I'm working on converting an external garage into an office. I've removed the original flat roof and replaced with a pitched roof. I designed and built the roof trusses myself. I then put underlayment on the timbers and tiles until the job was complete. The tiles overlap and are screwed to the woodwork.

The problem is that the pitch of the roof is such that when the rain is at an angle, water gets in at a few spots.

The question is, what's the best way to resolve this? I don't want to redo the roof, so I was thinking of using Marine plywood secured inside immediately under the tiles with felt on the plywood, creating a run off to the bottom of the roof and to the outside.

Would this work? Is there a better solution?

Another thing I was thinking of was using expanding foam instead between the tiles to seal up any gaps or places where water could get in.

[edit] Please see below a screen shot of the roof structure taken from 123D Design:

enter image description here

The trusses are 462mm apart, each truss is made from pressure treated timber 38mmx63mm. The root is 4840mmx4226mm.

The underlay is from Jewsons, http://www.jewson.co.uk/search-results/?keywords=roof%20underlay

The tiles overlay at least 2 inches. The underlay overlaps approx 3/4 inches. There is nothing under the underlay except the trusses. There are at least 2 leaks. The pitch of the root is approx. 30 degrees.

Hope this is the required information.

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    Caution about foam... it can become a sponge as it degrades. Commented Mar 24, 2016 at 16:11
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    The issue is the underlay. It is EXPECTED that rain will get in a little between tiles with some wind directions. The underlay should then direct the few drops of water that got in harmlessly into the gutter.
    – Walker
    Commented Mar 24, 2016 at 16:32
  • 1
    Please further describe the structure: How far apart are the trusses? What product/material did you use for the "underlay"? What size, product/material did you use for the tiles? How much do the tiles overlap vertically and horizontally? How much does the underlay overlap adjacent underlay? Is there anything supporting the underlay, or is it self-supporting? How large is the roof? How many leaks have you observed so far? How many bulges have you seen so far? What is the pitch of the roof?
    – Adam Davis
    Commented Mar 24, 2016 at 18:04
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    Ok the water will still cause damage over the years if not corrected. However if this only happens occasionally with a driving rain, and if you don't care at this time about the eventual damage, then you could install an absorbent on the underside. Like this: www.amazon.com/New-Pig-MAT3250-Medium-Weight-Polypropylene/dp/B00BUBPBEI/ref=sr_1_sc_3?ie=UTF8&qid=1458903717&sr=8-3-spell&keywords=Pig+roll+absorb+ant If this is stapled to the bottoms of the rafters with spacing from the roof it would have a chance to dry out between heavy rain storms. It should still be sealed from the outside.
    – ArchonOSX
    Commented Mar 25, 2016 at 11:06
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    The critical questions are 1) what's the pitch of the roof, and 2) what are the minimum pitch requirements of the tiles used? On roofs with less than a 3-1/2:12 pitch tile roofing is considered to be DECORATIVE and the waterproofing characteristics of the roof must be provided by whatever is under the tiles, e.g. a single-ply membrane over plywood. So if you've got a relatively flat roof put down over some type of underlayment with no waterproofing membrane, I think you're going to find you'll have to do it over. IMO tile roofing is more pain than it's worth, but YMMV. Best of luck. Commented Mar 25, 2016 at 12:39

3 Answers 3


You have either, too flat of a pitch in your roof or installed your roof tiles incorrectly. Just accept it and move on.

It's going to have to be redone. You would be wisest to tear it all out, then get someone who knows how to do it correctly in to do it. If you half-ass it now, it will just come back later, usually after causing structural damage for years.

Sorry, I know the truth hurts, but it's better you know now.

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    Adding marine plywood to the bottom would only trap the moisture that gets into the roof.
    – Dano0430
    Commented Mar 24, 2016 at 14:14
  • What @Dano0430 said x1000!! (Although that should have been on the OP, not this answer. Details... :)
    – FreeMan
    Commented Mar 24, 2016 at 17:52
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    The problem with a tile roof is that the tiles do not seal tight with the tiles around them. One of the reasons they are not used in areas that are famous for driving rain like the northwest US. Also, they work better on steeper pitches. So, PPA has some good advice here.
    – ArchonOSX
    Commented Mar 25, 2016 at 11:00

What is the pitch of your roof?

In the USA, this type of roof (most roof types, in fact) is installed over solid decking (usually plywood or OSB) with the underlayment nailed or stapled over the decking. Any water that gets under the tiles when the wind is blowing falls onto the underlayment and is directed down towards the eaves.

It seems that you have followed standard non-USA roofing practices and installed the underlayment right over the timbers with nothing solid underneath them. The downside to this approach, as you've discovered, is that with no support below the underlayment in most places, it can only shed a small volume of water before ponding. This type of roof is more sensitive to perfection in the choice and installation of the roofing material itself to keep out water. The underlayment is a last ditch defense backup. And it seems that you chose roofing tiles with too short a headlap (amount of each tile that is hidden below the tiles above it) for the pitch of roof you built: the shallower the pitch, the more headlap is needed for each tile. Clearly your roof is too shallowly-pitched for the headlap of the roofing tiles you chose.

At this point, you have no good options other than removing the tile and redoing the roof in one of several ways:

  • Add solid decking over the truss timbers and then reinstall the underlayment and tile
  • Rebuild the trusses to have a steeper pitch and then reinstall the underlayment and tile
  • Remove the existing tiles and install a type of roofing more suited to a shallow pitched roof in a rainy climate with no solid decking, such as metal panels
  • Remove the existing tiles and replace them with ones having a greater headlap that are designed for shallower roofs

There are various band-aid fixes you can try to apply such as caulking the tiles but they won't last. The roof has been built fundamentally incorrectly. It must be re-done to have any kind of longevity.


Just a caulk between tiles will last for decades. Pry the covering tile up as much as you can & either squirt the caulk in or load it on a 25mm putty knife to butter the seam of the abutting tiles beneath. Personally, I've found latex caulk or silicone to last exceedingly longer than any "specialty" roof garbage.

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