How to design and build skylights

If you are designing a skylight or glass structure that you would like us to build the following information will give you an idea of what the possibilities are. If you’re planning on building such a structure yourself  there are some ideas below that are a good start on the planning process.
The major manufacturers of silicone publish specifications for using their products for installing glass. The essence of what they are describing is using their product as a glue to hold glass in place. The system is known in the industry as “wet glazing”. NO HARDWARE OR COVERING BARS ARE NECESSARY TO RETAIN THE GLASS.  This may run counter to your intuition about a safe way to install glass but the fact is that this technique is superior to any fastener based or preformed dry gasket system in most ways that matter: 
1. It’s the easiest system to make waterproof, unlike preformed, dry gasket systems the seal is helped by the bond between the pumped into place rubber compounds and the glass/frame elements.
2. It’s very forgiving of  problems in how flat and planar the bedding surfaces are. Since the rubber compounds used are pumped into place, in gaps that the design provides,  the rubber conforms perfectly with the elements of the glazing system even if the frame or bedding surfaces are wracked, warped or out of true. Not only do they conform perfectly but they also bond to the glass and the frame – this makes getting a watertight assembly much easier! 
3. Point loads on the glass are eliminated – this makes the glass much more resistant to breakage. Every square inch of the edge of the glass is held with equal pressure in a  snug, tenacious, yet flexible embrace that distributes any load on the glass without creating a local stress to be the start of  a crack. Never again will your glass break because it came in contact with a fastener in the framing.
4. Since the glass is glued into place the glass can be attached to an almost unlimited choice of framing and substrate elements  - steel, aluminum, stainless steel and wood are all quite appropriate to the system – anything that the rubber will stick to.
5. Proprietary factory hardware or structural members are not necessary – everything can be done with  generic materials available at the yard or warehouse.  


If you’re like I used to be,  your use of caulk has consisted of sticking the nozzle of the caulk gun in a gap, crack, corner, or intersection of materials and squeezing out some goo while you used the gun tip to smear out the caulk as neatly (not very) as you could. 

Here’s a drawing of a properly designed caulk joint. This particular joint was used between the bottom sill plate of a skylight and the flashing below it. The caulk is the hour glass shaped part. The cross section of the caulk is about twice as long as it is wide. The hour glass shape provides maximum bonding surface while increasing the flexibility of the joint. Imagine griping a rubber band with the thumb and forefinger of both hands. Pinch the rubber band with   the fingers very close together – the rubber band will not have much stretch. If you separate the hands, pinching the rubber band so that there is an inch or two between  the pinch locations  then the rubber band is easily stretched. The key to good joint design is this stretchiness – it reduces the stress on the  bonding surfaces. REMEMBER – the stresses from expansion and contraction of construction materials  are very large forces acting through very small distances. The key to keeping any caulk sealed tight is design the joint so that it doesn’t stress the bonds.  
a drawing of a properly designed caulk joint 
 Since you usually will only have access to one side of the joint use foam backer rods behind the caulk to shape the back of the joint. The backer rods are soft foam that the caulk will not stick to. Backer rod comes spooled up in rolls in a wide assortment of diameters and should be sized so it is slightly compressed when forced into the joint. Set it deep enough into the joint so that there is room for the caulk on top of it. Masking tape on either side of the joint will provide clean edges after wiping the joint with a putty knife. Grind the putty knife to the cross section shape you need in the caulk. Use the putty knife to distribute the caulk so that one final full length pass tools the caulk into the right shape. Pull the masking tape while the caulk is wet. A little practice and you can get a  caulk joint that’s nearly as neat as if it were a factory made extrusion.
The next photos are a sample of what the above joint looks like  in real life and shows how flexible they are.
Flexible caulk joint. 
Flexible caulk joint. 
The following two images show two configurations for installing glass onto structural steel “T” sections.
The top setup allows the stem of an inverted “T” section to stick up above the glass. The bottom has all the steel underneath the glass. Technically, the bottom set up is superior, it provides a thermal break and keeps all  the steel out of the weather.  The top set up looks better and is the set up that most designers use - the end appearance of the structure is more interesting and experience shows that the problems are not serious. Epoxy coatings on the steel will keep rust from forming on the exposed portions of the steel for a long time.  Condensation will not form unless there are unusual sources of moisture in the interior space – a steamy bathroom, swimming pools or a kitchen that’s doing commercial scale cooking. Occasionally  during cold, rainy periods we’ve seen condensation on the inverted “T” system. A small fan in the room to provide a little circulation has been sufficient to prevent this condensation.   

