In this article I will share how to master the bare-liner pond floor finish. In 2006, I started construction of my prototype natural swimming pool in southeastern Australia using EPDM rubber liner as the waterproof membrane. At that time, I had no experience cutting and joining EPDM liner. I was concerned that a water depth of 10 feet was too much pressure for the joins, so I didn’t cut the liner and ended up with large pleats on the curved wall sections, starting at zero at the floor and getting bigger as the wall height increased.
Not long after finishing my pool, I discovered that joined sheets of EPDM rubber were used for large irrigation reservoirs that were more than 30 feet deep. If the joining system could cope with those pressures, 10 feet deep was not going to be an issue.
At that point, I realized that every natural swimming pool I built for clients with a bare-liner finish was going to need a custom-fit EPDM liner. I threw myself in the deep end with my first client’s natural pool, and every pool since has had a custom-fit EPDM liner.
EPDM Advantages
Not all natural swimming pool and pond liners end up covered in rocks and stones to hide the liner from view. So, when a design calls for bare EPDM as the finished surface, it needs to look neat and tidy, without acres of excess liner folded over and highly visible in clear water. This is when a liner needs to be custom fit by cutting and joining using the liner manufacturer’s proprietary joining products and methods.
It’s OK for an inexperienced do-it-yourself builder to have pleats and folds and be happy with what they’ve built, but not so much for a contractor charging tens of thousands of dollars to build a pond or pool. EPDM is just about the best liner to use for on-site cutting and joining, because the joining system comprising primer and seam tape are very good when used properly, and it also has the longest lifespan for a liner.
Another benefit to custom-fitting EPDM is that no expensive machinery for joining on site is required.
There’s almost no limit to the shape for a custom-fit EPDM install, although certain shapes will make it much easier or more difficult.
Flat Walls & Floors
Mostly flat floors and walls with well-formed corners (if square or rectangular) will make the joins easier. They allow the liner to fit with almost zero pleats or wrinkles. Line curved walls with a join at the floor, or cut the wall pleats out and join them to remove the pleats. This produces a very neat result. Avoid any large-bowl shaped surface, whether convex or concave, because it will not allow the liner to lay flat without pleats or wrinkles forming.
Earth walls can be hand trimmed after excavation to remove high points if the soil permits. Rocky soils will make hand trimming difficult, with generally a rougher finish to the dig. There are methods to making walls flat. Sprayed concrete or stabilized sand can be compacted behind form work or even rendered with bricklayer’s mortar for filling lesser irregularities. Some soil types will allow for a sufficiently flat and smooth finish to walls without any need to fix the excavation. Such soils may give a satisfactory finish to a floor without further repair work, and geofabric can be applied directly on top.
Otherwise, to make a flat floor, use concrete or stabilized sand (sand and cement about 1:10) — or even compacted masonry sand and crusher dust.
Another major benefit to having flat walls and floors (besides looking neat and professional) is the ability to effectively clean the liner of biofilm. This can be done by brushing or vacuuming.
Underlayment
Once the excavation has been repaired using the methods above and all loose material removed, it’s time to install a geofabric underlayment or suitable equivalent to protect the liner from the excavated ground. Some underlays sold for this purpose are almost see-through and almost totally ineffective at protecting a liner. A good product for liner underlay is non-woven geofabric made from recycled plastic with a weight of at least 300 grams per square meter (0.06 lbs. per square foot) as a minimum. Geofabric won’t degrade, providing long-term liner protection.
The underlay seen in most of the pictures is 510 grams per square meter (0.1 lbs. per square foot) — so a seriously good thickness. This will offer so much more protection from sharp stones than a thin layer of geofabric.
A thick geofabric layer in a bare-liner pond or pool also provides a safer surface for swimmers and divers. No more eggs on kid’s foreheads from hitting their heads on the bottom of a hard-surface pool. The finished EPDM-lined pool’s floor and walls are not too hard or soft.
Vacuum the underlay prior to installing the liner to remove grit or anything else under the liner that could potentially cause damage.
Best Practices to Master the the Bare-Liner Pond Floor Finish
Shallow water with an EPDM-lined floor is not a good mix, as the liner will become extremely slippery. About 3 feet of depth is a safe minimum. One can still slip and fall over at that depth, but at least there’s enough water to absorb the fall. It’s always good to have family or friends present for a good laugh if it happens.
It’s good practice to size the pond or pool excavation to match stock EPDM widths in order to reduce waste. As little as 1 foot of overhang is a good allowance to shoot for when calculating excavation width and depth. As for length, 50 and 100 feet are stock-roll lengths and the most economical way to buy. Cut-to-length is also available from most EPDM suppliers at an increased price.
A crane or other lifting device is often best practice to get the liner in the prepared and ready hole.
It’s not worth risking damage or holes to the heavy liner by using rough-handling methods.
Putting the liner in the hole in just the right place is reasonably important. As the liner is unrolled and unfolded, position the liner in basically the right place. This is not so important with a small roll of liner. However, with a 100-by-50-foot roll at several hundred pounds, every bit helps.
If the liner has manufacturing codes printed on it, turn it upside down. The white printing is very difficult to remove and doesn’t look good in a finished pool or pond. This is not a concern if the liner will be covered in rocks and stones.
When the liner is positioned and held in place using sandbags or similar, the excess liner can be accurately cut out and removed. Work one join at a time. Leave enough overlapping liner to complete the join. Cut with care to ensure cuts are in the right place, plus or minus 1/8 inch.
Keep it Clean
Prior to applying primer to the join, cleaned dirt, dust, oils or any other contaminant from the EPDM. In Australia, a product called Prepsol is ideal for this. It’s a clear liquid mostly used in automotive painting as a degreaser. Personal protective equipment such as nitrile rubber gloves are mandatory when using such chemicals. We don’t want that chemical absorbed by our skin.
When the liner is clean and ready for primer, accurately mark the extent of primer and apply no more than needed. A white wax crayon or engineering chalk used with a ruler or straight edge are good to use. Clean off any primer outside the join lines using a rag and Prepsol, after the join. If not removed, the primer will turn white underwater and won’t look very good.
It takes a fair effort to clean off the excess primer. This teaches the installer to be accurate with primer application. Using a contact adhesive to hold one side of the join in place makes it much easier to apply primer. This prevents the EPDM from curling up due to the applied primer. An aerosol can of rubber contact adhesive is the easiest and quickest product to use. Rubber contact adhesive in a tin is OK but much slower to apply. Complete the joins with EPDM in a relaxed state. It’s not good to have joins under tension, as the join may eventually fail.
Custom fitting an EPDM liner sure takes a lot more effort than covering the liner with rocks and stones. However, it provides a completely different finish and is perfect for clients who wish for a hybrid look — a cross between a normal swimming pool and a swimming pond. It takes dedication and practice, but the result is worth the effort.
About the Author: Jeff Knox is the owner of Riverwater Pools in New South Wales, Australia.
