How to treat cracked Merbau decking?

[David10]

Hi Community,

We've just had our deck redone and we've noticed a lot of cracks in the merbau where the screws are. Our carpenter has completed about 95% of the screws and there are about 100 cracked areas on our 60m2 deck. He's been using these screws, which do not require a pilot hole, as long as you're far enough away from the edge and ends of the boards, I believe:

• Simpson Strong-Tie 12G (5.5mm) x 65mm 316 Stainless Steel DHSD No-PreDrill Hardwood To Softwood Decking Screws

He's advised us that he hasn't been drilling pilot holes when in the middle of the boards, but has been on the ends of the boards with a regular drill bit the same size as the screws, but not a countersinking drill bit.

The decking is pre-oiled 90x19mm merbau.

He will likely replace the worst of the boards, but with about 100 cracks, we won't be able to get everything.

Reaching out to see if there's anything we can do with the cracked boards that are left. Currently planning to wash, oil and maintain regularly, but also considering trying to seal cracks with Selleys Aquadhere PVA Wood Glue Exterior Adhesive, removing adjacent screws, wedging boards closed, re drilling screw holes and then screwing again. To try to seal up the cracks as much as possible.

Any advice would be greatly appreciated.

We understand that predrilling all screws with a countersinking drill bit 15-25mm from the edges and ends of the boards is best practice and how to avoid this happening, but the deck is almost finished now and we're looking for a solution moving forward.

The wedges in the photos are marking some of the cracked areas.

David

[David10]

Hi @JacobZ,

Thanks very much for contacting Simpson.

The majority of the cracks are at screws that are closer than 20mm from the edge, however quite a few are at 20mm. I've attached a few more photos. 

Could the timber being very dry cause this? It was installed a few days after delivery.

David

[JacobZ]

Hi @David10,

Yes, dry timber will lose some of its flexibility and is likely a contributing factor to the cracking.

Ultimately, a combination of things will have caused the cracks and if the installer were to do it again, they should predrill all the boards to ensure it doesn't happen again.

Let me know if you have any further questions.

Jacob

[David10]

Hi @JacobZ,

It looks like pre-drilling with a countersink has taken care of most of the cracking, but our carpenter has had a look and was frustrated as at least one board has cracked, even with pre-drilling. So it seems like some cracking may be unavoidable? Or maybe too high setting on his impact driver could contribute? The board seems like it might crack easily as there's a crack in the second photo that's not even at the screw.

Are cracks like these normal or acceptable in a new merbau deck? Mostly keen to understand what's normal and what we should be telling our carpenter.

David

[Dave-1]

Afternoon @David10 

In all the times I have used merbau decking it has split on my twice, and thats because I was going to close to the edge on purpose (was making a bracket edge) and overtightened after the predrill.

Courtyard privacy fence and gate build, Part 2  Step 4 fourth photo up on the handrail is the split I caused. It was because I was predrilling the 19mm width to go into the flat handrail and I didnt have my drill absolutly vertical 😕 

Were they new boards that cracked? If so then possibly the torque/impact driver was too high. I have used Merbau that has sat in the garage for 6 months waiting to be used and there was no issue. I use a torqued driver not an impact drill. Impact drill, I have issues with screws shearing off when I go into hardwood posts through hardwood decking.

Dave

[JacobZ]

Hi @David10,

If your carpenter is predrilling and countersinking the screws, regardless of how dry the timber is, they should not split unless the screws are being overdriven.

Predrilling and countersinking will remove material so there is a clear passage for the screw to go through. The only way splitting will occur is if the screw is overdriven causing the timber to compress which will push sideways to find space and potentially crack down the line of the grain. The screw should be driven until it just applies pressure to the top of the board.

My impact driver, which I always use to install screws, has multiple power settings and when installing screws that had the potential to split things, I would always put it on the lowest power setting and take care not to overdrive the screw. This is not too much to ask of your carpenter when installing screws close to the end of the boards.

Let me know if you have any further questions.

Jacob