.

Bil

[DaveW]

Yes Bill, the Alton's Chris mentions have a wooden block to stop the glass sliding down, and the glass as I recall stops half way down the bottom rail, but they are rot proof Cedar. If you use that method with softwood the bottom rail would be rotten within a year or so. The glass overhanging the bottom rail protects it and also drips the water clear of the greenhouse.

I used to build quite a few wooden conservatories in the 1960's and they were based on conventional wooden greenhouse construction of the time. The glazing bar section was similar to this:-

[attachment 17099 Bar.jpg]

The glass was bedded on putty, but unlike conventional windows no putty fillets were used on glass roofs since it was considered if the fillet came away from the glazing bar after a time it held rainwater against it and rotted it.

Instead the glass was held in place with small panel pins, and the paint was taken over the bar and about 1/2" onto the glass each side of the bar (grey in my detail).

Greenhouse and conservatory roofs in those days needed a lot of maintenance, and usually the bars needed repainting every year or two to maintain the seal and stop the softwood rotting.

Possibly these days you could keep the glass level with the top of the bar and use a screwed on replaceable wooden capping strip to hold it down.

DaveW

[DaveW]

You could use this method also Flavio.

[attachment 17103 bar2.jpg]

The capillary grooves in the top capping stop vibration on the glass due to wind etc and capillary attraction sucking the water sideways. The ones in the bar itself act as a gutter should any manage to get past. If you are using softwood however I would use a sealant gun and run a bead of sealant down the side of the capping to make sure water does not get under or softwood will not last long.

The advantage of the top capping is it protects the bar and if it does rot can be replaced. If you intend to overlap glasses though you will have to make the rebate twice the glass thickness in depth at least.

If you are using a portable saw you can just make the capillary grooves with a shallow saw cut from that.

DaveW

[Mal]

Dave, your drawings look good to me. If only flat packs had instructions like that life would be so much easier. Mal.

[Flavio]

Hello all...Here it is May and I am still plugging away at my greenhouse project started months ago. Finally a solution to my challange of securing the glass on the roof from sliding off. Hopefully it will help someone else facing a similiar situation on their greenhouse project.

I will try to describe what I used and how I installed the glass along with the photos. I apologize for long post and also the quality of the photo's.

[IMG]http://i138.photobucket.com/albums/q262 ... 18-19A.jpg[/IMG]
This is an end view image of the aluminum strip that I found to hold the glass from sliding off the roof. As you can see it is fabricated so as to have two seperate what I call compartments. The outside edge is made to accept glass of different thickness up to 1/4 inch. The glass slides into this first compartment which is shown as a gap. The second compartment is independent from that on to which the glass fits. This makes it possible to drill holes to affix strip with the glass onto the 2x2 inch redwood framing on the lower edge of the roof. I also drilled two weep holes on this lower compartment in case some water would be collected there.

[IMG]http://i138.photobucket.com/albums/q262 ... 1_0001.jpg[/IMG]
This is a front view of the aluminum strip tha I am using. You can see the gap into which the bottom edge of the glass slides into. Note that I drilled three holdes through it to screw onto the 2x2 inch framing.

[IMG]http://i138.photobucket.com/albums/q262 ... -17A_1.jpg[/IMG]
This photo shows the 2x2 framing onto which the glass rests after it is insered into the gap on the aluminum strip. Note that I have put metal flashing on top of and on the downside of the 2x2 framing strip. This is done prior to mounting the glass. the butyl double faced sealing stip sits on top of the runners and the glass is set on top of the strip to form a seal.

[IMG]http://i138.photobucket.com/albums/q262 ... 21-22A.jpg[/IMG]
Photo of glass on roof taken from inside the greenhouse. A partial view of the 2x2 framing onto which the glass is resting is shown.

[IMG]http://i138.photobucket.com/albums/q262 ... A_0001.jpg[/IMG]

This shows a group of several of the glass sheets installed on the roof.

I put two inch neoprene spacers between the side edges of the glass and the rafters. I then cut a strip piece of redwood to fit on the top and sides of the glass to cover the gap which results once the spacers are installed. I then put calking around the side edges of the glass. a thin bead of calking is run into the gap in the aluminum piece before the glass is slid into the gap. I also put a dab of calking in each screw hole before screwing the aluminum piece onto the 2x2 bottom piece. In addition, I also calk the flashing edges formed on the bottom end of the rafters shown where there are gaps. On the top of the rafters which seperate the glass installation you can see a strip of plywood on top of the rafters. This is sheathing that will cover the remainder of the roof. I had some drip edge bent into an upside down "U" shape with a flange on each edge. The flanges fit over the wood strips installed earlier around the top sides of the glass. I also run a bead of calking along all exposed edges of the final installation.

A special thank you to all who have been helping with ideas to solve problems that I have encountered thus far on my project. Your help has keep me on track. Needless to say, I am anxiously awaiting the completion of this project.

[DaveW]

Remember with glass sizes that the normal manufacturers recommended clearance to allow for expansion and fitting etc on glass is 1/8" (4mm) smaller than the measured opening size.

Therefore if you plan to use standard 2'-0" x 2'-0" or other
2'-0" wide horticultural glasses the distance between rebates or grooves the glasses go in should be 2'-0" + 1/8".

I saw a toughened glass sheet once that had gone off bang in the summer showering fragments like confetti all over the plants because it was originally too tight in the aluminium glazing bars.

DaveW