Project: Site-Built Under-Deck Drainage System
Watch Mike Guertin demonstrate how to keep the space under a deck dry with materials that cost less than $1/sq. ft. in this FHB webinar.
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In this instructional webinar, FHB editorial adviser Mike Guertin discusses how he uses EPDM roofing or pond liner to drain water under the decking and keep the space beneath the deck dry. Whether you’re thinking of using the space beneath the deck for storage or outdoor living space, Mike’s simple approach is easy to install and cost effective (materials are usually less than $1/sq. ft). He covers how to lay out a pattern to cut successive sheets of EPDM from, install the sheets to create sloping swales between the deck joists, and how to tie the membrane into the wall of the house and the gutter.
Originally presented live on Thursday, April 2, 2020 at 6 PM EST.
Read the article: Site-Built Deck Drainage
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Incredibly informative, sorry I missed it live. How do you waterproof around the rail posts or with a ladder box for a double picture frame?
Around rail posts the detail will depend on whether or not post sleeves are installed. If so, then uncured EPDM flashing tape can be used to flex around the post about 4 in, up and onto the membrane beneath. If not using post sleeves then EPDM water cutoff sealant run between the post and epdm sheet.
If you install double joists or any other border board framing then you would cover them with membrane - either just the EPDM or a self-sealing membrane. Id run a strip of wood 1 x 1/8" around the outside edge so there's a little gap between the decking and the membrane. That will let any water that is in that area drain and dry.
Hi Mike
Thanks for this. Want to do a system with metal pans between joists. Would like to use coil trim stock and our siding brake. Have you ever done it that way ? Mahalo
Mark
I have not. But there are manufactured systems that sell track gutters that straddle the bottom of the joists and have slots that you insert aluminum coil you cut yourself into. Water drains into the joist straddling track/gutters under the joists and drain down to a gutter.
There should be an article covering all the different manufactured systems in the June FHB magazine.
There are also a way to use corrugated metal or plastic mounted to the bottom of the joists as the drainage system. You just apply perlins of different thickness across the bottom of the joists in succession to create a slope. It's like an upside down roof.
Mike,
Thanks for the great lecture. Any suggestion on how to get around all of the extra blocking for wood handrail posts in the outer most joist bays? Or is there a way to attach posts that work better for this drainage system?
Also do you have any suggestions for using this drainage system technique when there is a feature deck board in the center of the deck with ladder box framing under it?
-Mike
Other than installing the posts on the outside of the deck frame the only way I have been able to install any underdeck drainage system is beneath the bottom of the joists. I think I described doing this at the end of the program. The membrane is fastened to the underside of the joists and framing is installed beneath the joists if the ceiling will be finished. Posts inside the deck frame are a problem for all manufactured and site-built underdeck drainage systems.
A feature board only needs 2x blocks placed on the flat dimension and toe-screwed/nailed to the joists. I place the membrane and then install the blocking between joists. The epdm is pinched between the block ends and the joists. Another way to do this is to install the joists for the deck 1-1/2 in below the height you intend it. Then install the EPDM as I described. Then install blocking across the joists where you need the feature board. Finish by installing 2x2 s on top of joists (in line with joist direction). Then decking over the top.
Hi Mike,
I am really glad I found your article as this is exactly what I am looking to do. What is the best way to estimate the size EPDM that I need? I read your first article but am a little unsure exactly what you meant when you describe this.
thanks,
Eric
My deck is roughly 9'x14'
Eric, Figuring that each joist bay needs an extra 3 inches of EPDM to create the swale, and your deck has 11 joist bays - if you add an extra 3 ft. to the width of the deck you would have plenty lengthwise (14' + 3' = 17') And you need an extra 2+ft. more for the depth of the deck to account for the curve of the fan shape (9+2 = 11')
If you round those numbers up to match standard sizes of sheets - You should be good with a 12' x 20' piece. More likely to find that in pond liner as opposed to roofing unless there's a supplier local to you who sells cut roofing off a 20' wide roll.