How to fix drainage under replacement deck?

[Daz591]

Hello. I am in the process of repairing/replacing my deck due to rotting joists. Upon removal of some decking boards, I noticed that the original joist construction involves 100 x 50 joists with a sheet of gyprock/villa board??, then a 50 x 50 joist on top. This has resulted over time of debris and water building up around the smaller joists and causing rot. I am guessing the original builder has laid this gyprock in order to slow down rainwater drenching under the house and possibly flooding the garage during heavy rain. I am mindful that a low-lying deck should have ventilation, and it seems to me that this gyprock is preventing this and obviously catching not only water but debris, causing the rotting of the joists. I am of a mind not to replace this gyprock ( if that is what it is??). Any ideas?  Cheers...Darryl

[JacobZ]

Hi @Daz591,

Welcome to the Bunnings Workshop community. We're happy to have you here.

It appears to be a fibre cement product, which in older homes, can certainly contain asbestos. You should definitely have this material tested to ensure it is not asbestos. It is always better to be safe than sorry.
 

If it does not contain asbestos, feel free to remove it. If it contains asbestos, please contact an asbestos removal company for their assistance.

I can't see any benefit in replacing this material, nor do I understand why they would have attached it in the first place. All it is doing is creating a surface that water will build up on and increase the potential for rot.

Please exhibit caution, but I would suggest it should be removed.

Jacob

[Daz591]

Hi Jacob. 

Thanks for your reply. The house was built only 13 years ago (9 years after asbestos was banned) so very unlikely that it contains asbestos. Have showed samples to a number of builders and they are of the opinion that it is gyprock or villa wood with possibly a waterproof membrane painted over it. I guess the only issue if I don't replace it is how to control heavy rainwater from flowing through the deck-boards into the area under the house. I guess that is another problem. One problem at a time!! Thanks

[JacobZ]

Hi @Daz591,

As long as you're confident, you are safe to remove it. I just would hate to say go right ahead, and it turns out it was an issue.  

What is underneath the rubble in the photo you uploaded?

Is it the earth or is there a concrete pad or something similar?

Grading the ground so that any water that drips through the deck flows away from the house would certainly help. Some added drainage measures like covering the ground in drainage gravel or installing some agi pipe could also help depending on what you're working with.

If you have some photos of what is beneath the deck, this would certainly help us work on the drainage.

Let me know what you think.

Jacob

[Daz591]

Hi Jacob. 

Underneath the rubble is just earth. As you move toward the white door in the middle, the ground underneath the deck drops away to about 2.5 metres. Underneath the deck at this point is a rock face and earth with a brick wall that leads to the garage. At the moment, when it rains the water leaks through to under the house and most of it drains away through a small drain under the house. If the gyprock was not there, the rain would rush through a lot faster and possibly enter the garage. 

[JacobZ]

Hi @Daz591,

I'd have to see what is underneath the deck to try to offer a better solution, but as I've already mentioned, adding sheet products beneath the decking boards is not what I'd advise, as it traps the water against the timber, which will, in turn, rot prematurely.

Is there any way to get into that area and waterproof the surface of the brick wall with something like this Crommelin 1L Exterior Grade Brushable Waterproofer?

Is there any way you could add drainage that would divert the water away from the garage?

Allow me to tag @Nailbag and @Dave-1 for their thoughts.

Let me know what you think.

Jacob

[Daz591]

No worries Jacob

Happy to hear anyone's thoughts.

I'll try and describe the issue if I do not replace the sheets under the decking boards.

Picture 1.

This is the view from underneath the deck. I am under the house, just in front of the garage just underneath the white back door in the previous picture. You can see the underside of the decking sheets.  (also note the water stains.) This underside of the deck is about 3 metres from the ground under the house. These sheets obviously slow down the water from cascading down when it rains. (Doesn't stop it, just slows it down).

Next picture:

You can see the underside of the deck in the top of the picture. The rainwater cascades down this rockface. Usually carries with it some silt and stones in heavy falls.

Next picture:

The water ends up on the dirt ground at the base of the rock face in the previous picture. This is the view on the other side of the manhole from my garage. Where the black plastic is there is a drain which carries some water away under the house and down the front of the house. (Very steep drop in the front. I am concerned that if I don't replace the sheeting under the deck boards, the heavy rainwater may be too much for the drain and then overflow into the garage. This has happened once with an extreme rainfall event, but without the sheeting slowing down the rainfall, it may happen often. I can not make the drain bigger as it is constructed through a concrete wall nearly a foot thick. I guess the worse case scenario if I don't replace the sheeting is managing a flooded garage. Cheers

[EricL]

Hello @Daz591 

It is a rather complex issue that has many parts interconnected. If the villaboard is not reinstalled the amount of rainwater entering the bottom of your house will increase tremendously and might negatively impact the foundation of your home. However, reinstalling the villaboard means that your decking joist will have a very short lifespan as water will get trapped between the materials.

One way to provide a gap between the joist and villaboard is to use spacers or something similar. This will prevent the joist from making direct contact with the villaboard. I also recommend using Gripset Betta 1L Waterproofing Membrane Bitumen Rubber both on the surface of the villaboard and the bottom of the joist. This will provide a waterproof membrane that will repel any rainwater that it encounters and thereby extending the service life of the joist and villaboard at the same time. 

To increase the water tightness of the build, I suggest using Sika 310ml White Sikaflex Pro+ Polyurethane Sealant - 310ml White on the edge of the villaboard to prevent water from trickling down under the house. The more complex solution is to build an elaborate draining system under your house that also involves a ventilation system to prevent trapped moisture from spawning mould under your house.

Please note that adding the spacers will increase the height of your deck. Make sure to take this into account when you start rebuilding your deck. 

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

[Daz591]

Hi Eric.

Thanks for your suggestions. The spacer idea is great as it solves two problems. The original battens sitting on top of the joists were 50mm x 50mm ( a difficult size of timber) I was going to have to replace with 45mm which meant my deck would drop by 5mm. The spacers fix that problem and the future rotting problem. I was never comfortable with not replacing the sheets due to the problem you discussed about the amount of water, and retro fitting an elaborate water drainage would be just too complex. The current sheets have been painted with a waterproof membrane and I have used the product you suggested before with outdoor projects.

Thanks for your suggestions.

Cheers,

Darryl

[Dave-1]

Good Afternoon @Daz591 

Now thats a pain, stuck between a rock and rain pretty much 😕

I agree that the villaboard material cant go back, thats asking fro trouble in another 9 years.

I wouldnt want to add more water to my garage and wouldnt be happy with what you describe as already entering the garage.

I have seen a flexible membrane used that hangs over joists under the decking boards to catch and divert rainwater on first floor decks. If you could install something that catches and then funnels the rainwater to a drain under the decking and slopped the right way may be an answer. Here is a link to the type of thing I mean. I havnt used them or seen them in the big blue F social media pages so it may be an idea to check reviews if you go that way. Trex Rain Escape Trough 

You will still need to have the bottom end of that drainage system drain to a stormwater point. I would figure a large area such as the deck size you have would catch a fair amount of water.

Dave