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Question DetailsAsked on 11/3/2015

I am looking to cover an existing pergola with a material that would let in sunlight and keep out rain. Thanks

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Assuming it is fairly flat (not necessarily level) surface, acrylic greenhouse plastic is fairly cheap, holds up fairly well even in snow as long as you give it a 20% slope or better (thaws from bottom due to solar heating and slides the snow off) and support it every 2 feet or less. If you plan on sitting under it when it rains or has snow on it, use rubber gasket screws or greenhouse plastic gasketed nails to fasten it down, and never in the troughs (comes in rounded and beveled rectangular trough shapes) - run the fasteners through the ridges, being careful not to drive in far enough to crimp it - so screws are better for controlling that. Use cad plated or stainless if you don't want them discoloring and staining the wood. 2x2 cedar works well for support pieces (need on under each longitudinal joint, which should go uphill to downhill, then depending on spacing 2x2 or 2x4 to 2x6 crosswise to provide intermediate support for the 2x2 "rafters" as needed. Comes in lengths from 8 to 12' - as do the wood.

Price around, as runs everywhere from about $25 to $70 for 8', $35 to $100 per 2' wide sheet for 12' long pieces - available at many home improvment box stores and lumber yards. Polycarbontate plastic is also available in same dimensions, costs about 25% more but lasts longer too - more like 15-25 years instead of 10'ish. Fiberglass cheaper but discolors to yellowish within a couple of years, and breaks easier. All come 2' at most places that carry them, available in around (not exactly - varies by product) 4' width on special order or at specialty greenhouse places.

Don't forget overhang on front to serve as dripedge, otherwise it will wick back underneath and stain the pergola leading edge badly. Not a bad idea to put a piece of metal prepainted drip edge there anyway, especially if overhang is not about 6 inches or more (about all the snow load it will handle on an overhang).

If flatter than about 20% slope or in area with real icing conditions so snow does not slide off, you need more intermediate support, or use roof rake or broom to remove the snow before it gets over about a foot thick.

For flat pergola will work in rain country, not snow - dams up and leaks too easy. Will NOT handle very high winds/tornadoes/hurricanes unless you put framing over the top too, screwed through to wood framing underneath - which works OK but catches tree debris like crazy, so not great for that. I have seen (not done) 2x2 stringers up to down slope over the "rafters" and screwed through both to cleamp it between them - at about 2' spacing that will take a lot of wind load before it starts tearing loose.

On slope issue - commonly easier on existing pergola to raise or lower one edge to provide slope for the entire thing than to overlay a slope on top of it.

Note they have a solar barrier layer on them (thin plastic sheeting adhered to it) to cut the ultraviolet damasge - so pay attention to which way is "up".

More expensive by a factor or 2-3 - plexiglass sheeting with available roll gasketing for the edge joints, screwed down to framing with gasketed screws (have to predrill to avoid splitting) or held down with framing over-and-under method described above to clamp it down between the wood layers. Not so much a DIY job - and sheets are pretty pricey to be taking DIY chances with if you have not used it before.

Bear in mind - is pergola was not originally covered, might not have been designed to carry water or snow or the wind load from being roofed. Structural Engineer is the person to address that issue if needed.

Answered 4 years ago by LCD

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