pergola anyone?

craig shields

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6
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edmonton alberta canada
ok i guess this would be my frist post here and i have to start somewhere soooo... my backyard deck is 23ft x 16ft and the wife says she wants a pergola above it. I will need 3 or 4 beams 23-24 ft long to span the lenght, there is no option of spanning the other direction and no chance i will put support post in the middle of my deck. does anybody here have and experience making their own glue lam beams? i was thinking 4-6x 3/4 by 7 1/2 inch strips of plywood glued, then encasing it in cedar with some west system 2 part epoxy. am i way off base here or do i get a second and third mortgage on the place and just order 4 solid cedar beams?
 
Might work, but I think I would bolt them together also. Just to be on the safe side. Recess the head and nut, then cover it with the cedar. I would even go as far as just to use 2x10's or something. That is the way basement beams used to be done. Long as you could keep the water out of it, I really don't see a problem.

Oh and welcome.
 
thanks steve i thought about solid wood 2x8 or 2x10 but came to think that plywood is way more structurally stable and if im not mistaken a beam made of plywood 7 1/2 inches by 4 1/2 inches (6 layers thick x 3/4 inch) should be darn close to tank proof? idk just a thought :dunno:
 
Glue lam beams are not meant for outdoor use. Youy are going to need on heck of a chunk of timber to spaned over 20'. I dont think I would even try to spand that with out getting someone with exp in support loads.
 
the beam will be in a totally weather/waterproof enclosure, the west system epoxy is the same stuff that the wood boat builders use and the weightload is not much 40 or so 2x8x16 cedar planks on their edge so no snow load or waterload just their weight i could even put fiberglass matt under the epoxy but i really dont think that it would need it :dunno:
 
I would tend to agree here with Al, delamination due to moisture/humidity would be your big enemy. You don't need a post directly in the center, but a pair of posts with a beam spanning their top for the long beams to lay crosswise onto would help. (clear as mud???):rofl::rofl: Pergolas don't really hold much of a snow load as they aren't a solid roof, only slatted for shade correct?:huh: What about a gazebo over half of the deck?
By the way, welcome to the family!!
 
that is the plan just a slatted roof. You think that delam would be a problem even if its totally encased in a fiberglass resin without the cloth? I would much rather see the cedar than the fiberglass mesh so i would skip the cloth
 
Yes, I think delamination would happen eventually. Snow and ice sitting on top of the beams, a warm day, wet puddles, freezing cold night causing a crack to develop, water seepage, a couple of days later warm again, laminated beam soaking up water through the crack in the fiberglass, freezing night again, delamination occurring. Might be years before you realize it, but don't think I would take the chance. Around here many of the old electric poles were cedar. If you could get a sawyer to square up a cedar telephone/electric poles for you then you would be in business. :thumb:
 
Welcome Craig!

I feel that you are over thinking this. Keep it simple
Your load is going to be negligible since you are not going to put a solid roof over it. that said you can span 20 ft with a 2x12 no problem. If you used pressure treated 2x12 and pressure treated 1/2" plywood laminated together, it would support the "joists" just fine. Also, dont wrap them so tight that they cant breathe you will increase the speed of degradation and it will want to warp more due to the disparity of moisture between the inside and outside. Water WILL get in there, you need to provide a way for it to get out of there so the beam can dry out. One way would be to put your cedar cap on it and when you put the cedar sides space them off the beam 1/8 to 3/16" put a cedar bottom the width of the beam and let the sides run past. You dont need to leave a gap where the bottom meets the sides just dont seal it.
Another thought. instead of putting your posts all the way to the extreme set them in a couple of feet from each end which will shorten the span to about 16 feet and give you some nice archtectural interest, and adding a third post in the middle of the run further decreases the size of the necessary beam.
The run out from the house to overhang the beam is just standard dimensional stock. Use stainless joist hangers against the house to tie it in and support that end.
 
Agree with rich on the pt 2x12's for the span. I think outdoors the plywood would have major problems over time, and a 7-1/2" glue lam beam is not going to be big enough for a 20-23' span.

Not sure what the sag on a 2x12, but in steel a W8x15 (15 lbs/ft) beam or 8" Channel x 18.5 (18.5 lbs/ft) is going to have about 3/16" sag in the middle with 1000 lbs load if that gives you any idea. I think a 7.5" tall wood beam is going to struggle on a 20' span even without the plywood issues, you need a taller beam or shorter span.

Another option that will help a lot is some loaded 45 degree support beams off the post. If you can put a piece 3-4' from the top that hits the overhead beams 3-4' from the posts, this will shorten the effective open span, and make your structure a lot more rigid.
 
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