The most important issue when building a wheelchair ramp is to make sure that the incline gradient is not too steep. Remember that ADA guidelines require that the ramp has at least a 1:12 gradient, meaning that for every inch of height, the ramp should have at least 12 inches of length. If you live in a snowy or icy area, the length of the ramp should be even longer.
Use the Sandpaper like Shingles. The kinds the roofers use. It's pretty much waterproof, inexpensive and can be purchased by the smaller boxes. If your friends can come back and put a simple cover over the ramp, that might be your best bet.
We have pressure treated wood for a ramp and front deck. So when it snows I just use the shovel, and sometimes I need to put salt on it to help scrap it. If there's a problem at all, it's usually someone walking on it, not my husband's chair. You might want to test it before the snow comes!
You could take a gallon of outdoor paint and add a cup or two of sand into it. This will give it some grit for traction when it is painted onto the ramp. Good Luck.
This is just an example of treads you can put on there to help with your issue. Other places sell them too also dark colors will help the snow melt when the sun shines on them, much like black paved driveways melt much faster than lighter colors.
http://www.collectionsetc.com/4PC_S ... &No=0&N=0&productid=2051
You say the guys used underlaymant for the top surface. Question is what kind of underlayment. If it is of the OBS type (Oriented Strand Board) it is not recommended for external construction and that is why they said not to salt or use a shovel. It is water resistant but not for out door use.
I think it would be better to take up the underlayment and replace it with pressure treated wood. You can shovel it and it will last for years. Not knowing what the underlayment is it is difficult to give a good answer.
I am not an wheel chair ramp expert, although I've worked part time in health care for many years.
My first thought was a no-brainer: for you to sweep the ramp with a broom often, epecially when the snow is "dry" so it won't melt later and form ice.
My other idea is to attach something like astro turf to the ramp, wich I think would make it less slippery come winter.
I hope somebody else here has more experience with these things.
p.s. you can also improve footing by applying grainy, non-skid outdoor stair treads, such as are available at large hardware stores.
I have a client who screws a tarp along one side and then holds down the other edge with a couple of bricks. This, or some variation of it might work for your son.
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