 Glazing detail 1. 
Glazing detail 2. 
The steel “T” in the lower drawing could just as easily be an “I” beam.  This opens up the possibility of long span rafters. In fact, since the glass is bedded onto a plain flat surface the rafter could be wood with 
a plate screwed on top or just about any other structural material that silicone can be adhered to. 

The exterior seal in the wet glazing system is set up the same way as the caulk joint  shown in the previous discussion. There is a backer rod behind an hour glass shaped silicone seal. This seal is tooled
with purpose shaped putty knives and kept neat by using masking tape.  The silicone used is a softer, more elastic formulation. Dow Corning’s #775 formulation is appropriate. The set up used on the interior, to glue the glass down is quite different. It’s set up to allow expansion and contraction of the glass without defeating the bond. It also provides a secondary seal against leakage. The first step is to apply a silicone compatible glazing tape. Don’t use butyl tape - it interacts badly with the silicone. We use Norton’s 3/8” wide  x ¼” thick neoprene foam tape.  It’s a  peel and stick tape and comes in a version that’s sticky on both sides. It also comes in two hardnesses – we like the harder one. The tape is laid down on the bedding surface. There should be a  substantial gap between the edge of the glass and adjacent materials (glass or mullion parts). The silicone manufacturers recommend 3/8”.  Sometimes in the inverted “T” system there’s only room for a ¼” gap. We have found that this works OK but the outer seal needs to be shaped so that 2:1 ratio between the length and the thickness of the silicone cross section is maintained. The hourglass shape helps here. In any case the edge of the tape needs to be set back from the edge of the bedding bar a ½”. This leaves a ½” by ¼” channel to put the structural glazing silicone into – this is the glue that holds the glass down.. Once the tape is laid down the next problem is positioning the glass. We cut some small wood shims to hold the glass off the glazing tape until the glass is in position. If the shims are 1/16” thicker than the glazing tape this allows you to slide the glass around until its in position, then you  pull out the shims and the glass sticks to the tape as soon as it contacts the tape.  Once the glass is adhered to the tape it’s surprisingly strong.  Although the glazing tape could be peeled off pretty easily once it’s stuck to both the glass and the bedding bar peeling is not possible. Both parts are stiff and so to separate the glass from the bedding bar (or mullion) the whole length of the adhered surface would have to separate at once – the glass will break first. We’ve endured some severe windstorms with just the glazing tape holding the glass.  Once the glass is in place and adhered to the glazing tape the next step is to gun in the structural glazing adhesive. Dow Corning’s #995 is good for this. It’s a little tougher than the #775.  The technique here is much the same as for the exterior seal. Mask off the glazing bar and the glass. Fill the ½” x ¼” channel with the 995 silicone. Use a flat putty knife to distribute the silicone. Make sure that the silicone fully wets out all surfaces of the channel. We stick the putty knife edgewise into the channel to get rid of bubbles and make sure that the silicone fully wets the glass and bedding bar. Distribute the silicone so that there is a slight excess then finish of the whole length of this joint with one pass of the putty knife, letting any excess collect on the masking tape. Pull the masking tape and the joint is done.                  

We are sometimes asked about condensation channels. This is a feature of many old designs.  Give a little thought  on this and you realize that there’s something really fishy about condensation channels. Firstly in an old skylight with single glazing any condensation that formed would also form on the glass. How would this condensation, forming on sloped glazing magically run sideways to get to the condensation channels on the mullions? It wouldn’t.  The so called condensation channels are really leak channels. Older design skylights that used hard glazing compounds to set the glass  were chronic leakers at the joints in the glass.  Newer designs that use dry, preformed extruded gaskets are not much better. The condensation channels are really there to catch the leaks that always occur in these systems. The system described above, properly installed DOES NOT LEAK and doesn’t need “condensation” channels.     
Make a test of your system. There are compatibility problems with some paints and substrates. There are also compatibility problems with some edge seals on insulated glass. You don’t want to find this out after having built your whole job. Wait a few days for the silicone to cure and make sure that the bonds are satisfactory.  This is not a complete discussion of all the elements of designing glass structures. Some of the other things that need to be considered are the stiffness of  mullion or rafter elements. Acceptable spans for the glass are also important. Finally, it’s important to know that if you are using the system on vertical glazing the bottom edge of the glass must be supported. mechanically. The silicone tends to creep and does not resist sheer forces well.